![]() ![]() We then compare results with published EF determinations, including those used as a basis for current IPCC tier 1 methodologies ( 4, 10) and carbon credit markets ( 26, 27). We also test for possible biases in sampling factors, including the duration and number of measurements, flux chamber area, number of samples per flux measurement, and numbers of replicates. Here, we report the results of a metaanalysis of this global dataset and investigate the potential interaction of ∆EF with other factors, such as crop type, fertilizer type, and available environmental factors. We then evaluated the presence and direction of a slope, and calculated ∆EF as the percent change in EF per unit of additional N input (measured in kilograms per hectare). Although there are very few N 2O response studies with a sufficient number of N-input levels to characterize a nonlinear response with precision, we located over 200 studies with two or more N inputs in addition to a zero N control, which allows the determination of two or more EFs for the same site-year. Here, we test the generality of these findings globally. In short, for every 100 kg of N input, 1.0 kg of N in the form of N 2O is estimated to be emitted directly from soil. This value for fertilizer-induced emissions is an approximate average of emissions induced by synthetic fertilizer (1.0%) and animal manure (0.8%), and it has been rounded by the IPCC ( 5) to 1% due to uncertainties and the inclusion of other N inputs, such as crop residues ( 12) and soil organic matter mineralization ( 1). The current global mean value, derived from over 1,000 field measurements of N 2O emissions, is ∼0.9% ( 10, 11). ( 4, 10), and Stehfest and Bouwman ( 11). Global EFs for fertilizer-induced direct N 2O emissions have been determined by Eichner ( 6), Bouwman ( 7, 8), Mosier et al. ![]() The N 2O EF is the percentage of fertilizer N applied that is transformed into fertilizer-induced emissions, which is calculated for Intergovernmental Panel on Climate Change (IPCC) GHG inventories as the difference in emission between fertilized and unfertilized soil under otherwise identical conditions. In low-input systems typical of sub-Saharan Africa, for example, modest N additions will have little impact on estimated N 2O emissions, whereas equivalent additions (or reductions) in excessively fertilized systems will have a disproportionately major impact. Use of this knowledge in GHG inventories should improve assessments of fertilizer-derived N 2O emissions, help address disparities in the global N 2O budget, and refine the accuracy of N 2O mitigation protocols. Our results suggest a general trend of exponentially increasing N 2O emissions as N inputs increase to exceed crop needs. A higher ΔEF was also evident in soils with carbon >1.5% and soils with pH <7, and where fertilizer was applied only once annually. N-fixing crops had a higher rate of change in EF (ΔEF) than others. We found that the N 2O response to N inputs grew significantly faster than linear for synthetic fertilizers and for most crop types. From 78 published studies (233 site-years) with at least three N-input levels, we calculated N 2O emission factors (EFs) for each nonzero input level as a percentage of N input converted to N 2O emissions. We performed a metaanalysis to test the generalizability of this pattern. Accumulating evidence suggests that the emission response to increasing N input is exponential rather than linear, as assumed by Intergovernmental Panel on Climate Change methodologies. Nitrogen (N) fertilizer rate is the best single predictor of N 2O emissions from agricultural soils, which are responsible for ∼50% of the total global anthropogenic flux, but it is a relatively imprecise estimator. Nitrous oxide (N 2O) is a potent greenhouse gas (GHG) that also depletes stratospheric ozone. ![]()
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