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

323 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. Th e objective of this study was to investigate the potential of litterfall to contribute to N2O emissions in a fi eld study located on a pasture site in Canterbury, New Zealand (43°38.50′ S, 172°27.17′ E). We applied 15N-labeled perennial ryegrass (Lolium perenne L.) to the surface of a pastoral soil (Temuka clay loam) and, for up to 139 d thereaft er, quantifi ed the contribution of herbage decomposition to N2O production and soil N dynamics. Litterfall contributed to the 15N enrichment of soil NO3–N and N2O–N pools. Aft er 49 d, 15N recovery as N2O equated to 0.93% of the surface-applied litter 15N, with 38 to 75% of the cumulative N2O fl ux occurring within 4 to 10 d of treatment application. Emissions of N2O likely resulted from ammonifi cation followed by a coupling of nitrifi cation and denitrifi cation during litter decomposition on the soil surface. Th e 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 diff erent pasture species and litterfall rates are required to understand the microbial processes responsible for litter-induced N2O emissions. Nitrous Oxide Emissions from In Situ Deposition of 15N-Labeled Ryegrass Litter in a Pasture Soil