Among the various consequences stemming from global warming: Arctic ice melt, plant/animal extinction, sea level rise, increasing ocean acidity, etc, one of the most important is how climate changes will influence agriculture. With a world population slightly above 7 billion and expected to increase to between 9 and 10 billion by 2050 identifying how agricultural practices will need to adapt to a changing climate is essential to avoid large periods of food instability and even starvation which could lead to outbreaks of violence like those seen in 2008 when food prices increased sharply over a short period of time. While the probability of more extreme weather events will increase as global warming progresses, it would be difficult to design methodologies to account for these events for standard agricultural practice due to associated costs and divergence from more stable weather events; thus adaptation strategies should be designed under the assumption of a general anticipated progression for changes in the climate with the flexibility to compensate for an extreme weather event. To accomplish this goal it is important to investigate how the climate will change in response to global warming and how those changes will affect agriculture because combining climate vulnerability and low adaptive capacity will create an environment of food scarcity, which will lead to significant reductions in global quality of life.
The 2007 IPCC assessment report comes to the conclusion that despite increasing temperatures agriculture will experience increases in cereal crop, wheat and rice yields in mid and high-latitude regions with reduced yields in tropical regions over the short to mid-term.1 The net result of these changes is presumed to be a net increase in global food production. This conclusion is born from numerous enclosed experiments (open-top chambers) that attempted to isolate the affects of doubling CO2 concentration (from pre-industrial levels with a consistent temperature of 25 degrees C) and demonstrated an overall average increase for C3 crops of about 30% 25% with a 99% confidence range of 18-32%.2-6 More specifically at 550 ppm yield increases were 21%-29% for soybean, 17%-21% for wheat, 17% for rice and 6%-10% for maize.9-11 Other experiments concluded that due to better water utilization and absorption rain-fed crops will experience higher yields than irrigated crops.7 Also low fertilizer exposure reduced the effectiveness of crops responding to increased CO2, thus to realize these gains fertilizer deposition needs to remain at modern day rates.5,8
An increasing ambient CO2 concentration typically increases yield through augmenting photosynthesis and stomatal conductance;12 note that this effect is much more prominent in C3 plants over C4 plants. Plants perceive a change in atmospheric conditions, including CO2, through the inner surfaces of the guard cells of stomata and the mesophyll due to the protective cuticle of the leaves and other photosynthetic areas.12 Some believe that part of the detection could also be influenced by an unknown reaction that influences stomatal aperture size.13 C3 plants have sources of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) in mesophyll cells that are in direct contact with the intercellular air space that receive ambient air through stomatal pores in the epidermis.14
Due to a lack of atmospheric saturation of CO2 increasing the concentration of CO2 increases the overall level of reaction between CO2 and ribulose bisphosphate forming more 3-phosphoglycerate and other photosynthetic byproducts that later enter the Calvin Cycle. The inhibitory properties of CO2 on photorespiration further increase photosynthetic efficiency in a CO2 elevated environment.12,13 Note that photorespiration involves RuBisCO fixing oxygen instead of carbon dioxide, which limits photosynthetic efficiency. Some argue that an increase in ambient CO2 concentration from 380 ppm to 550 ppm would increase C3-based photosynthesis by 38%.14
C4 plants do not experience significant photosynthetic enhancements in an elevated CO2 environment because the C4-based photosynthetic pathway derives the principle supply of CO2 that interacts with RuBisCO from malate or asparate intermediates not directly from the ambient air. This substitution occurs because in C4 plants RuBisCO is typically isolated away from mesophyll cells. The enzymes pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase, which converts absorbed atmospheric CO2 to oxaloacetate and eventually malate or asparate, interacts with the CO2 that enters the plant from ambient air.15 This product (malate or asparate) is then transported to chloroplasts in the bundle sheath cells to interact with RuBisCO and begin the photosynthetic process. Due to this process the CO2 concentration C4 RuBiSCO is exposed to is typically three to six times current atmospheric CO2 concentrations saturating RuBisCO making increases in atmospheric CO2 inconsequential.15
Note that CO2 enhancement of C3 plants is not governed solely by its own increase. Photosynthesis in C3 plants is governed by two limitations: RuBisCO availability and ribulose bisphosphate availability. The higher temperatures that associated with increased CO2 concentrations also increase ribulose bisphosphate regeneration,16 which should also enhanced photosynthesis at higher temperatures and higher CO2 concentrations, to a certain temperature ceiling for eventually temperature related stressors cause the plant to divert resources to survival.
Unfortunately despite the favorable biochemistry there may be inaccuracy regarding how beneficial increased CO2 concentrations will be on crop yield in actual practice. Most of the favorable information regarding CO2 concentration influence comes from open-top chamber experiments, but these experimental environments have demonstrated some concerns with their field replication accuracies. First, the chambers have a lower transmission of sunlight, which creates a higher ratio of diffuse to direct light versus outside the chamber.17 Diffuse light increases photosynthetic efficiency over direct light due to reduction of saturation probability. Second, the chambers alter airflow to conditions that may not be regarded as natural. Third, the interaction between the plants and available water is somewhat dissimilar than in field plots.18 Fourth, a number of chamber plants are grown inside of pots and it has been demonstrated that rooting volumes are altered in response to increased CO2 concentration and also have strong feedbacks when roots encounter a physical barrier.19 Overall there are legitimate concerns about how the microenvironments of these chambers change the ability of the plants to interact with the elevated levels of CO2.
These concerns are supported in the few free-air CO2 enrichment (FACE) studies that have been conducted. FACE experiments involves exposure of plants to evaluated levels of some gas (typically CO2) in natural and fully open-air conditions thought the elimination of any confinement structures. Instead of confining the plants they are planted in a typical field plot and the enhanced element is added through a series of vertical or horizontal pipes that exist on the periphery of the plots relying on diffusion and passive elements (like the wind) to disperse the added gas. While initial concerns were given about temporary concentration spikes on the plants closest in proximity to the gas release points, these concerns do not appear to have any significant influence on long-term FACE studies.
While the FACE studies are few and far between versus the amount of chamber experiments, C3 plant FACE results fall well below (50-75% less yield increases) than those results seen in chambers.12 Specifically wheat yield increases were 33% chamber yield increases, rice was 67%, soybean 80% and corn had no significant change in yield with enhanced CO2 conditions versus normal CO2 conditions (basically a 100% drop).13
This reduction was seen in both yield/biomass and overall photosynthesis. The photosynthesis result may genuinely facilitate concern about the diffuse light generation inside the chamber. For example chamber studies using rice have demonstrated a level of photosynthesis that is four times greater than FACE studies using rice.12 As mentioned above the biochemical pathway for C4 plants suggests no significant increase in yield due to increased CO2 concentration and this result is supported by FACE studies, but not supported by chamber studies.20 Some question this difference,21 but including all of the available studies lower yield does appear in the FACE studies.14,22
A secondary potential benefit of increased CO2 concentration comes from an improved drought response in both C3 and C4 plants. For example under drought conditions elevated CO2 concentrations increased photosynthesis by 23%.23 Unfortunately whether or not this increased photosynthesis activity results in a higher direct yield remains unclear.24 One could conclude that increased CO2 concentrations will facilitate an indirect increase in yields through increased survival rates of C3 plants in drought conditions.
Unfortunately a direct environmental consequence to an increased concentration of CO2 is an increase surface temperature, which will have a negative influence on crop yields. In fact from the mid 1970s to 2002 crop yields in wheat, maize, soybeans and barley have all decreased due to what is believed to be rising temperatures.25,26 The bigger problem may be the fact that wheat, maize and soybeans represent a large portion of the food supply and the influence of temperature on their yields appears to be non-linear.27 This non-linearity was partially demonstrated empirically on a large-scale during the European heat wave in 2003.
