of precipitation are predicted in high latitudes, while decreases of up to 20% by 2100 are predicted in most sub-tropical land regions (IPCC 2007a). Changes in the seasonal pattern of precipitation are also likely. In Northern Hemisphere winter (DJF), precipitation increase of over 20% are predicted for Northern Hemisphere high latitudes and Eastern Africa, whilst decreases of over 20% are predicted for the Southern USA, North Africa and the Middle East, by 2100. In Northern Hemisphere summer (JJA), precipitation increases of up to 20% by 2100 are predicted for Northern Hemisphere high latitudes, and decreases of up to 20% for Southern Europe, North Africa, South Africa, Brazil and Central America (IPCC 2007a). The regions where there is least agreement between GCM projections of changes in precipitation are the USA, Russia, South Asia, Australia, tropical Africa in summer, and Southern Africa, Australia, South Asia, mid USA and South America in winter (IPCC 2007a). Figure 5.2 shows predicted changes in temperature, soil moisture and litter C inputs under a coupled climate-carbon cycle simulation (Cox et al. 2000)
These changes in annual totals and seasonal patterns and in the magnitude and frequency of extreme climate events are also predicted, including warmer and fewer cold days and nights, warmer and more frequent hot days and nights, an increased frequency of warm spells/heat waves and heavy precipitation events, and a general increase in the areas affected by droughts. Atmospheric CO2 concentrations were approximately 280 ppm during the pre-industrial period, and 379 ppm in 2005, while under the IPCC SRES scenarios, they are predicted to rise between 730 and 1020 ppm dependent on scenario and the magnitude of climate-carbon cycle feedbacks (IPCC 2007a). The GCMs generally agree in the sign of soil moisture changes in many regions, although the magnitudes of change are more uncertain. Annual mean decreases of up to 20% in soil moisture are predicted in the sub-tropics, the Mediterranean region and in high latitudes, where snow cover diminishes (Fig. 5.2b), whilst increases of over 20% in precipitation are predicted in east Africa, Central Asia, and some other regions (IPCC 2007a). Jones and Fal-loon (2007) discuss the implications of future climate uncertainty in determining future SOC storage.
5.2.6 Predicted Changes in Land Use, Cropland Productivity and Management
Future changes in the amount and quality of plant C and N inputs returned to cropland soils depend on several factors, including changes in large scale land use, crop suitability (crop types grown in particular regions), crop productivity, and crop-specific factors (root to shoot ratios, and litter quality).
In 1990, ~12% of the global land area was in crop and energy use, ~33% in forests and ~55% in other uses including grasslands. The IPCC SRES scenarios suggest a wide range of possible future mixes of global land use, dependent upon the scenario chosen. By 2100, global land use could consist of ~5-25% crop and energy use, ~25-45% forests and ~47-62% other land uses including grasslands (IPCC 2000). Thus, both decreases and increases in the present global land area under crops are possible. Large regional differences in land use changes are anticipated - for instance, the A2 scenario includes widespread deforestation in Amazonia and Africa and some reforestation in Europe, whilst the A1B scenario suggests much less deforestation in Amazonia and Africa, and more widespread afforestation especially in regions of China and Europe (Falloon and Betts 2006).
Whilst climate change could alter the suitability of crops and thereby, the geographic areas occupied by particular cropping systems (Smith et al. 2007b). However, there is a little information on the nature of these changes, not least because economics and agricultural policy are strong drivers, and subject to considerable uncertainty. For instance, increasing temperatures are likely to have a positive effect on crop production in colder regions due to a longer growing season, (Smith et al. 2005), allowing crop production zones to shift northward. However, increases in productivity may not necessarily lead to increases in carbon storage, since climate change could also increase the length of the season when respiration occurs (Harrison et al. 2008). In mid- to high latitude regions, a combination of local temperature increases of 1-3°C, carbon dioxide (CO2) increase and rainfall changes could have small beneficial impacts on crop yields, although in low-latitudes, similar changes are considered likely to have negative yield impacts for major cereals (Fischlin et al. 2007). Further, warming is projected to have increasingly negative impacts in all regions and global production potential is likely to decline as global average temperature rises above +3°C. Air pollution could also reduce crop yields, since tropospheric ozone has negative effects on biomass productivity (Booker and Fiscus 2005; Liu et al. 2005).
There is a little information concerning climate impacts on the crop-specific factors which influence C and N inputs to soil, and given the complexities discussed above, there is considerable uncertainty in likely responses. For example, the predicted doubling of atmospheric CO2 concentrations in the next century will alter plant growth rates, plant litter decomposition, drought tolerance, and nitrogen demands (Torbert et al. 2000; Norby et al. 2001; Jensen and Christensen 2004; Henry et al. 2005; van Groenigen et al. 2005; Long et al. 2006). Given that changes in crop breeding and production systems are also likely, it is difficult to speculate how climate change might alter factors, such as root to shoot ratios and litter quality. However, in many regions, improvement in cultivars has a far greater impact on yields in the past few decades than any other factor (Amthor 1998). Where maximum potential yields are not yet attained, this could continue to be the case in the coming decades. With more of the NPP being directed toward harvestable yield, carbon content produced in other components of crop plants has decreased during recent decades. This trend could continue over the coming decades, resulting in lower C and N returns to soil each year.
5.2.7 Climate Change Impacts on Cropland Greenhouse Gas Fluxes
Greenhouse gas fluxes from croplands will be affected by the changes in climate, cropping systems and management drivers discussed above. Increases in temperature alone are likely to accelerate decomposition of soil organic matter, resulting in soil carbon losses (Knorr et al. 2005; Fang et al. 2005; Smith et al. 2005). The global scale assessments made by Cox et al. (2000) and Jones et al. (2003), using a coupled climate-C cycle Global Circulation Model (GCM), HadCM3LC, predicted decreases in soil C stocks across most of the globe by 2100, even in regions where C inputs to soil from vegetation had increased (Jones et al. 2003). Regionally, large decreases in soil carbon for the Amazon region (over 4 kg C m-2), Southern Africa (2-4 kg Cm-2) and Eastern USA (2-4 kg Cm-2) and increases in soil carbon for Siberia, Alaska and Northern Canada and much of Eurasia (1-3kgCm-2) are predicted (Jones et al. 2004; Falloon et al. 2007a). In the C4MIP study of Friedlingstein et al. (2006), all coupled climate-carbon cycle models showed a decrease in soil carbon storage globally due to climate change. Higher temperatures will also increase N mineralisation rates, which may increase N2O fluxes and nitrogen leaching, especially in N saturated systems.
How changes in precipitation (and hence soil moisture) alone will affect GHG fluxes from cropland soils is more complex than as previously discussed, future predictions of precipitation (and soil moisture) are less certain than future changes in temperature. Globally, overall decreases in soil moisture alone could lead to increased global soil carbon storage because drying will reduce respiration rates (Falloon et al. 2007a). Regional scale studies in the UK also found that future moisture changes alone can increase cropland soil C storage (Falloon 2004). Hence, the predicted annual decreases in soil moisture for the sub-tropics and Mediterranean region are likely to lead to increased soil C storage, whilst the predicted increases in soil moisture for East Africa and Central Asia are predicted to enhance reduced soil C stocks. The impact of seasonal moisture changes is less certain, although the predicted increases in winter precipitation for Northern hemisphere high latitudes and East Africa, and summer precipitation for East Africa and Central Asia, could lead to reduced soil C stocks. The predicted decreases in summer precipitation for Southern USA, North African and the Middle East, and winter precipitation for Southern Europe, South Africa, North Africa, Brazil and Central America, may act to increase soil C stocks. Higher winter rainfall could also increase N2O production and emission, and permanent waterlogging of wet soils may increase CH4 emissions. Conversely, the drying, as discussed above, may lead to reduced N2O and CH4 emissions. How these seasonal changes balance annually and in the long-term is very complex and will depend upon the relative influence of wetting/drying patterns on GHG fluxes in each season.
