Low N2O emissions induced by root-derived residues compared to aboveground residues of red clover or grass mixed into soil

Abstract

The default The Intergovernmental Panel on Climate Change (IPCC) guidelines assume a constant N₂O emission factor (EF_N2O) for both belowground crop residues (BGR) and aboveground residues (AGR), and that ∼70 % of total N₂O emissions following renewal of temporary grasslands come from BGR. However, empirical evidence is lacking, which motivated this study. BGR-free and BGR-rich clay loam collected in grass or red clover leys were incubated alone or mixed with AGR and different doses of nitrate over 107 days. The average EF_N2O of BGR was around 18 % of that of AGR, and remained low even when soil nitrate concentration was very high, whereas EF_N2O of AGR varied largely and rocketed even with a small increase in soil nitrate. The decomposition of the carbon present in crop residues was critical for N₂O emissions. Lower EF_N2O of BGR relative to AGR were related to slower C decomposition, which was not predicted by the biochemical characteristics. It is also likely that BGR were less conducive than AGR to develop into hotspots for N₂O emission because of the roots’ finer distribution and closer contact with soil particles. Differences in EF_N2O among AGR were mostly linked to the availability of N, either derived from residue mineralization or present in the soil. In conclusion, N₂O accountings based on present IPCC default methodology likely overestimate the contribution by crops’ BGR.