Temperature is a problem not only through long-term application, but also through acute snaps. For example even a few days of high temperatures can negatively affect the grain-filling period of cereals and wheat. Every degree C beyond 25 C can reduce the reproductive phase by 6% and shortens the grain-filling duration by 5% reducing yield and harvest index.28 These acute periods of high temperatures also apply stressors that negatively affect pollen viability, fertilization and grain formation among other things. These negative influences last even if average temperatures for the growing season end up in a ‘safe average’ range. Also long-term higher average temperatures lead to faster maturation of crops reducing the total amount of carbohydrate accumulation and overall grain production. Currently warmer tropical climate address this issue by growing crops that require longer times to mature. Unfortunately this strategy does not appear applicable to a world warmed by global warming because certain tropical regions will be eliminated as food producers increasing the production burden on other areas, areas that cannot afford to spend more time cultivating a single cycle of crops.
In a progressively warming environment, changes will need to be made with regards to when and/or how crops are planted. Unfortunately due to the anticipated losses in the tropical regions and increases in world population, temperate yields need to increase to cover these responses. This additional yield can be achieved one of three ways; first, more land is cleared and prepared for cultivation. The immediate problem with this strategy is that creating more cultivatable land would typically lead to more deforestation, which would result in ecological damage, increased probability for species extinction and a further increase in CO2 concentrations, both from the deforestation and cultivating the new land, thus resulting in a further temperature increase both in magnitude and duration.
Second, change global eating habits dramatically reducing the amount of meat consumption. While the exact numbers and quality are under debate there is no doubt that the production of one pound of meat costs at least three pounds of human viable grain feed. Under this formula each pound of meat that is not consumed frees up at least three pounds of grains for human consumption. Unfortunately while this mindset is easy to theorize it appears incredibly difficult to execute because decreasing poverty levels in China and India are creating more middle class income households and these individuals are developing a taste for meat. Therefore, it is expected that demand for meat products will increase in the future not decrease.
Third, existing arable land in temperate regions need to become more productive. The most direct way to accomplish this efficiency is to double crop all existing land. Double cropping involves planting two different types of crops on the same parcel of land within a single year. The most popular form of currently executed double cropping involves growing wheat in the spring and later planting corn or soybeans in late summer. However, with the understanding of the negative influence of temperature on crops and the progressive increase of temperature during the normal growing season (April – September) in the future, it seems that adjusting the planting schedule to avoid these future negative consequences would be wise.
The execution of a newly timed double cropping strategy will be a little tricky because of the germination and maturation times that are allotted for a given crop versus the temperature changes that need to be accommodated. For example one strategy may be to plant an initial crop of wheat, barley or maize in mid March and harvest in late June and wait until mid August to plant a second crop of soybeans or high value crop and harvest in early November. The idea behind such a strategy is to attempt to avoid late June to early August where temperatures will favor higher ranges that may result in significant crop losses. The general increase in temperature will limit the negatives associated with starting the first crop in March with irrigation additions addressing soil moisture and early photosynthesis.
Harvesting the second crop in early November creates two concerns. First, depending on the exact dates of planting and harvest there will be about a 90-95 day window, which based on existing experience is a short window of maturation for most crops (100+ days is preferred). One hope is that higher August and early September temperatures will speed maturation. Second, such a strategy depends on gaining experience of trends in late October and November with regards to both temperature and sunlight availability. If growth functions normally the double cropping strategy can be expanded to include a primary season for normal food stuffs and a secondary season for food stuffs that provide additional soil benefits like nitrogen fixation because increased temperatures will hasten nutrient loss from the soil.
There is a concern that a second planting during the mid-fall may not be effective because farmers in these regions already have little success planting cultivars with low vernalization requirements. If this is the case then this crop planting will have to wait for new breeding programs to produce new cultivars with even lower vernalization requirements, a process that will more than likely take around a decade. Note that vernalization is the cooling of seed during germination to accelerate flowering. One way to address this issue beyond breeding may be to cover the second cultivation with a biological residue, either manure or residue from the previous harvest to limit temperature exposure.
Models demonstrate that shifting planting to earlier times flip yield losses to small yield gains in temperate regions, but tropical regions are more problematic due to limitations due to moisture versus temperature.29 This limitation creates a two-pronged required analysis for changing planting dates between soil moisture and ambient temperature. The moisture limitation may be eased by more efficient irrigation techniques, but the costs associated with their application may be too large. If irrigation proves too expensive then another strategy for tropical regions could use faster maturing crops planted early, which would avoid the most of the negative elements associated with drought or heat stress, especially during the flowering and grain filling stages of growth. Based on increasing temperatures this strategy may need to be applied to more temperate regions as well.
There are still some significant questions regarding how global warming will alter precipitation. A common rule of thumb is that under ‘normal’ conditions wet areas will see an increase in precipitation and dry areas will see a decrease in precipitation, but all areas will see increases in the probability of extreme precipitation events.30 Basically precipitation will increase in the deep tropics and extratropics and decreases in the subtropics and most temperate regions, but flooding probability will increase universally. However, one side element to this rule of thumb is that it is relative because higher temperatures will result in greater overall levels of evaporation reducing the overall influence of the increased precipitation. There is some question to this result due to the Clausius– Clapyeron relationship in that specific humidity increases exponentially relative to temperature.
A problem with applying the C-C relationship is one cannot do so in a vacuum, existing empirical evidence suggests an overall global drying trend, especially since the 1970s when global warming started to significantly influence temperatures.31 In fact the IPCC reports conclude that models suggest increased aridity in the 21st century in most of Africa, most of North, Central and South America, Australia, southeast Asia and southern Europe.31 Also it is important to note that there is some question to whether the influencing factor on the occurrence of these extreme events is near-surface water vapor concentrations or total atmospheric water vapor content.30 Finally a tricky issue in evaluating precipitation is how cloud formation, both in number and type, will change in a global warming influenced environment, which will also influence the level of precipitation. Overall at the moment most signs do not point to an increase in rainfall for most large scale growing areas throughout the globe.
Based on how global warming will influence precipitation levels in the future within high yield regions across the globe, dependence on rain-fed crops will become more and more risky when attempting to maintain yield stability. Most high volume cultivation areas are expected to see a decrease in precipitation in the near future. Due to this expected inconsistency and anticipated reduction in average precipitation an important adaptation would be for farmers to install drip irrigation systems. Drip irrigation systems have demonstrated water conservation superiority over flood and spray irrigation system while not sacrificing total water distribution to crops. The mindset to install a drip irrigation system is understandable, but some believe that the cost could be prohibitive. There are two major types of drip irrigation: surface and subsurface with subsurface being a more efficient and the focus of this discussion.
Subsurface drip irrigation utilizes subterranean tubing, usually made from plastic, to deliver a slow and even application of low-pressure water to the root zones of plants. The interaction of water directly on the root system reduces water contact with crop leaves, stems and fruit reduces the probability of disease development.32 One of the chief advantages of drip irrigation is the flexibility of its application, which allows operators to adjust to crop water demands increasing yield and individual plant quality. Drip irrigation typically has an application efficiency 95-99% where sprinkler irrigation efficiency is 85% and flood irrigation efficiency is 65%.32 Some estimate a 25% reduction in net irrigation that result in 35-55% income savings relative to irrigation costs versus sprinkler or flood irrigation.32
One concern with drip irrigation is a lower rate of crop rooting because of a smaller volume of soil wetting. Some believe that a smaller root system would demand more frequent irrigation limiting the water savings provided by drip irrigation. Fortunately some evidence exists that limits the impact of this concern as irrigation frequencies of 1, 3, 5 or 7 days have shown to produce similar corn yields.33 In fact there is some indication that higher water-use efficiencies were achieved in the 7-day test due to reduction in deep percolation below the root zone and better overall water storage.33 However, this lack of significance in frequency may be specific to deeper-rooted crops like corn over crops that form naturally shorter root systems.
