Bias in IPCC Methodologies for Assessment of N20 Emissions and NO3 Leaching From Crop Residue.

Worldwide studies (20+) conducted with precise ¹⁵N techniques that trace the fate of N found that an average of 66% of the fertilizer was recovered in crops and soils. In other words, 34% of the added N was lost from the cropping systems. There are no extensive ¹⁵N recovery data tracing the fate of organic N from crop residues after a year of crop residue incorporation. Four rotations using the Delgado et al. (2004) large ¹⁵N cover crop residue exchange design resulted in an average recovery of 87% of the organic N from crop residues in soil and plants. The average losses from organic N added with crop residue were about 13%, much lower than the 31% N lost from inorganic N fertilizer in these four cropping systems. Additionally, we conducted DAYCENT evaluations on the effects of adding or removing crop residue on N₂O emissions and nitrate (NO₃-N) leaching. We used these ¹⁵N findings and DAYCENT simulations to evaluate the accuracy of the methodologies currently used by the Intergovernmental Panel on Climate Change (IPCC). The current approach for measuring N₂O-N emissions, NO₃-N leaching and indirect N₂O-N emissions from NO₃-N leaching does not reflect the relatively higher N losses from inorganic N fertilizer compared to the lower N losses from the much slower decomposition of the organic crop N residue pool. Default IPCC methodology uses the same N₂O emission factor (1%) and 30% NO₃-N leaching losses for N from crop residues, as for N from applied fertilizer. These unique ¹⁵N crop residue exchange studies and our DAYCENT simulation evaluations support the suggestion that the current IPCC methodology should be changed by lowering the N₂O-N emission and NO₃-N leaching losses coefficients in order to reflect lower N₂O-N emissions and leaching from crop residue N inputs when compared to N fertilizer.