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

Aim Incorporating 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 N 2 O production. We studied how urine urea N concentration affects soil nitrifier and denitrifier abundances, activities, and N 2 O 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. CO 2 , N 2 O, 13 C-CO 2 , and 15 N-N 2 O production were quantified over 240 h, along with nitrifier and denitrifier N-cycling genes and mRNA transcripts. Results High urea urine increased total N 2 O relative to the control; low urea was not significantly different from the control or the high urea treatment. As a result, N 2 O-N emission factors were not significantly different between the low urea treatment (1.17%) and high urea treatment (0.94%). Doubling urea concentration doubled CO 2 -C urea and N 2 O-N urea but not total N 2 O-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 N 2 O production. Urine addition increased nosZ-II vs. nosZ-I transcripts, improving soil N 2 O reduction potential. Conclusion Characterizing this interplay between nitrifiers and denitrifiers improves the understanding of urine patch N 2 O sinks and source dynamics. This mechanistic information helps to explain the constrained short-term N 2 O emissions observed in response to excess urine N excretion from cattle consuming high protein diets, e.g. non-bloat legumes.