One important issue when applying a drip irrigation system is how to space the driplines. Ideally one line should be placed per row, but such a strategy could increase costs, so some argue one line per two rows (alternating rows)34,35 Studies have suggested that alternating dripline installation leads to a reduction of 30-40% in capital costs.36-38 However, alternating driplines may not provide enough water for some studies have concluded an excessive water requirement to plants both before planting and during growth because of evaporation.36,37,39 This excessive water requirement is more pronounced in soil with a higher clay content or coarser soil.39
One strategy has highlighted the installation of driplines to each row when introducing drip irrigation and then after some time (2-3 years) driplines can be reduced to alternating rows.32 Unfortunately such a system is only slightly more cost effective because of the full capital cost for the per row system is still applied. Another possibility is that long root crops can better manage alternating rows, but shorter root crops will require a per row system.
An interesting cost reduction aspect of either type of drip irrigation is that the conserved water can actually be reassigned to areas that have not incorporated a drip irrigation system. Therefore, even if a drip irrigation system cannot be applied to an entire plot its partial application is still beneficial for in long-term economics and both the portion of the field that receives its effects and the portion that does not.
The prohibitive nature of the drip irrigation economics stems from the high initial capital costs, the absence of a general economic methodology for purchasing and installing the drip irrigation system and some difficulties regarding long-term economic analysis for determining a rate of return. Some of the problems in the economic analysis are uncertainties in water resource availability and cost, commodity prices and changes in site-specific soil characteristics.40 One study did determine a 450% net return on the overall investment over a 16-year period.41
Overall while it is only one study the length of time associated with that rate of return is generally appropriate as a guideline for the future for large plots. At least it would be if global warming were only a temporary occurrence. Applying global warming to the analysis should decrease the years required to generate a net gain from the installation of a drip irrigation system because of greater strain on soil health and water resources. In fact it stands to reason that installation of drip irrigation systems may become required to turn a consistent annual profit, especially for the second planting as the application of new drip irrigation systems should address the concern about providing enough water to winter crops in a given season. One of the biggest disadvantages of a sub-surface drip irrigation system that can increase costs is that the tubing must be maintained to eliminate any leaking or plugging. Most plugging is either caused by silt when it is not filtered out by the irrigation water or algae. Fortunately in the new double cropping schedule plenty of down time exists for cleaning and maintenance.
Irrigation is the most important and expansive element in the water consumption sector accounting for almost 70% of global freshwater use and over 90% of consumptive water use.42 Expanding agricultural growth has increased the need for irrigation water and increased the amount of groundwater use. Groundwater use is effective because it provides flexibility in use and availability; however, this flexibility has costs and have lead to a significant decline in groundwater tables along with reductions of river base flow, which not only increase costs, but also have a negative impact on aquatic ecosystems and even soil.42
In the future the probability of consistent fresh water stress will be much higher due to warmer temperatures and more sporadic precipitation. While drip irrigation lowers water use, another aspect involving the flow of water below the soil in subsurface drip irrigation expediting the movement of salts away from the root zone towards the surface of the plot36 could offer another water reduction strategy. What if drip irrigation could not only reduce the water requirement for growing crops, but could also do it with salt water instead of fresh water? Using salt water for crop irrigation would free-up fresh water supplies for human and animal consumption. Also developing a system for the utilization of salt water will better manage areas of crop growth where water is scarce. There is some evidence that suggests such a strategy may not be so far-fetched.
Saline water with a concentration of 11 dS/m has been used in commercial irrigation for successful crop growth and maturation.43-45 Of course using saline water requires a specific management procedure, otherwise multiple seasons will typically result in complete yield collapse. The factors that have been identified as important for saline irrigation are: 1) water use – clearly reducing the amount of water reduces the total amount of salt, but enough water must be provided to ensure proper growth; 2) Ensure leaf health – most notably avoiding leaf burn; 3) frequency of irrigation application – increasing salt concentration of the soil will hasten water loss from the soil, thus avoiding peaks in salt concentration and concomitant high osmotic potentials is important; 4) salt location – salts will be continuously leached out from wetted areas accumulating at the wetted front away from the active root zone.36
As mentioned above frequency of irrigation is an important issue. Low discharge rates result in longer irrigation application periods and depending on the soil quality may even result in higher salt concentrations compared to high discharge rates. Also low discharge rates can increase soil evaporation rates due to longer soil wetting reducing irrigation efficiency. Evaporation rates can be controlled somewhat by starting the major portion of the irrigation process at dusk and ending it at sunrise limiting the total temperature exposure due to limited sunlight. The advantage to a low discharge rate is a more consistent and linear change in soil salinity, which reduces the overall salinity stress on the soil and the plant.43 However, low discharge rates seem to be better with a residue cover of mulch or other plant matter with respects to retaining water content in the soil over high discharge rates.
The biggest concern with using saline water is salt accumulation in the soil over time. There appear to be two ways to address this concern. First, in areas that receive a sufficient amount of rainfall the concern may not become a problem because rain can lead to leaching salts from the soil. Second, the drip irrigation system can switch between saline water and fresh water for each crop rotation to alleviate the salt build-up. Doping soil with potassium may also reduce the severity of salinity stress from both salt and other types of soil salts like chlorides, sulfates, carbonates, and bicarbonates improving overall plant health.46,47
A possible side advantage of drip irrigation is its role in the process of fertigation, which is the process of delivering fertilizers or other water-soluble products (including chemicals) through the irrigation system. Such a system provides nutrients to the center of the root system for a given plant, the subterranean application reduces soil surface fertility reducing weed germination and can stimulate root growth increasing survival probability and crop quality.48 There is evidence to suggest that fertigation can acquire the same soil nitrogen concentrations at lower base applications.49
In the heat of the Green revolution little attention was paid to the soil and how the large monoclonal cultivation seasons affected its nutrient composition. Some of the negatives of this strategy have been compensated for by the addition of industrial nitrogen/phosphate fertilizers, but unfortunately utilization of nitrogen fertilizers is only a temporary measure under the current methodology due to phosphate limitations and increasing costs. The key element regarding soil quality is the amount of soil organic carbon (SOC). There is a direct correlation between the amount of SOC and soil quality. The SOC pool is comprised to two chief components: 1) the inert component, like humus or biochar, that is not involved in the mineralization process; 2) the labile/active fraction that dynamically changes in the soil.50,51 Note that neither component is permanent in that humus can breakdown into a more active component and more active components can solidify into an inert component. However, the dynamic change of a component from one to another is not a common occurrence and typically occurs over long periods of time.
Changing the SOC pool is though to increase soil quality and crop yield through three separate mechanisms: 1) increasing the available water capacity for the soil and thereby the crops; 2) enhancing soil structure (porosity, etc.) and other physical properties which allow for more microbacterial intrusion and growth; 3) improving the supply and access of nutrients;50 The increase in water capacity has been especially noteworthy in restricting soil loss from excess rain and flooding. In fact it has been thought that the soil available moisture content increases by 1-10 grams per every 1 gram increase in SOC.52 With more short-term excessive rain event probable in some regions resulting in a higher probability of flooding, better soil water retention would prove useful. Also increasing soil-based microbacterial activity is important for increasing the nutrient availability, preventing against diseases and aiding root-based water collection and extraction.50
The two main methods of SOC loss are actually divided between the developed world and the developing world. In the developed world the main method of loss involves the removal of crop residue for use as fodder for animals or bio-fuel/energy leaving the soil exposed to various weather conditions resulting in soil loss through erosion and nutrient loss through evaporation. In the developing world the main method of loss involves the use of animal waste-based manure in cooking instead of returning it to the fields. This is a problem because developed countries typically use chemical fertilizers in lieu of manure, but in developing countries these fertilizers are cost prohibitive, thus if waste-based manure is not used the soil is less able to recoup nitrogen and phosphate losses from harvests, which reduces SOC.50
The most common method for restoring SOC is to utilize a crop residue cover, which reduces both SOC losses and leads to SOC gains after decomposition of the crop residue. However, there are some planting drawbacks that need to be accounted for in such a strategy and will be discussed later. Another strategy involved crop rotation by switching between nitrogen consuming and nitrogen fixing crops or between deep-rooted and shallow-rooted crops. Early versions of crop rotation focused on incorporating periods of ‘resting’ the land where a tract of land would be divided into a multiple number of plots with some percentage being cultivated and another percentage being left unsown for that growing season. Unfortunately in modern times the world population is large enough that resting land during growing seasons is more difficult, thus natural rejuvenation is more conducive to double cropping with crop residue and nitrogen fixing crops.
Another strategy to restore SOC is to supplement soil with biochar. Biochar is basically charcoal created through the pyrolysis of biomass that is placed in the soil. The idea is based on terra preta in the Amazon, which has thought to be a stable source of carbon for over 6000 years. Numerous studies have demonstrated that when biochar is integrated into soil, it increases the quality of that soil enhancing the growth rates and yields of any future crops. There is little doubt that biochar improves the quality of soil on a general level by aiding in the supply and retention of nutrients, controlling soil acidity, increasing microbacterial activity, nitrous oxide emission reduction and reduced nitrogen and phosphate losses through leaching and increases in these attributes will increase SOC.53,54
Unfortunately one of the chief problems of biochar is source material. A number of biochar supporters want to use agricultural and forestry residue as a source reactant to create biochar. However, such a strategy increases the probability of erosion damage and reduces soil bacterium reducing the overall effectiveness of creating and applying the biochar in the first place. It could be said that such a strategy is akin to the parable, ‘Robbing Peter to pay Paul’.
Another issue that must be addressed when considering a biochar application strategy is any influence that large deposits of biochar would have on the average albedo of the field plot. Large deposits of biochar could darken the Earth’s surface at the point of entry, which will increase the probability of thermal absorption of sunlight at those areas and increase localized surface temperature. This change in surface albedo could neutralize some of the soil benefits derived from the biochar due to higher evaporation probabilities.
An official procedure for the application of biochar needs to be developed as well. Currently no ‘ideal’ time has been developed with regards to the application of biochar (before irrigation, after irrigation, before fertilizer application, after fertilizer application, etc.). Such a strategy is important for understanding biochar stability in the soil both as a single compound that could move through the soil and a compound that could, although unlikely, breakdown. Some lesser concerns relate to the health of those directly handling biochar as at times particulate matter can be released from biochar during the application process and could enter an individual’s lungs. However, as mentioned such a concern is not a large one.
Finally a side note about biochar application is the question of augmenting cation exchange capacity of the soil. Cation exchange capacity is important in mineralization and making nutrients available from the soil to aid plant growth. However, although modern synthesized biochar does not have the exchange capacity of terra preta it does have more capacity than other soil organic matter due to its greater surface area, charge density and negative surface charge. Currently modern synthesized biochar has been unable to achieve anything remotely similar in cation exchange properties to natural terra preta.53 One thought is that cation exchange capacity only increases with dynamic experience and cannot be replicated in a chemical reaction. Basically the terra preta is better simply because it has had thousands of years of experience with cation exchange which has slightly changed the properties of the terra preta.53 Temperature may increase this process because high exposure temperatures (30-70 C) increases cation retention and the overall cation exchange capacity.53v
Another potential adaptation method could be to culture certain beneficial soil bacteria and fungus in effort to introduce them to soil when soil health is reduced. While higher temperatures will place a greater strain on soil health, the addition of double cropping will further strain soil, especially if farmers and agricultural corporations forgo using soil positive crops in favor of more high value crops. Therefore, to have a stock of symbiotic soil bacteria could prove to be useful.
One valuable soil fungus is the Mycorrhiza family, which form a symbiotic relationship with roots of most plants. The beneficial relationship involves the Mycorrhiza receiving a constant supply of glucose and sucrose from plant photosynthesis and the plant receiving enhanced water and mineral absorption from the large surface area of the mycelium portion of the Mycorrhizas versus its natural absorption via its roots.55,56 The smaller diameter of the mycelium allow for greater soil exploration and resultant nutrient acquisition. This increased absorption is most notable with respect to phosphate.56,57 In addition plant disease probability decreases when mycorrhiza are co-localized with plants.57 The ability of mycorrhizas to improve the growth probability of co-localized plants in poor soil could open up more northern latitude soils like those above the shield zone in Canada. However, such a strategy should probably be undertaken only in an emergency because of the general poor quality of soil and the costs associated with beginning cultivation.
A second important soil bacteria is rhizobacteria, which is also referred to as plant growth-promoting rhizobacteria (PGPR). The chief advantage provided by PGPRs is the solubilization of unusable nutrients in order to extract free elements or other more useful compounds.58 This solubilization is most notable in creating additional supplies of phosphorus and nitrogen.58,59 PGPRs are able to fix nitrogen into more usable ammonia and synthesize phosphoric acid (H2PO4) from other phosphate containing compounds.58 A cooperative relationship between legumes and rhizobacteria significantly increases nitrogen fixation apart from isolated legume growth. This relationship is so well regarded that rhizobacteria are already cultivated and have been inoculated with various legumes like peas for decades. While inoculation to other crops have been prohibited by costs, with increasing temperatures placing greater strain on soil health these costs may be better justified.
There are three common tilling methods, conventional tillage (CT), reduced tillage (RT) and no-till (NT). CT is as a practice that leaves less than 15% residue cover after planting, RT leaves 15-30% residue cover after planting and NT leaves greater than 30% residue cover.60,61 To achieve this result CT involves plowing, RT involves disks or chisels and NT does not significantly disturb the soil. The chief driving factor for the introduction of no-till techniques was to improve soil quality.
In some areas NT has expanded rapidly, most notably in South America; however, globally NT techniques only encompass 5-10% of land utilized for food production.60 The reason NT has not progressed quickly globally are the short-term economic losses associated with moving from CT to NT due to reduced seedling emergence lowering yield and expansion of weed/crop competition. The change in temperature and moisture of soil demands an examination of available tilling methods to determine if existing advantages and disadvantage will change with the climate for these different methods.
The principal advantage of CT is to soften the soil, which prepares a seedbed allowing seeds to be placed into the soil at suitable depth for efficient growth origination. A side benefit of CT is that any weeds are typically buried beneath the soil preventing their germination and limiting their ability to compete against the seed crop. More efficient soil placement of the seed also helps the seed out-compete weeds that are not disadvantaged by the tillage. In addition CT is though to increase usable nutrients in soil through mineralization and oxidation due to greater exposure of organic matter to ambient air. Breaking up soil compaction is another advantage of CT, which better aerates the soil providing nutrients and controlling soil-born diseases and insects over the long-term.
