Roots and other residues from leys with or without red clover: Quality and effects on NO emission factors in a partly frozen soil following autumn ploughing.

Revised IPCC guidelines assume that a constant share of N in decomposing crop residues is directly emitted as NO (emission factor: EF), and calculate the amount of nitrogen (N) in non-removable residues of temporary grasslands proportionally to the average annual herbage yield. However, EF depends on the intrinsic quality of the residues and their interactions with environmental conditions. Only a few field studies on NO emissions from grassland renewal are available, and none have simultaneously quantified the N amount and quality of non-removable residues (roots and stubble). To gain insight into the effect of non-removable residue quality on EF, we studied the amount and quality of roots and stubble and their effect on EF following the ploughing of three-year-old swards. The measured amount of N in non-removable residues was approximately 20, 25, and 31 kg N per 1 Mg average annual dry matter yield in grass, red clover-grass, and red clover, and 70-83% of it was below ground. However, the EF of non-removable residues measured over 252 days was lower (0.24%, SE = 14% for grass and red clover-grass) than the IPCC default value (0.6%, CV: 50%) for wet regions, although within the uncertainty margin, and was significantly lower than the EF of incorporated herbage, which was related to differences in EF. We advocate for more specific studies that separate the effects of belowground and aboveground residues (AGR), considering the possibility of simplifying the accounting of NO emissions from belowground residues while improving that of non-removable AGR from temporary grasslands and other green crops. We observed the accumulation of NO in the frozen soil under snow, which was released during diurnal percolation of meltwater. NO emissions from frozen soil accounted for 30% or more of the total emissions.