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 (NO) emissions. Of the many biotic and abiotic NO production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid NO, is one of the least understood. Codenitrification forms hybrid NO via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce NO. 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. N-labelled ammonium, hydroxylamine (NHOH) 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 NO fluxes were monitored and the codenitrified NO fluxes determined. Fungal inhibition decreased NO fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the NO flux of the glycine treatment, by 14%. Hydroxylamine (NHOH) generated the highest NO fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the NO flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NHOH substrate treatment followed those of total NO fluxes (85.7% of total NO flux). Codenitrification fluxes under non-NHOH substrate treatments (0.7-1.2% of total NO 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.