However, there are disadvantages to CT including the increased probability for release of carbon sequestered in the soil, disruption of soil aggregates, which increase organic matter decomposition and increase in erosion (both wind or water) probability reducing top soil availability.50,62,63 Soil moisture over the long term is greater in NT than CT due to greater water incorporation.64,65
Most NT strategies involve the creation of crop residues either by leaving the previous crop loosely anchored after harvest or by creating mulch by killing the previous crop. Other strategies elect to apply external sources of mulch from compost and/or manure. However, externalized crop residues have problems due to additional economic costs. The residue also moderates soil temperature reducing water loss due to evaporation and promotes biological activity; this biological activity then enhances carbon and nitrogen mineralization, most notably in the surface layers, and aids nutrient recycling.66,67 This soil temperature control has been thought of as a double-edged sword because it aids growth in tropical and sub-tropical environments, but hinders growth in more temperate climates due to slower soil warming in the spring which delays germination.64 NT erosion control is largely dependent on two factors: the slope of the utilized land (how fast will excess water run-off) and the total residue cover. 60 to 70% cover is typically required to create a significant difference in erosion and soil moisture over CT in most situations.64
Some NT supporters argue that a cover crop helps reduce weed infestation by not allowing weed seeds the necessary light for germination and/or allelopathic properties of certain residues (notably cereal) reducing weed germination.68 However, this argument does not appear to occur in practice because NT techniques use more herbicide for weed control than CT techniques.60,69
The biggest advantage of NT is its positive effect on soil microbes as NT typically has a 7-36% greater amount of soil microbial biomass than CT.70 The reason is largely attributed to tilling destroying soil pore networks, which create living spaces for microbes. This enhancement is especially important for rhizobacteria, which as mentioned previously play a significant role in maintaining crop health and overall soil quality through nutrient cycling and nitrogen fixing among other things.71
The two major disadvantages of NT are increased soil compaction and planting requirements due to the lack of seedbeds and light availability. Lacking seeded demands farmers apply a higher seeding rate (20% more) to increase the probability of effective yield.72 Additional seeding could also interfere with proper interspacing between crops reducing yield. Compaction is a problem because it reduces germination probability as well as limits the soil microbial and water retention advantages of NT by eliminating the micropores within the soil. A third minor disadvantage is a higher probability for weed/crop competition under a NT strategy over a CT strategy.
The inconsistent NT yields and nutrient loading due to sub-optimal seedbeds and un-supplemented nutrient availability have created a concern beyond costs for switching directly from CT to NT. However, some believe that the addition of solid compost can neutralize significant portions of this inconsistency over the long-term.73 Overall devotion to one methodology over the other in the future under a global warming influenced climate may be the wrong strategy; similar to crop rotation, rotating the tillage mechanism may be the best strategy with higher societal value crops with respects to food staples (corn, wheat and rice) receiving CT to ensure high yield and secondary crop rotations using NT to restore/maintain soil health.
Among the different types of air pollution, ozone is important because of its large, relatively speaking, atmospheric concentrations and its known detrimental influence on both vegetation and animal (including humans) health.74,75 In fact current fluxes of ozone concentrations in the last two-three decades is thought to have reduced yields in crops in the United States ranging from rice and wheat to corn and potato.76,77 Also China, Japan and South Korea lost 1-9% of wheat, rice and corn and 23-27% of soybeans in the 1990s due to ozone.78,79 Overall research has divided a large number of crops into three categories of sensitivity regarding ozone damage. Wheat, watermelon, pulses, cotton, turnip, tomato, onion, soybean and lettuce are seriously ozone-sensitive crops; sugar beet, potato, oilseed rape, tobacco, rice, maize, grape and broccoli are moderately ozone-sensitive; barley, peanuts and fruits like plums and strawberries are generally ozone-tolerant.80
Ozone exerts its influence through different pathways including photosynthesis, biomass, leaf area index (LAI), grain mass and grain number.81 Influence of action takes place first through ozone entry via the stomata. The most common action pathway is ozone-derived production of reactive oxygen species (ROS), which reduces photosynthetic efficiency through multiple pathways: reducing RuBisCO activity, stomatal absorption of CO2 and acceleration of leaf senescence causing chlorophyll degradation.82 Cholorphyll degradation is usually measured by analyzing the light interception and light-to-biomass conversion efficiency, it can be seen visually in leaves that are not green, but have rusty or dark red color.83 Ozone can also reduce reproduction efficiency by reducing pollen germination and tube growth or increasing the probability of flower abscission or abortion of pods and seeds.84 Also ozone inhibits phloem loading and influences root and grain partitioning increasing carbohydrate retention in leaves.85
These negative outcomes expand further problems like a higher shoot/root biomass ratio, altered leaf chemistry and lower harvest index. Some believe that these outcomes of ozone damage are more damaging with respect to changes in yield than photosynthesis and biomass losses.86 Ozone may also induce genetic mutations over long-term exposure.87 Finally there are some concerns that these influences on root partitioning can decrease carbon flux into soils negatively affecting long-term system carbon balance between the soil and the crops.88 Research have demonstrated that after long-term exposure (4-5 years) ozone is able to strongly inhibit stable soil carbon formation by changing soil composition to higher molecular weight aromatic compounds.89,90
However, while ozone may reduce yields there is some question to its influence on individual crop quality. Studies have demonstrated that increases in atmospheric ozone concentration increase individual protein concentration in grains while reducing overall protein yield for the crop as a whole.91,92 Basically more crops die, but those that survive are of higher quality. Interestingly enough increased levels of CO2 appear to have the opposite effect increasing overall protein yield, but reducing individual protein yield.93 Another element of concern is that ozone damage can severely impact crop residues that are used to grant advantages to NT strategies.
One of the tricky problems with ozone is that while CO2 concentrations are increasing, that increase has been a somewhat controlled linear one with slight slope adjustments, ozone levels can change dramatically in a positive or negative manner in a localized region based on existing weather. Ozone is synthesized in the lower atmosphere through a photochemical reaction involving nitrogen oxides (NOx), methane (CH4), carbon monoxide (CO) and various volatile organic compounds (VOCs). Ozone concentrations are largely dependent on temperature, humidity and air mixing. High latitude ozone concentrations can also be influenced by stratospheric ozone concentrations.94,95 Overall ozone concentrations that do not undergo stratospheric mixing generally demonstrate a diurnal variation where due to a lack of sunlight ozone destruction dominates during the night with increasing concentrations as the day passes due to vertical mixing and an increased level of sunlight peaking in the afternoon.
In a warmer global climate increases in surface temperatures should also increase atmospheric humidity, but also decrease cloudiness. Decreased cloudiness will increase solar radiation penetration, which will increase ozone reactant reactivity increasing ozone concentration. However, the increased humidity is thought to decrease ozone concentration because of increased ozone destruction.96 Models of this competitive behavior have demonstrated a dominance of ozone synthesis over ozone destruction in temperate and arctic regions and a dominance of ozone destruction over ozone synthesis in tropical regions.97 This result is discouraging because with increasing temperatures tropical regions will become less viable food producers, so limited ozone concentrations will be of little consequence.
