Urease Inhibitors Weaken the Efficiency of Nitrification Inhibitors in Mitigating N2O Emissions from Soils Irrigated with Alternative Water Resources

It is generally accepted that inhibitors are effective in reducing agricultural nitrous oxide (N2O) emissions from soils irrigated by groundwater. However, it was unclear whether these inhibitors effectively regulate N2O emissions from soils irrigated with alternative waters, like reclaimed water and livestock wastewater. To clarify this, nitrapyrin, a nitrification inhibitor, and N-(N-butyl) thiophosphoric triamide, a urease inhibitor, were added separately or jointly to the soils irrigated by groundwater, reclaimed water and livestock wastewater through two consecutive cycles of pot experiment. Both the single and combined addition of inhibitors lowered N2O emissions from soils irrigated with alternative water, while the reduction effect of the combined application decreased relative to that of the single application. The using of combined inhibitors did reduce the enrichment level of nitrification genes and slow down the nitrification process, but the associated relatively high nirS/nosZ ratio potentially discounted its ability to prevent N2O emissions. Whereas under groundwater irrigation, treatment with combined inhibitors only decreased N2O emissions in the first cycle but not in the second cycle. Inhibitor application affected the composition of soil bacterial communities, and in particular, urease inhibitor application increased community differences across the two cycles. Moreover, using inhibitors led to a general reduction in the enrichment level of the denitrification genes narG and nosZ, and we speculate that inhibitors could also indirectly manipulate N2O release by involving the denitrification process. Structural equation model results further displayed that the relative abundance of the nxrA and narG genes and NH4+-N concentration played a vital role in the regulation of N2O release from the alternative water-irrigated soils applied with inhibitors.