Quantitative responses of potential nitrification and denitrification rates to the size of microbial communities in rice paddy soils

Abstract

Nitrification and denitrification are important to nitrogen balance in agricultural ecosystems. However, the molecular drivers and limiting steps for these microbial processes in rice paddy soils are not well understood. Here, we assessed soil properties and abundances of functional genes affiliated with nitrification (amoA and nxrA), denitrification (nirS, nirK and nosZ), nitrate reduction (narG and napA) processes, and measured potential nitrification and denitrification rates (PNRs and PDRs) at 15 sites in Xiamen, China. The soil properties imposed indirect impacts on the potential rates by mediating the relative abundances of microbial communities. No significant relationships between the size of microbial communities and the potential rates were observed. Instead, we found the variables that best explained the variations in the PNRs and PDRs were AOB/nirS and (nirK + nirS)/nosZ, respectively. The PNRs were mainly limited by the relative strength of two steps, namely bacterial ammonium oxidation and nitrite into nitric oxide reduction, whereas the PDRs were mainly limited by the relative strength of the second and last denitrification steps. These results indicated that the dynamics of microbial communities based on the relative gene abundances are valuable in integrating fluctuations in soil physicochemical properties and are indictive of potential rates in paddy soils. Results of this study contribute to our quantitative understanding of the relative importance of soil physicochemical and biological factors in driving microbial potential in paddy soils.