Fortunately due to efforts to reduce VOCs and CO, regions with the highest ozone concentrations have seen a declining trend in the last two decades.98,99 Unfortunately due to increasing NOx emissions and air mixing the annual mean concentrations at background sites in mid-latitude regions have seen ozone concentration increases of between 0.5-2% per year.100,101 This is a troubling trend from a standpoint of agriculture because ozone concentrations are decreasing in high level areas (largely where food is not grown) and are increasing in lower level areas (including those where food is grown). In fact numerous model simulations suggest that tropospheric ozone concentrations will increase an additional 6-15% by 2030.102
Another source of ozone synthesis in a warmer climate would be biogenic emissions. Biogenic emissions are emissions which originate from biological sources like isoprene or terpene from forest trees and these emissions are highly temperature dependent. Studies suggest that biogenic emissions will increase between 27-59% and be responsible for a 30-50% of the increase in ozone formation in the future.103,104 Further complications may come from increased CO2 concentrations increasing the total leaf surface area resulting in more emissions, but the validity of this increase is currently under debate.105,106
Atmospheric ozone concentration is only part of the issue pertaining to the influence of ozone on crops; another important aspect is how ozone is managed entering the plant and once it enters the plants. Due to the fact that ozone must enter through the stomata surface leaf characteristics are important, especially wetness. Through an unknown mechanism surface wetness appears to significantly increase total ozone flux into the plant.107 Therefore, ozone damage will be mediated based on the total precipitation in a given region, the greater amount of precipitation the higher the probability for ozone damage. Once in the plant ozone enters the apoplastic fluid where it decomposes into hydrogen peroxide, an oxygen free radical and hydroxyl radicals.108 These ROS species can be removed by various components most notably ascorbate.109 In fact symplastic ascorbate accounts for about 66% of the variation in ozone damage flux. Other enzymes that are involved in ozone protection are glutathione peroxidase, superoxide dismutase, catalase and dehydrascorbate reductase.110,111
An interesting path of research is how increased levels of CO2 affect antioxidant activity in plants. Early results demonstrate increased antioxidant levels in leaves due to increased photosynthesis and assimilate availability.74 Also increased CO2 reduced antioxidant activity, despite their increased number, which could suggest a decreased basal rate of oxygen activation and hydrogen peroxide formation.112 Therefore, this interaction with a plant’s antioxidant system may be how CO2 is able to neutralize some of the detrimental influence of ozone.
All things considered some estimates place global yield losses of 5%, 4%, 9% and 12% for maize, rice wheat and soybeans respectively with a 20% increase in ozone concentration, which is assumed by the IPCC to occur in 2050.74 Unfortunately the occurrence timing for numerous predictions made by the IPCC do not inspire confidence in their accuracy, a vast majority appear to be occurring at a faster rate, so it stands to reason that these ozone losses will be attained at a faster rate as well without adaptation.
A concerning thing about increased ozone is the susceptibility of soybeans. Soybeans are one of the big four food crops in the world (rice, maize and wheat being the other three) and the soybean-maize agricultural rotation system occupied 63.2 million ha in the USA in 2004 and has only grown since. Similar to the confliction between chambers and FACE in observing the influence of increased CO2 concentrations, there is confliction between chambers and FACE in observing the influence of increased ozone concentrations on soybean yields. FACE studies over the course of two years, 2002 and 2003, identified yield decreases of 15% and 25% for ozone increases of 21% and 25% respectively.82 Chamber study equations using similar increases in ozone concentration only demonstrated yield decreases of 12% and 9%.76,113 Fortunately or unfortunately depending on the point of view soybean yield decline is generally linear between ozone concentrations of 30 ppb and 130 ppb, so there is little reason to suspect a concentration ceiling for negative influences to be attained in the near-future.
Determining whether the chamber studies or the FACE studies are accurate demands identifying how they influence soybean yield loss. Chamber studies identify a decreased amount of photosynthesis as the principle reason for decreased yield with leaf loss occurring at very high ozone concentrations.114 Also the end of the growing season saw decreases in leaf area index and individual leaf duration.115 FACE studies demonstrated no significant changes in photosynthesis during the growing season.82 Instead photosynthesis changes only started when the leaf neared senescence, the rate of which was increased with increased concentrations of ozone.115,116 The increased leaf senescence leads to greater dry-mass loss and decreased leaf number, which would affect canopy photosynthesis and explain the greater yield loss.117
The most interesting aspect of the increasing CO2 and increasing ozone environment of the future may be the apparent competition between their respective yield increasing and yield decreasing influences. Chamber experiments have shown that increasing CO2 concentrations decrease stomatal conductance reducing ozone uptake. The only major FACE study confirmed this result, but there were questions about whether or not the reduced conductance reduced yield loss.118 One explanation is that while CO2 may reduce ozone uptake it weakens the plant itself making it more receptive to ozone damage.74 Another possibility is that ozone attacking ovule fertilization is effective enough to overcome the loss of uptake.
Most responses to increased ozone concentrations involve pollution mitigation, but the execution of such a strategy will not influence concentrations in the short-term or even perhaps the mid-term. With ozone concentrations continuing to rise in most regions of the world there is a high probability that agriculture will have to address this additional ozone and its detrimental influences. Unfortunately adapting agriculture to higher ozone concentrations is quite problematic because of the dilute, yet destructive nature of the low atmospheric ozone. It has been demonstrated that packed beds of granular activated carbon (GAC) can remove volatile organic compounds (VOCs).118,119 Eliminating these VOCs will reduce available reactants for ozone formation. The GAC beds are placed in a zig-zag pattern to increase VOC sorption capacity and limit airflow resistance. However, there is some concern in that these GAC beds are only comprised of granules that are a few centimeters thick and are quickly consumed because the VOC removal process is based on physical adsorption instead of a catalytic reaction.120 Therefore, the packed beds will quickly become saturated and need to be replaced. Other concerns stem from the overall expense, weight and pressure of such a system.
Another option could be to use catalysts in combination with GAC to destroy ozone outright. The development of such a device would not need to be as large or heavy due to the inclusion of the catalysts. Note that the activated carbons will still need to be replaced because they are not the catalytic element and like VOCs and ozone interact and are consumed with ozone interactions. Typical catalysts for this type of reaction are either palladium or platinum, but these catalysts require a temperature of at least 100 degrees C.121 Therefore, it may be better to use metal oxide materials like carbon impregnated with iron and manganese oxides.120 Exploration of bed configuration has determined that thin fabric sandwiches holding small particles of activated carbon appear to have superior function relative to the currently used packed beds.119
Overall based on existing information there are three basic strategies that can be applied to address low atmospheric ozone pollution in a field environment: 1) Use the standard protocol GAC device; the overall size and weight should not be an significant issue because of its outdoor use, space confinements are usually reserved for indoor units. 2) Use a smaller catalytic based GAC unit; 3) Use a unit which utilizes biochar or some other form of charcol. Each option can either rely on passive airflow (ambient wind) or can use active airflow (a weak vacuum) to drive ozone through the unit.
If an active airflow system is utilized, which it probably should be, it would need to be powered by a trace carbon energy provider so not to significantly increase greenhouse gas emissions. Attaching a small PV solar unit with a small storage battery to the device makes the most sense because deactivation during the night should be suitable because atmospheric ozone layers fall significantly without available sunlight to drive photolysing reactions, thus ozone destruction derived from these units would be less efficient. The units should be placed on the outer perimeter of the plots in order to limit the disruption of airflow over the crop bed itself. The biggest concern with such a system is to ensure that the vacuum is weak enough that it does not increase fatality rates of small mammals, birds and beneficial insects like bubble bees.
Another problem for agriculture due to global warming is an increase in detrimental events associated with insects. The combination of warmer temperatures in the spring speed development rates of some species and less harsh winters, which reduce insect death, increase overall insect numbers during the growing season. One important example of this characteristic is the empirical response of mountain pine beetle to warmer temperatures through its increased reproductive activity. This combination is especially problematic because of increased numbers and activity during the initial planting phase of the season when crops are more vulnerable. Higher temperatures will also increase feeding in most insect species.
One point of interest with regards to an increase in feeding is that due to increased photosynthesis from increased CO2 plants will have a higher carbon/nitrogen ratio (assuming similar fertilizer use to current applications). A higher carbon to nitrogen ratio reduces the overall nutrition of the plant. Reductions in plant nutrition may stimulate greater insect feeding in order to make up for the reduction in nutrients per unit consumed exacerbating insect related losses.122 It is important to note that this increased feeding rate is thought to apply more to adult insects because of their ability to survive and reproduce with less nitrogen than caterpillars and other larvae. For non-adult insects there is question whether or not the nitrogen available will be worth the effort applied to consume the plant.
