Orthophosphate enhances N 2 O production from aerobic hydroxylamine decomposition: implications to N 2 O emissions from nitrification in ornithogenic and manure-fertilized soils

Soils affected by animal wastes have simultaneously high N and P contents. Despite the reports of high nitrous oxide (N2O) emissions in such environments, the role that P plays in N2O dynamics has not yet been systematically investigated. Here, we report the enhancement effect of orthophosphate (PO43−-P) abundance on N2O yields from abiotic NH2OH decomposition, which may have substantial implications to N2O emissions from nitrification in such P-rich soils. The axenic cultures of Nitrosomonas europaea, an ammonia-oxidizing bacterium previously reported to leak NH2OH, exhibited significantly higher N2O yields when incubated at higher PO43−-P concentrations. As NH4+-to-NO2− turnover and growth rates were unaffected even at the highest PO43−-P concentration examined, the abiotic interaction between extracellularly released NH2OH and PO43−-P was the most plausible mechanism of enhanced N2O emission in these nitrifier cultures. This proposed mechanism was supported by the results of abiotic NH2OH incubation whereby higher PO43−-P concentration resulted in higher N2O yield. Orthophosphate enhancement of NH2OH-to-N2O turnover was then simulated with addition of 5 μmol NH2OH to an ornithogenic soil with high PO43−-P content (23.9 ± 6.7 g/kg wet soil) and active nitrification activity after sterilization. The N2O yield, 69.0 ± 4.6%, was significantly higher than the N2O yields for other examined soils with lower PO43−-P contents (0–1.94 g/kg wet soil), and the PO43−-P contents of the examined soils exhibited strong correlation with the N2O yields. These findings suggest that N2O production from nitrification via abiotic turnover of released NH2OH may be a consequential mechanism of N2O emissions in PO43−-P-rich soils.