Nitrous Oxide Emissions from In Situ Deposition of 15N-Labeled Ryegrass Litter in a Pasture Soil

During pasture grazing, freshly harvested herbage (litterfall) is dropped onto soils from the mouths of dairy cattle, potentially inducing nitrous oxide (N2O) emissions. Although the Intergovernmental Panel on Climate Change (IPCC) recommends accounting for N2O emissions from arable crop residues in national inventories, emissions from the litterfall of grazed pasture systems are not recognized. The objective of this study was to investigate the potential of litterfall to contribute to N2O emissions in a field study located on a pasture site in Canterbury, New Zealand (43 degrees 38.50 ' S, 172 degrees 27.17 ' E). We applied N-15-labeled perennial ryegrass (Lolium perenne L.) to the surface of a pastoral soil (Temuka clay loam) and, for up to 139 d thereafter, quantified the contribution of herbage decomposition to N2O production and soil N dynamics. Litterfall contributed to the N-15 enrichment of soil NO3-N and N2O-N pools. After 49 d, N-15 recovery as N2O equated to 0.93% of the surface-applied litter N-15, with 38 to 75% of the cumulative N2O flux occurring within 4 to 10 d of treatment application. Emissions of N2O likely resulted from ammonification followed by a coupling of nitrification and denitrification during litter decomposition on the soil surface. The emission factor of the litter deposited in situ was 1.2 +/- 0.2%, which is not substantially greater than the IPCC default emission factor value of 1% for crop residues. Further in situ studies using different pasture species and litterfall rates are required to understand the microbial processes responsible for litter-induced N2O emissions.