Impact of nitrogen compounds on fungal and bacterial contributions to codenitrification in a pasture soil

Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N(2)O) emissions. Of the many biotic and abiotic N(2)O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N(2)O, is one of the least understood. Codenitrification forms hybrid N(2)O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N(2)O. The objective of this study was to assess the relative significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. (15)N-labelled ammonium, hydroxylamine (NH(2)OH) and two amino acids (phenylalanine or glycine) were applied, separately, to sieved soil mesocosms eight days after a simulated urine event, in the absence or presence of bacterial and fungal inhibitors. Soil chemical variables and N(2)O fluxes were monitored and the codenitrified N(2)O fluxes determined. Fungal inhibition decreased N(2)O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N(2)O flux of the glycine treatment, by 14%. Hydroxylamine (NH(2)OH) generated the highest N(2)O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N(2)O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH(2)OH substrate treatment followed those of total N(2)O fluxes (85.7% of total N(2)O flux). Codenitrification fluxes under non-NH(2)OH substrate treatments (0.7–1.2% of total N(2)O flux) were two orders of magnitude lower, and significant decreases in these treatments only occurred with fungal inhibition in the amino acid substrate treatments. These results demonstrate that in situ studies are required to better understand the dynamics of codenitrification substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrification.