Soil denitrification response to increased urea concentration constrains nitrous oxide emissions in a simulated cattle urine patch

AimIncorporating non-bloat legumes into grass pastures can reduce enteric methane and alter cattle urinary urea N output by increasing protein intake. Deposition of high urea N urine influences soil N-cycling microbes and potentially N2O production. We studied how urine urea N concentration affects soil nitrifier and denitrifier abundances, activities, and N2O production.Methods(15)N(13)C-labelled urea dissolved in cattle urine was added at 3.5 and 7.0 g L-1 to soils from a grazed, non-bloat legume pasture and incubated under controlled conditions. CO2, N2O, C-13-CO2, and N-15-N2O production were quantified over 240 h, along with nitrifier and denitrifier N-cycling genes and mRNA transcripts.ResultsHigh urea urine increased total N2O relative to the control; low urea was not significantly different from the control or the high urea treatment. As a result, N2O-N emission factors were not significantly different between the low urea treatment (1.17%) and high urea treatment (0.94%). Doubling urea concentration doubled CO2-C-urea and N2O-N-urea but not total N2O-N. Urine addition initially inhibited then increased AOB transcripts and gene abundances. nirK and nirS transcript abundances indicated that denitrification by ammonia oxidizers and/or heterotrophic denitrifiers dominated N2O production. Urine addition increased nosZ-II vs. nosZ-I transcripts, improving soil N2O reduction potential.ConclusionCharacterizing this interplay between nitrifiers and denitrifiers improves the understanding of urine patch N2O sinks and source dynamics. This mechanistic information helps to explain the constrained short-term N2O emissions observed in response to excess urine N excretion from cattle consuming high protein diets, e.g. non-bloat legumes.