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

The default The Intergovernmental Panel on Climate Change (IPCC) 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 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 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 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, N 2 O accountings based on present IPCC default methodology likely overestimate the contribution by crops’ BGR. • N 2 O emission factors of roots (BGR) around 1/5 those of stubble and herbage (AGR). • Nitrate highly increased N 2 O emission factors of AGR, but not of BGR. • Diverse effects of BGR were related to lower C decomposition.