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Ammonia volatilisation is not the dominant factor in determining the soil nitrate isotopic composition of pasture systems

Wells, Naomi S., Baisden, W. Troy, Clough, Tim J.
Agriculture, ecosystems & environment 2015 v.199 pp. 290-300
agroecosystems, ammonia, cattle, fractionation, freshwater, mineralization, nitrates, nitrification, nitrites, nitrogen, pastures, pollution, soil, stable isotopes, urea fertilizers, urine, volatilization
Nitrate dual isotopes (δ15NNO3− and δ18ONO3−) are increasingly used to assess the sources and sinks of nitrogen (N) pollution in freshwater systems. However, the application of this methodology to pasture agroecosystems is currently limited by the lack of information on how, or even if, the primary N inputs to the systems (livestock urine and urea fertiliser) are expressed in the isotopic signature of exported NO3−. To remedy this gap, direct measurements of fractionation during ammonia volatilisation were linked with changes in the concentration and isotopic composition of the residual soil inorganic N pool (NO3−, nitrite, and ammonium) following the addition of differing levels of bovine urine and urea fertiliser. Ammonia volatilisation, with a δ15N enrichment factor of +35±5‰, removed from 5 to 40% of N inputs from the different treatments, which should have enriched the residual inorganic N pool to 25‰ and 3‰, respectively. However, this fractionation did not propagate into the soil NO3− pool due to a combination of urine-induced mineralisation (up to 120μgNgsoil−1day−1 in the high urine treatment) and on-going nitrification. Consequently, NO3− measured within the treatments was not as enriched in 15N as the values typically ascribed to excreta-N sources. Up-scaling these results, the whole-pasture NO3− isotopic composition primarily reflected time since fertilisation, regardless of urine inputs. These findings necessitate expanding the range of δ15NNO3− values ascribed to livestock sources to encompass values as low as −10‰, highlighting the need to account for post-deposition soil N cycling in order to accurately define NO3− isotopic source ranges.