Objectives The abundance of nitrification and denitrification microorganisms under different soil moistures was studied to reveal the response of soil microbial community structure to soil moisture content, and serve the water management of farmland in black soil area.

Methods The tested soil was collected from a long-term experiment station in a typical black soil. Microcosmic controlled laboratory culture experiment was conducted and soil samples were loaded into tubes, four soil moisture treatments were setup to simulate field soil moisture environments of 40%WHC, 60%WHC, 80%WHC and 100%WHC. After incubated for 7 days at (25±1)℃, the abundance of nitrification and denitrification microorganisms in soils was determined using real-time fluorescence quantitative q-PCR and high-throughput sequencing technology, to reveal the change of microbial community structure under different soil moisture contents.

Results Soil moisture significantly affected the abundance of nitrification and denitrification microorganisms in black soil. For nitrification microorganisms, the copy number of AOA gene was the lowest at 60%WHC, and then increased significantly with the increase of soil moisture. The AOB gene copy number was in descending order of 100%WHC, 60%WHC, 80%WHC and 40%WHC. For denitrification microorganisms the copy number of nirS and nosZ genes increased with the increase of soil water content. The highest values were under 100%WHC, 38 and 2.7 times of those under 40%WHC. The results of high-throughput sequencing analysis showed that soil microbial community structure and composition changed significantly under different water conditions. The species richness and diversity of soil microbial community under 80%WHC treatment were relatively high, while the diversity of soil microbial community under 100%WHC treatment was relatively low. RB41 and Nitrospira were the dominant genera under 40%WHC and 60%WHC, while Rhizobacter,Pseudomonas and Flavisolibacter were the dominant genera under 80%WHC and 100%WHC. Proteobacteria had a strong positive correlation with soil water content, and the dominant genera Rhizobacter and Pseudomonas showed a significant positive correlation with soil moisture content.

Conclusions Lower soil moisture increases the abundance of nitrification microorganisms, and higher soil moisture, especially under saturated water conditions, significantly increases the abundance of denitrification microorganisms nirS and nosZ. The increase of water content promotes the further reduction of N₂O to N₂ by increasing the abundance of nosZ gene. Water content changes the dominant species of nitrification and denitrification microorganisms in soil. Nitrospira abundance is negatively, while Rhizobacter and Pseudomonas abundance is significantly and positively correlated with soil moisture content. High water content further promotes the denitrification.