Abstract
Urine patches contribute greatly to greenhouse gas emissions within livestock grazed ecosystems. The effective area of a ruminant urine patch comprises the wetted area, the diffusional area and the pasture response area. This study specifically assesses the importance of considering the diffusional area for monitoring urine patch N2O emissions. Spatial and temporal changes in N2O emissions and potential drivers of emissions (soil pH, EC, redox potential, dissolved organic carbon and nitrogen, NO3− and NH4+) were measured in sheep urine amended Eutric Cambisol mesocosms, maintained at 50% or 70% water-filled pore space (WFPS). At 70% WFPS, over 10 weeks, the emission factor (EF) was greater when considering the wetted area plus a 9 cm diffusional area (EF = 2.75 ± 0.72% of applied N) than when considering the wetted area alone (EF = 1.44 ± 0.30% of applied N); differences were not statistically significant at 50% WFPS. Redox potential, total extractable N and WFPS contributed significantly to the observed variation in daily N2O fluxes from the urine patch. We conclude that the urine patch diffusional area is an extremely important source of emissions from urine patches. This has implications when measuring EFs, as the lateral diffusion of solutes may be restricted by chamber walls resulting in an underestimate of N2O emissions, particularly at higher soil moisture contents. Site-specific assessments of the urine patch diffusional area should be made, and accounted for, prior to monitoring emissions and calculating emission factors from urine patches applied within chambers.
Highlights
Grazing returns of excreta to pasture soils are estimated to account for 40% of the total nitrous oxide (N2O) emissions from animal production systems, globally (Oenema et al, 2005)
The current default IPCC emission factors used in national inventories for excretal deposition to soils are 1% and 2% of deposited N for sheep and cattle, respectively (IPCC, 2006); yet, this Tier 1 approach lacks accuracy as it fails to account for variation in N2O emissions due to environmental, edaphic or management related factors (Skiba and Smith, 2000; Skiba et al, 2012; Buckingham et al, 2014)
Spatial and temporal variation in soil pH, electrical conductivity (EC) and oxidationereduction potential (ORP) was observed in the Eutric Cambisol following sheep urine application (Fig. 2)
Summary
Grazing returns of excreta to pasture soils are estimated to account for 40% of the total (direct and indirect) nitrous oxide (N2O) emissions from animal production systems, globally (Oenema et al, 2005). Additions of labile carbon (C) and nitrogen (N) to soil in the form of urine (van Groenigen et al, 2005) fuel the major microbial N2O producing processes of nitrification and denitrification, creating “hot spots” and “hot moments” for emissions within pastures (McClain et al, 2003; Groffman et al, 2009). Microbial N2O production and consumption processes depend on several interacting environmental controls (Bouwman et al, 2013) such as N supply, soil temperature, soil moisture, oxidationereduction potential (ORP), the availability of labile organic compounds, soil type, soil pH and climate (Skiba and Smith, 2000; Butterbach-bahl et al, 2013). Urine patches offer potential for emission reductions and improvements of nitrogen use efficiency (NUE) within the agricultural sector, yet a greater understanding of the spatial and temporal variability in N2O emissions from urine patches (at several scales of magnitude) is required to improve emission estimates and provide information for emission reduction strategies, such as the use of nitrification inhibitors
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