Low N2O emissions induced by root-derived residues compared to aboveground residues of red clover or grasses mixed in the soil

Abstract IPCC default guidelines assume a constant N 2 O emission factor (EF N2O ) for both belowground crop residues (BGR) and aboveground residues (AGR), and that ~70 % of total N 2 O emissions following renewal of temporary grasslands come from BGR. However, empirical evidence is lacking, which motivated this study. BGR-free soil 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 N 2 O emissions. Lower emission factors 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 N 2 O 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, belowground residues likely contribute less to N 2 O emissions than calculated by the IPCC default methodology, whereas the ‘carbon effect’ and potentially high emissions factors of non-removable stubble interacting with the fertilizer management are neglected.