15N isotopic crop residue cycling studies and modeling suggest that IPCC methodologies to assess residue contributions to N2O-N emissions should be reevaluated

Abstract

It is difficult to quantify nitrogen (N) losses from agricultural systems; however, we can use 15N isotopic techniques to conduct site-specific studies to increase our knowledge about N management and fate. Our manuscript analyzes two reviews of selected 15N isotopic studies conducted to monitor N fate. The mechanistic foci of these studies include crop residue exchange and N fate in farming systems. Analysis of the data presented in these studies supports the claim that the average N losses are greater from inorganic N fertilizer inputs than organic crop residue N inputs. Additionally we conducted unique DAYCENT simulations of the effects of crop residue on nitrous oxide (N2O-N) emissions and nitrate (NO3-N) leaching. The simulation evaluations support the crop residue 15N exchange studies and show lower leaching and N2O-N emissions from crop residue sources when compared to N fertilizer. The 15N data suggest that the N in the crop residue pool must be recycled, and that this is a slower and more protected pool when compared to the readily available fertilizer. The results suggest that the Intergovernmental Panel on Climate Change (IPCC) methodology should be reevaluated to determine whether the direct and indirect N2O-N emission coefficients need to be lowered to reflect fewer N2O-N emissions from high C/N crop residue N inputs. The data suggest that accounting for nutrient cycling has implications for public policy associated with the United Nations Framework Convention on Climate Change (UNFCCC) and mitigation of N2O-N emissions from agricultural soils. Additional crop residue exchange studies, field N2O-N and NO3-N leaching and support model evaluations are needed across different worldwide agroecosystems.