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

Abstract

Abstract 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, N2O production. We studied how urea-N concentration affects soil nitrifier and denitrifier abundance and activity and CO2 and N2O production. Methods 15N13C-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, 13CO2, and 15N2O production were quantified over 240 h, along with nitrifer and denitrifier N-cycling genes and mRNA transcripts. Results High 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 decreased from 1.25–0.74% in low vs. high urea treatments. Doubling urea concentration doubled urea-derived 13CO2 and 15N2O but not total N2O. Urine addition initially inhibited, but then increased AOB transcription and abundance. nirK and nirS transcript abundances indicated that denitrification by ammonia oxidizers and/or heterotrophic denitrifiers likely dominated N2O production at a lower-than-expected WFPS (47% & 65%). Urine addition increased nosZ-II vs. nosZ-I transcription, improving soil N2O reduction potential. Conclusion Characterizing this interplay between nitrifiers and denitrifiers improves the understanding of urine patch N2O sinks and sources. The smaller emission factor from high urea urine suggests that current inventory methods using N deposition rates could overestimate soil N2O emissions from cattle consuming non-bloat legumes.