Abstract
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.
Highlights
Mountains, moorlands and heath comprise 18% of the total UK land area (Van der Wal et al, 2011) and extensive livestock grazing in these ecosystems allows the maintenance of an open habitat of grass and heath (Worrall and Clay, 2012; Leiber-Sauheitl et al, 2015)
We focused on the urine fraction of excreta as, in comparison to faeces, it is more susceptible to N2O losses due to the highly labile nature of the substrates added
Fluxes were not monitored for a full year, which is recommended to provide IPCC compliant N2O-N emission factors (EFs), we believe we have captured the main N2O emission window caused by the urine application, as concentrations of both NH4+ and NO3− were similar to control values by the end of the summer study
Summary
Moorlands and heath comprise 18% of the total UK land area (Van der Wal et al, 2011) and extensive livestock grazing in these ecosystems allows the maintenance of an open habitat of grass and heath (Worrall and Clay, 2012; Leiber-Sauheitl et al, 2015). The impact of livestock urine on greenhouse gas (GHG) emissions from extensively grazed agroecosystems is understudied, especially those from organic soils (e.g. Histosols). Organic soils are renowned for either being large sources or sinks of GHGs e.g. under water-saturated conditions they are a source of CH4 and a sink for CO2, due to the retarded degradation of plant residues (Martikainen et al, 1995; Berglund and Berglund, 2011). Organic soils drained for agriculture, forestry or peat extraction produce large amounts of the powerful GHG nitrous oxide (N2O; Regina et al, 1999; Andert et al, 2011; Taft et al, 2017). Pristine peat soils have negligible N2O emissions (Regina et al, 2004), due to the highly competitive demand for available N between plants and microorganisms (Repo et al, 2009). Atmospheric N deposition is the only major input of N to these systems i.e. inputs of N as fertilisers do not occur (Batey, 1982; Chapman et al, 2001)
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