Also the carbon/nitrogen ratio affects the type of decomposition experienced by the plant with lower ratios increasing the probability of mineralization and higher ratios increasing the probability of stable humus formation. Among other things humus is thought to help suppress plant diseases, help soil retain moisture by increasing microporosity and aid the formation of quality soil structure in general. So an increasing carbon/nitrogen ratio may be a mixed bag in that the potential for insect losses are increased, but there is also an increased probability in better soil quality. However, if a biochar strategy is executed there would be some questions to how the biochar would be loaded into a higher humus concentrated soil.
While most of the direct influences associated with an increasing CO2 concentration presented so far have been positive, one concern is that there is evidence to suggest that elevated CO2 levels reduce the probability of plants to defend themselves against insects. It appears that increasing the ambient concentration of CO2 decreases the ability of soybeans to produce jasmonic acid, which is an insect defense precursor.122 Jasmonic acid facilitates the expression of protease inhibitors, which prevent proteolytic activity for insect derived proteases. Without the ability to breakdown proteins the level of nitrogen acquisition for the insects is significantly reduced reducing the nutrient acquired to energy consumed ratio reducing the lifespan of the insect. Decrease jasmonic acid concentration in leaves indirectly increases insect lifespan and the probability of greater insect loss in crops. Jasmonic acid can be applied by farmers in a spray form, but doing so obviously increases total operational costs.
While the overall influence on insects, the winners/losers in global warming so to speak, varies between species one loser may be fruit flies. Fruit flies like to breed when temperatures are below 32.2 degrees C and relative humidity ranges between 60-70%.123 Also they are rather sensitive to temperature with regards to their lifespans as their life cycles are completed in 36.3, 23.6 and 11.2 at 15, 20 and 27.5 degrees C respectively.124 Reductions in fruit fly populations will be an overall boon to fruits, especially those with tender skins.123
The reduction in fruit fly activity due to increasing temperatures may limit the need for lure traps. Parapheromone lure traps are commonly used to eliminate males that can damage crops (though not as badly as egg laying females) or mate with females. These pheromone traps are regarded as superior to food attractant tephrit-lure traps.125,126 While lure traps create very limited ecological damage their deployment reduction can slightly reduce operational costs for farmers and probably reduce some psychological stress.
The influence of increased temperatures and CO2 concentrations on weeds is similar to the influence on crops in that influence is much greater on those weeds using a C3 photosynthetic pathway over a C4 photosynthetic pathway. However, C4 weeds are thought to have a greater competitive advantage against C3 crops at higher temperatures and will also extend their geographic range towards higher latitudes due to their ability to resist those higher temperatures.127 Due to the higher CO2 concentrations stimulating photosynthesis and growth C3 weeds are thought to have higher growth rates leading to more invasive behavior (Patterson 1993), but the influence of this change will be muted due to higher temperatures. This CO2-temperature contrast unfortunately is thought to benefit weeds over crops because most weeds are C4 where most food crops are C3. The adaptability of weeds also gives them an advantage over crops when facing other stressors like increased ozone concentrations and soil salinization.128 Finally if NT strategies are utilized, weed expansion may increase further depending on why types of control methods are executed. Overall the biggest concern with weeds is the uncertainty in how their advantages in a warmer world will allow them to compete against food crops.
Some individuals hold out hope that humans can genetically engineer a way out of the crop losses in the warmer and ozone richer future. Transgenic crops do provide an interesting avenue of action against yield losses and can even improve growth in effort to create the more secure food supply that is required for the future 8-9 billion humans.129,130 The methodology of genetic engineering surrounds insect resilience and abiotic stress resistance.
Most genetic engineering for pest control use biopesticides like Bacillus thuringiensis (Bt). Bt is sporulating, Gram-positive facultative-aerobic soil bacterium that has larvicidal activity with a effective focus on numerous harmful pests due to decades of bacterial evolution and refinements, yet does little harm to beneficial insects and no reported harm to humans.131 Due to this useful activity, Bt genes have been incorporated into transgenic plants since early 2000 and expanded significantly since reaching over 32 million global hectares in 2006 and even greater since.132 These transgenic plants can be further subdivided into 19 million hectares of insect-resistant crops and 13.1 million hectares of with a combination of transgenic traits (insect resistance and herbicide tolerance).132 Overall Bt has demonstrate significant effectiveness for most plants, especially for cotton, significantly reducing insecticide use.
Ignoring complaints about Bt incorporation on the grounds of ‘franken food’, there are legitimate concerns about insects building a resistance for the Bt rendering the transgenic protections useless. Such a mindset is understandable because various insects, especially aphids, have developed resistances to numerous insecticides. However, while Bt crops have been in use since 2000 and despite development of resistance in laboratory experiments, field resistances have been limited to only diamondback moths located on watercress in Hawaii.131,133
Most believe that the main reason for this lack of field resistance is the use of the “high dose-refuge” (HDR) strategy. HDR consists of growing two sequential plots where one consists of Bt crops producing a large amount of toxin and a second that consists no Bt crops. The plot without Bt crops is regarded as a refuge zone because larvae of pest insects are not exposed to toxin, thus creating a reservoir of non-resistant larvae. Laboratory tests have determined that Bt resistance is rarely dominant.134,135 (basically it is a recessive gene, thus to confer resistance offspring needs to receive a double recessive allele). Therefore, the Bt crops will kill off all of the double (homozygote) no resistance and the heterozygote (1 resistance and 1 no resistance) leaving only a very small number of homozygote resistant pests alive. However, due to the existence of the non-Bt crop plot these resistant pests will almost always mate with these non-resistant individuals, which will not pass on Bt resistance to its offspring.
Overall there is no reason to suggest that this strategy will not continue to work eliminating insect resistance to Bt crops. In addition to existing Bt crops the next generation of transgenic plants are being developed. These plants are being designed to express two cry genes encoding multiple toxins that recognize different receptors making the development of resistance even more difficult.131
Natural plant counters for abiotic stressors are complex and coordinated responses involving hundreds of gene responses.136 The type of response is also predicated on the developmental stage of the plant, which can heavily shift the potential life cycle of the plant actions including reduced or aborted seed production or accelerated senescence.
