Abstract
Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N2O) emissions. Of the many biotic and abiotic N2O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N2O, is one of the least understood. Codenitrification 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 significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. 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 fluxes were monitored and the codenitrified N2O fluxes determined. Fungal inhibition decreased N2O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N2O flux of the glycine treatment, by 14%. Hydroxylamine (NH2OH) generated the highest N2O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N2O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH2OH substrate treatment followed those of total N2O fluxes (85.7% of total N2O flux). Codenitrification fluxes under non-NH2OH substrate treatments (0.7–1.2% of total N2O 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.
Highlights
The nitrous oxide (N2O) molecule is a potent greenhouse gas, with a global warming potential 298 times that of carbon dioxide over a 100 year time period[1]
Using selective microbial inhibition treatments, and simulating a ruminant urine patch environment, we demonstrated that all the used 15N-labelled N substrates contributed to codenitrification in a soil matrix
Hydroxylamine was the most important N substrate with respect to increasing the N2O flux and contributing to codenitrification (85.7% of total flux), likely because of its more reactive character compared to the other N substrates
Summary
The nitrous oxide (N2O) molecule is a potent greenhouse gas, with a global warming potential 298 times that of carbon dioxide over a 100 year time period[1]. In order to conserve both energy and oxygen, nitrifier-denitrification may occur in response to limited soil oxygen conditions[21], whereupon nitrifiers convert NO2− to NO, N2O and N212 the significance of this process may have been overestimated in some studies[22] In addition to these N2O production pathways, N2O may be produced as ‘hybrid’ N2O via codenitrification, a process involving two different N pools[20,23]. Abiotic reactions involving reduced iron (Fe2+) and NO2−, may occur at the interface between an aerobic zone overlying an anaerobic zone when NO2− diffusing downwards meets Fe2+24,25 This process is unlikely to contribute significantly to N2O emissions due to insufficient Fe2+ ion concentrations in most soils[26,27]. We hypothesise that in a soil matrix under simulated ruminant urine deposition the N substrates applied in this study will be utilized for codenitrification reactions, with a microbial preference for NH2OH and that these reactions would be mainly fungi driven
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