There has been little research on the impacts of changes in climate extremes on GHG emissions from cropland soils, although the recent European heatwave of 2003 led to significant carbon fluxes from terrestrial ecosystems (Ciais et al. 2005). Extreme increases in soil temperatures and drought events may also have implications on soil biological activity, reducing the decomposition capability of bacteria, ultimately reducing biomass growth and soil fertility. An increased frequency and magnitude of heavy rainfall events could impact cropland GHG fluxes by increasing soil erosion (and thus losses of soil C to watercourses), or by increasing the occurrence of short periods of warm and wet conditions suitable for N2O production. The predicted increase in atmospheric CO2 concentrations is likely to have only small overall impacts on soil carbon storage (Smith 2005). Although soil organic matter is often increased under elevated CO2 in the short-term (Allard et al. 2004), the long-term soil carbon sink may saturate at elevated CO2 concentrations, especially when nutrient inputs are low (Gill et al. 2002; van Groenigen et al. 2006).
The uncertainties in future cropping and land use patterns make it difficult to postulate their likely impacts on cropland GHG fluxes. However, impacts of the predicted changes in crop productivity on GHG fluxes can be inferred. Assuming that the fraction of C returned to soil remains unchanged, the small mid-term increases in yield predicted for Mid-High latitudes may lead to small increases in C inputs to soil over the next few decades and so increased soil C storage. In the longer term, the predicted decreases in crop yields would lead to reduced C inputs to soil, and thus reduced soil C storage. However, the general trend to reduce below-ground C allocation in crops is considered likely to continue (Amthor 1998). Thus, overall response will be a net decrease in C inputs to soil, and so in soil C storage, unless crops are specifically bred to allocate more C below ground. A decrease in C and N inputs to soil is expected to reduce cropland N2O emissions. The impacts of air pollution on crop production could also indirectly affect soil carbon storage. Recent researches suggest that tropospheric ozone could significantly reduce carbon-sequestration rates under elevated CO2 (Loya et al. 2003; Sitch et al. 2007), due to reduced biomass productivity and altered litter chemistry (Booker and Fis-cus 2005; Liu et al. 2005). This would further reduce soil C storage in croplands.
The combined impact of changes in temperature, precipitation, atmospheric CO2 concentration, crop productivity, land use change and other factors is difficult to predict without holistic studies incorporating all of the above factors. Falloon (2004) investigated the impact of climate change on UK arable soils with the RothCUK soil carbon model (Falloon et al. 2006a), changing either only single climate variable, or all variables simultaneously. This showed that whilst temperature changes acted to decrease, SDC stocks precipitation and evapotranspiration changes acted to increase. This led to an overall decrease in SOC stocks. Moreover, the effects were not simply additive - summing the outputs of runs changing only single climate variable did not produce the same result as runs changing all climate variables at the same time (Fig. 5.3). This non-linearity in the response of soil C storage to different driving factors reinforces the need for a holistic systems modelling approach to assess climate impacts on C sequestration.
A number of studies, such as those of Cox et al. (2000) and Jones et al. (2003), discussed earlier in this chapter, have used coupled climate-carbon cycle GCMs which are able to capture the feedbacks between soils, vegetation and the atmosphere, although these models presently only represent natural ecosystems and not croplands. Further examination of these results (Falloon et al. 2007a) indicated that the regional response of soil C to precipitation differed from the global response - although there was a slight increase in precipitation globally, a decrease in C stocks was predicted. The reason for this may be been that whilst temperature increases under climate change were predicted everywhere, the nature of precipitation changes varied greatly between regions. Thus precipitation may control the sign of regional soil C changes under climate change with wetter conditions resulting in
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