One genetic engineering strategy has focused on enhancing carbon-rich osmolytes, like pinnitol mannitol, trehalose, etc. could stabilize protein structures during water deficit, increasing nitrogen use and the scavenging of ROS species increasing the individual protein content that is lost at higher CO2 concentrations and improving drought tolerance.136-138 Genetic engineering for salt resistance is more difficult in that while these plants may be protected against NaCl, they are not resistant against other salts, sometimes categorized as sodicity.136 In these situations nutrient content and soil pH have a more important influence over survival than genetic action.136,139
While focusing on one condition like drought, salt, carbon enhancement, etc. is standard operating procedure, most of this focus is not appropriate because in the future plants are going to be experiencing multiple stressors at the same time that work in combination, not isolation. There are the rare times where one stressor will limit damage from another stressor, but most situations the stressors will complement each other doing more damage than individually summed. One empirical example is that under high enough temperatures to induce heat stress plants increases their stomatal conductance to cool leaves through transpiration. However, if the plant is also experiencing a drought the plant would refrain from opening their stomata in order to conserve water, thus resulting in higher leaf temperatures.140,141 Increased heavy metal concentration in soils is also problematic under heat stress conditions because increased transpiration increases heavy metal uptake.142
Therefore, genetic engineering must be aware of these compounding effects and how independent genetic changes may not be the best way to handle them; i.e. if changing gene A improves drought response and changing gene B improves heat response changing gene A and B may not work at combating drought and heat stress at the same time. For example it wouldn’t be surprising to see Monsanto’s new drought-resistant hybrid corn, DroughtGard, fail in long-term tests because of its breeding against only one stressor. On a positive note at least one gene cytosolic ascorbate peroxidase 1 (Apx1) has be confirmed as required for the tolerance of Arabidopsis plants to the combination of drought and heat stress.143
The most important element in applying genetic engineering to crops is to understand how plants sense and acclimate to stress condition. With this information theories and strategies can be developed to enhance tolerance. One of the more encouraging elements to genetic engineering is how new testing methodologies have improved genetic testing. Whole-genome sequencing with chromatin immunoprecipitation has aided research into epigenetic control of stress-based gene expression, microarrays have significantly improved speed and accuracy of transcriptional network study and metabolic profiling has increased understanding of metabolic response to stress.136,144 This increased understanding has lead to new research in gene networks and upstream regulators, retrograde signaling in balancing stress and energy signaling, metabolomics and system biology, and epigenetic controls of gene expression while under stress.136
With all of the promise of genetic engineering one of the lingering problems is the tendency of annual crops to undergo early flowering and accelerated senescence when exposed to stress. This tendency may make evolutionary sense, but leads to significant yield losses. Some success has been demonstrated in using gene mediating cytokinin biosynthesis associated with a drought-stress inducible promoter, but initial testing has only examined such a response under independent drought conditions.145,146
Overall the biggest genetic engineering execution may come from transgene expression. Transgene expression is the introduction of genes from other species that have demonstrated a better ability to neutralize the negative influences of certain stressors like drought tolerance from desert plants or freezing tolerance from certain types of fish. An exciting element to transgene expression based genetic engineering is that there are numerous genes of unknown function in each genome that has been completely sequenced with some estimates ranging from 20-40%.147,148 Investigation into the functionality of these species specific genes may lead to new stress adaptive strategies for other species.
A potential important aspect of adaptation can come from a simple change in U.S. federal policy and could be extended elsewhere in the world. Farm subsidies have always been a controversial issue, especially those subsidies which award farmers for leaving their fields barren of crops (land idling) discouraging growth of a certain crop due to market economics (i.e. in effort to maintain a certain price floor). Millions of dollars are given so that thousands of acres either sit empty providing nothing of value or grow something that is generally irrelevant only so supply of a particular food will not escalate to a point where prices will drop making the production of the particular crop unprofitable.
While it would be very difficult to eliminate the subsidies because of the powerful agricultural lobby in Congress the operational condition of the subsidy could still be changed. For example instead of growing nothing owners of the land receiving these types subsidies would be mandated to grow a faster maturing feedstock like switchgrass. After maturing this switchgrass could then be harvested and burned in a slow pyrolytic process to create biochar. This biochar could then be transported to high yield fields with a localized proximity, for the U.S. it would be fields in the Midwest. A couple of side benefits to this subsidy switch are that the switchgrass would be a net negative carbon element reducing a small amount of atmospheric CO2 and the cultivation would not require the use of industrial fertilizers or pesticides because of the end result of the growth (pyrolyzation into biochar), thus some yield loss would be acceptable.
One of the first elements to adaptation is not developing a specific strategy, but instead developing the framework to ensure that the appropriate strategy is executed properly and universally. These adaptations will need to be applied to each sector for their effects to be realized. For example for far too long water conservation has been viewed as a luxury instead of a necessity. With drought conditions already affecting the Southwest and the Southeast portions of the United States and their probable future exacerbation, the development of a cohesive and thorough national water conservation strategy will be required to effectively deal with future shortages. While some individuals break out in hives whenever the government is responsible for anything, administration of intelligent and reasonable water conservation for 300+ million people is something only a centralized government has the ability to effectively manage. The ongoing droughts have demonstrated that states have been unable to rein in water usage and private enterprise is not appropriate or equipped to do so in an ethical and reasonable way.
Not surprisingly one of the key features of adaptation will be heterogeneity. The types of crops that are grown in a double cropping strategy need to go through crop rotation. Tilling will benefit from an alteration between no-till and convention till, leaning more heavily on no-till. Both of these rotation strategies will help soil health as well as limit pathogen and other pest losses because it reduces the probability of these agents establishing themselves due to the changing characteristics of the plant that is being grown at the given time and the soil. Also further incorporation of Bt and other transgenic crops with appropriate paring of HDR to offset increased insect incursion will be advisable.
However, while heterogeneity will play a part, so will homogeneity. With the reduction of available water resources for most areas drip irrigation systems will have be incorporated into numerous flood or spray irrigated field along with even those fields that are rain-fed. In addition it may be useful for the global community to create a universal methodology for the growth of feedstock, production, distribution, application and even transportation of biochar for incorporated into various fields to aid yield stabilization and expansion. Existing evidence suggests that incorporation of biochar by using fast growing feedstock in poorer soils can help both adaptation and mitigation strategies. Finally water supplies may need to be augmented not just through drip irrigation, but also through a series of atmospheric condensers located in close proximity to farms.
Also one of the keys to both adaptation and mitigation will be a new level of global cooperation and structure. Despite making adaptations certain areas will experience extreme weather conditions that cannot be properly prepared for and result in serious crop losses and the rest of the world needs to step in and fill in these gaps. In addition to CO2 mitigation global communities need to work on reducing nitrogen oxides and other VOCs to stabilize and even reduce atmospheric ozone concentrations. However, similar to existing temperature increases, existing ozone concentrations need to be addressed due to the damage they can inflict upon agriculture as well as other plant and animal life because damage to forests can still indirectly damage agriculture by eliminating other insect defense and targets among other things. An ozone purifier as discussed above may be the best way to neutralize ozone before mitigation takes effect.
Note that this discussion has only covered adaptations that are directly related to food production in the agricultural field. Production is only the first step of the process for the distribution of that food to a point of sale is also an important step. While it goes beyond the scope of this discussion there are some important elements that will need to be addressed in the distribution of food. One of the biggest problems is the dual nature of food waste both before reaching the point of sale, but after harvest and after extended time at the point of sale. Post-harvest losses are large due to poorly timed harvests, improper storage before transport which exposes crops to excess rain or heat, results in microorganism contamination, or excess/improper handling leading to damage. Waste at the point of sale largely stems from consumers refusing to purchase an item, usually a fruit or vegetable, due to an imperfection in its appearance. Clearly consumers need to be educated, probably by in-store signage, regarding the differences between a physical sign of contamination and a benign physical imperfection.
Distribution from the place of growth to a point of sale as well as the use of heavy farm equipment will also need to adapt due to changes in gasoline supplies influenced by oil availability. Advancements in electrical powered farm equipment should address the immediate issue of changing gasoline availability on the farm because there will be few charge issues due to the short proximity of operation for these vehicles. The transport issue is a little more complicated. Some argue that individuals should just purchase food locally, but this strategy is entirely dependent on where one lives. It is difficult to buy locally in Southwestern China, Siberia, Alaska, etc. and purchasing locally is also hindered by a significant lack of variety. While purchases should be made as locally as possible it is important that other means of transportation that do not utilize gasoline be developed for food stuffs as well.
Overall there are some serious adaptation issues in agriculture that have to be addressed in a global warming influenced future. It must be acknowledged that adaptation and CO2 mitigation must work together because enough CO2 has been and will be emitted to increase temperatures at sufficient magnitude at long enough to alter the agriculture in a negative manner. Therefore, adaptation must be incorporated to address the existing and near-future expectations and mitigation must be incorporated to guarantee a viable temperature ceiling. Global cooperation and a level of uniformity will be required in both of ventures and should begin as soon as possible.
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