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

Abstract

Ruminant urine patches on grazed grassland are a signifcant source of agricultural nitrous oxide (N2O) emissions. Of the many biotic and abiotic N2O production mechanisms initiated following urine-urea deposition, codenitrifcation resulting in the formation of hybrid N2O, is one of the least understood. Codenitrifcation forms hybrid N2O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N2O. The objective of this study was to assess the relative signifcance of diferent N substrates on codenitrifcation and to determine the contributions of fungi and bacteria to codenitrifcation. 15N-labelled ammonium, hydroxylamine (NH2OH) 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 N2O fuxes were monitored and the codenitrifed N2O fuxes determined. Fungal inhibition decreased N2O fuxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N2O fux of the glycine treatment, by 14%. Hydroxylamine (NH2OH) generated the highest N2O fuxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N2O fux. All the N substrates examined participated to some extent in codenitrifcation. Trends for codenitrifcation under the NH2OH substrate treatment followed those of total N2O fuxes (85.7% of total N2O fux). Codenitrifcation fuxes under non-NH2OH substrate treatments (0.7–1.2% of total N2O fux) were two orders of magnitude lower, and signifcant 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 codenitrifcation substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrifcation.