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

Abstract

• NH 3 volatilisation in urine patches is not directly reflected in δ 15 N of residual soil N. • NO 3 − isotopic enrichment depends on mineralisation, nitrification, and denitrification. • NO 3 − in grazed pastures can range from −10‰ to +15‰ (δ 15 N) and −5‰ to +5‰ (δ 18 O). Nitrate dual isotopes (δ 15 N NO 3 − and δ 18 O NO 3 − ) 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 NO 3 − . 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 (NO 3 − , nitrite, and ammonium) following the addition of differing levels of bovine urine and urea fertiliser. Ammonia volatilisation, with a δ 15 N 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 NO 3 − pool due to a combination of urine-induced mineralisation (up to 120 μg N g soil −1 day −1 in the high urine treatment) and on-going nitrification. Consequently, NO 3 − measured within the treatments was not as enriched in 15 N as the values typically ascribed to excreta-N sources. Up-scaling these results, the whole-pasture NO 3 − isotopic composition primarily reflected time since fertilisation, regardless of urine inputs. These findings necessitate expanding the range of δ 15 N NO 3 − 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 NO 3 − isotopic source ranges.