Litter-fall causes nitrous oxide emissions

Abstract

Our previous study showed that significant quantities of litter-fall (harvested but unconsumed plant material dropped during grazing) can be deposited onto the soil surface during a grazing event. However, the contribution of in situ decomposition of this litter-fall to nitrous oxide (N₂O) emissions is unknown. We applied 15N-labelled ryegrass (Lolium perenne L.) to the surface of a pastoral soil and for up to 139 days thereafter, quantified the contribution of herbage decomposition to N₂O production and soil N dynamics in field conditions. Approximately 70% of the total N₂O originated from the surface-applied litter treatment with 38–75% of the cumulative emissions occurring within 4–10 d of treatment application. After 66 d, dry matter loss from the litterbags equated to 46–82% of the pasture dry matter applied. Emissions of N₂O likely resulted from ammonification followed by a coupling of nitrification and denitrification during litter decomposition. The litter contributed to both the 15N enrichment of the soil NO₃ ⁻−N and N₂O–N pools. The emission factor (EF) of the in situ placed litter was 1.2%; similar to the IPCC default EF value of 1% for crop residues. Further in situ studies using different pasture species and litter-fall rates are required to understand the microbial processes responsible for litter-induced N₂O emissions.