The role of different exogenous NO concentrations on C and N biogeochemistry of an agricultural soil

The signaling compound nitric oxide (NO) might play an important, yet unquantified role in mediating soil biogeochemical Carbon and Nitrogen cycles. This study quantified the effects of different soil-typical exogenous NO concentrations on the microbial community, on fertilizer N turnover, and on C and N trace gas fluxes of agricultural soil. For this, we repeatedly established soil NO concentrations of either 0, 200, 400, and ppbv˗NO in soil mesocosms for in total of 12 days, followed by high-resolution automated measurements of CO₂, NO, CH₄, and N₂O fluxes, molecular analysis of microbial community composition and ¹⁵N-isotope-tracing based assessment of fertilizer N turnover. We found no effects of different NO levels on microbial communities and CO₂, CH₄, and NO fluxes. However, at 200 ppbv concentration, exogenous NO promoted microbial assimilation of fertilizer N. In contrast, at 400 ppbv˗NO concentration, microbial biomass N was reduced, and microbial uptake of fertilizer N was inhibited, accompanied by a 33% reduction of N₂O emissions. This suggested a promoting effect of 200 ppbv˗NO on the physiology of cells involved in heterotrophic microbial N turnover, probably reinforcing the role of cell-endogenous NO. In contrast, the higher exogenous NO concentrations of 400 ppbv seemed to inhibit heterotrophic microbial inorganic N assimilation, with however no increase in N₂O emissions due to detoxification mechanisms. In conclusion, our pioneering study provides first insights into impacts of exogenous NO on soil C and N biogeochemistry in natural soil systems and reveals a NO concentration-dependent regulation of microbial N retention.