Salinity decouples the relationships between microbial functional gene abundance and N2O emissions in subtropical agricultural soils

Secondary salinization resulting from long–term greenhouse cultivation has become a major factor adversely influencing soil functions such as nitrous oxide (N2O) emissions. Nevertheless, how salinity effects N2O emissions and underlying microbial mechanisms remains largely unclear. We carried out an investigation over one growing season into in situ N2O flux under long–term greenhouse cultivation. After the trials, we collected soil samples to measure potential nitrification and denitrification rates via laboratory incubations using 15N–labelled KNO3 in combination with acetylene inhibition. High–throughput qPCR was used to measure the changes in the abundance of N cycling–related functional genes in the soils. Soil salinity significantly promoted N2O emissions (P < 0.001), and the increase in N2O emissions was mainly attributed to the nitrification process, not the denitrification process. However, we found that the abundance of N–cycling–related genes had a downward trend with increasing soil salinity, and showed different patterns with the corresponding changes in soil potential nitrification rates across the treatments. In addition, the structural equation modelling showed that aboveground crop biomass was the second most important factor driving soil N2O emissions. These findings show that salt stress can increase N2O emissions via nitrification processes in subtropical agricultural soils. Given the prevalence of salt stress worldwide, it is necessary to incorporate soil salinity into models of global N2O emissions.