Sheep urine patch N₂O emissions are lower from extensively-managed than intensively-managed grasslands

A large number of sheep graze extensively managed grasslands, including upland and hill areas. Excretal deposition of nitrogen (N) to upland soil is a potentially large source of the powerful greenhouse gas (GHG), nitrous oxide (N₂O), however, few studies have assessed urine-patch N₂O emissions from upland areas. Current default excretal N₂O emission factors (EFs) are based on intensively managed lowland systems, with cattle excreta as the N source. We hypothesised that N₂O emissions could differ substantially from those of lowland systems, due to differences in soil type, climate, topography, pasture composition and management factors along altitudinal and productivity gradients. We investigated N₂O emission factors across two seasons (spring and autumn), for an extensive semi-improved, temperate grassland using IPCC-compliant and representative sheep urine patches (in terms of urine chemical composition, urine patch size and N loading rates). An automated GHG monitoring system provided high-frequency GHG data from sheep urine patches (756 and 1112 kg N ha⁻¹ applied in spring and autumn, respectively), reference artificial sheep urine patches (1066 and 1004 kg N ha⁻¹ applied in spring and autumn, respectively) and control treatments. In spring, urine patch N₂O emission factors were −0.02 ± 0.04 (artificial sheep urine) and 0.03 ± 0.09% (real sheep urine) of the applied N; in autumn emission factors were 0.02 ± 0.03 (artificial sheep urine) and 0.08 ± 0.04% (real sheep urine) of the applied N. These values are much lower than default inventory values (1% of applied N) for excreta deposited by grazing livestock. There was a greater pasture foliar N content following urine application in spring as opposed to autumn, and a significantly longer residence time of extractable mineral N in autumn. Our findings demonstrate the importance of generating country-specific N₂O EFs based on altitude/productivity gradients of livestock production, with implications for national inventories and the accuracy of sustainability metrics of lamb produced in the UK uplands.