Nitrification represents the bottle-neck of sheep urine patch N₂O emissions from extensively grazed organic soils

Extensively grazed grasslands are understudied in terms of their contribution to greenhouse gas (GHG) emissions from livestock production. Mountains, moorlands and heath occupy 18% of the UK land area, however, in situ studies providing high frequency N₂O emissions from sheep urine deposited to such areas are lacking. Organic soils typical of these regions may provide substrates for denitrification-related N₂O emissions, however, acidic and anoxic conditions may inhibit nitrification (and associated emissions from nitrification and denitrification). We hypothesised urine N₂O-N emission factors (EFs) would be lower than the UK country-specific and IPCC default value for urine, which is based on lowland measurements. Using automated GHG sampling chambers, N₂O emissions were determined from real sheep urine (930 kg N ha^-1) and artificial urine (920 kg N ha^-1) applied in summer, and from an artificial urine treatment (1120 kg N ha^-1) and a combined NO₃^- and glucose treatment (106 kg N ha^-1; 213 kg C ha^-1) in autumn. The latter treatment provided an assessment of the soils capacity for denitrification under non-substrate limiting conditions. The artificial urine-N₂O EF was 0.01 ± 0.00% of the N applied in summer and 0.00 ± 0.00% of the N applied in autumn. The N₂O EF for real sheep urine applied in summer was 0.01 ± 0.02%. A higher flux was observed in only one replicate of the real urine treatment, relating to one chamber where an increase in soil solution NO₃^- was observed. No lag phase in N₂O emission was evident following application of the NO₃^- and glucose treatment, which emitted 0.69 ± 0.15% of the N applied. This indicates nitrification rates are the bottle-neck for N₂O emissions in upland organic soils. We calculated the potential impact of using hill-grazing specific urine N₂O EFs on the UK inventory of N₂O emissions from sheep excreta, and found a reduction of ca. 43% in comparison to the use of a country-specific excretal EF.