Fungal and bacterial contributions to codenitrification emissions of N2O and N2 following urea deposition to soil

Abstract

Grazed pastures contribute significantly to anthropogenic emissions of N2O but the respective contributions of archaea, bacteria and fungi to codenitrification in such systems is unresolved. This study examined the relative contributions of bacteria and fungi to rates of denitrification and codenitrification under a simulated ruminant urine event. It was hypothesised that fungi would be primarily responsible for both codenitrification and total N2O and N2 emissions. The effects of bacterial (streptomycin), fungal (cycloheximide), and combined inhibitor treatments were measured in a laboratory mesocosm experiment, on soil that had received 15N labelled urea. Soil inorganic-N concentrations, N2O and N2 gas fluxes were measured over 51 days. On Days 42 and 51, when nitrification was actively proceeding in the positive control, the inhibitor treatments inhibited nitrification as evidenced by increased soil NH 4 + -N concentrations and decreased soil NO 2 − -N and NO 3 − -N concentrations. Codenitrification was observed to contribute to total fluxes of both N2O (≥ 33%) and N2 (≥ 3%) in urine-amended grassland soils. Cycloheximide inhibition decreased NH 4 + –15N enrichment and reduced N2O fluxes while reducing the contribution of codenitrification to total N2O fluxes by ≥ 66 and ≥ 42%, respectively. Thus, given archaea do not respond to significant urea deposition, it is proposed that fungi, not bacteria, dominated total N2O fluxes, and the codenitrification N2O fluxes, from a simulated urine amended pasture soil.