Impacts on productivity
Although there is some level of certainty associated with increasing temperature and climate change, predictions of rainfall change are far less clear. It is evident that there will be some regions where rainfall will increase and seasonal patterns will change, and other regions where rainfall will decrease. These changing patterns of rainfall will affect N2O emissions, particularly where associated with extreme events such as protracted droughts (for example in Australia) and extreme flooding. For example, N fertilizer use in southern and eastern Australia has decreased since 1997 with decreasing rainfall. This has inevitably resulted in less total N2O emission, but also impacted on potential production from both pasture and crop production. Plant responses to elevated carbon dioxide
Assuming no nutrient and water limitations, plants have been shown to increase their growth rates in the presence of elevated atmospheric CO2 concentrations. The three principal physiological plant responses to elevated CO2 concentration are:
1 an increase in leaf photosynthetic potential (especially in C3 pastures);
2 a decrease in stomatal, and therefore canopy, conductance;
3 a decrease in plant nitrogen content (Long et al, 2004).
Elevated CO2 levels can induce a biogeochemical feedback mechanism, resulting in a reduction in nitrogen and/or phosphorus availability. This mechanism, referred to as progressive nutrient limitation (PNL) has been examined in recent studies (Finzi et al, 2006; Gill et al, 2006; Hungate et al, 2006) that suggest that the CO2 fertilization effect on plant growth could be restricted due to the reduced availability of N and/or P. In legume-based pasture, where external N inputs occur via biological N fixation, phosphorus is likely to be the main limiting factor on plant growth. Although the PNL effect can be alleviated by external inputs of these nutrients, a better understanding of the full impacts of elevated CO2 and/or increased temperatures on pastoral grazing systems is required to allow the development of management practices that are adapted to global changes (Soussana and Lüscher, 2007).
The net effect of warmer winter temperatures, higher plant growth rates under elevated CO2, and PNL may mean that more N fertilizer is applied to crops and pastures in the future. This increased N input may well result in greater N2O emissions in areas with increased rainfall, but may have limited impact in regions where water increasingly becomes a limiting factor.
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