The denitrification process is one of the important processes in the soil nitrogen (N) cycle, and is closely related to the loss of soil N fertilizer. Five treatments were included in this study: N0 (control, no N application); N90 (N application rate 90 kg ha⁻¹); N150 (N application rate 150 kg ha⁻¹); N210 (N application rate 210 kg ha⁻¹); and N270 (N application rate 270 kg ha⁻¹), to study the effect of different N application rates on the soil nirS-type denitrifying bacterial community structure, the influence of key enzyme activities during the denitrification process, and the main environmental factors affecting the variation of the denitrifying bacterial community in maize field soil under the mulched fertigation system in the semi-arid region of Northeast China. The results showed that increasing N fertilizer application significantly increased the contents of soil inorganic N and acid-hydrolyzable organic N, but significantly decreased pH. N fertilizer significantly increased nitrate reductase (NAR) activity and nitrite reductase (NIR) activity. Excessive application of N fertilizer significantly increased the nirS gene copy numbers, and, at the same time, significantly decreased the diversity of nirS-type soil denitrifying bacteria. Proteobacteria was the dominant denitrifying phylum in all treatments, and N application promoted the growth of Bradyrhizobium belonging to this phylum. The application of N fertilizer significantly changed the community structure of nirS denitrifying bacteria, and the NO₃⁻-N content was the most important factor for this observation. Soil organic matter (SOM) and non-hydrolyzable N (NHN) indirectly affected the denitrifying bacterial community structure through NAR activity and NIR activity, while soil total N (TN) and nitrate N (NO₃⁻-N) indirectly affected yield through denitrifying bacterial community structure. Although increasing N fertilizer was beneficial in increasing soil nutrients, the community structure of nirS-type denitrifying bacteria changed significantly. This is attributed to the increase in soil NO₃⁻-N accumulation caused by a large amount of N application. The results of this research provide an important scientific basis for further research on the response mechanism of farmland soil denitrifying microorganisms to different N fertilizer managements under the mulched fertigation system in the semi-arid region of Northeast China.

The soil nitrogen (N) cycle is an essential role of the biogeochemical cycle. Bacteria play an irreplaceable part in the soil N cycle, but the impact of different N gradients on bacterial communities remains unclear. The purpose of this research was to explore the bacterial abundance, community composition, and diversity under different N application rates in a water-limited area. We investigated the bacterial abundance, diversity, community composition, and structure under five different N gradients (0, 90, 150, 210, and 270 kg ha⁻¹) using real-time quantitative PCR and high-throughput sequencing, and then explored bacterial functional groups with FAPROTAX. N application significantly affected bacterial abundance and community composition. Bacterial diversity was enhanced at low N application rates and reduced at higher N application rates. Principal coordinate analysis showed that bacterial community structure was separated into two groups between low N application rates and high N application rates; these differences in bacterial community structure may be driven by available nitrogen (AN). The results of FAPROTAX revealed that N application promoted the functions of Aerobic_nitrite_oxidation, Nitrate_reduction, and Aerobic_ammonia_oxidation, but inhibited the Nitrogen_fixation function of the bacterial community. The high N network caused the reduction of network structure stability. Our results revealed that N fertilizer driven bacterial community structure and soil nutrients were the main influential factors in the variation of bacterial community structure. We suggest that the optimal N application rate in this study may be approximately 150 kg ha⁻¹, based on the variations of soil properties and bacterial community structure in semi-arid areas.

Adopting an optimal cropping regime is crucial for sustainable soil use. However, how different cropping regimes impact phosphorus (P) availability and the underlying mechanism remain unclear. Here, a 10-year field experiment was performed to examine the influence of different cropping regimes, including maize–soybean rotation (MSR), continuous maize cropping (CMC), and farmland fallow (FALL), under unfertilized and fertilized conditions in Northeast China. The P forms were analyzed using chemical fractionation and solution phosphorus-31 nuclear magnetic resonance. Compared to FALL, total P and different forms of P contents were significantly lower under MSR and CMC systems. Moreover, the contents of total P and different forms of P were higher under MSR than those under CMC. Correlation analysis showed that there were significant and positive correlations between total P and different forms of P contents. Redundancy analysis revealed soil organic carbon (SOC) as the most significant factor influencing total P and different forms of P. Structural equation modeling demonstrated the direct positive impacts of SOC, total nitrogen, total phosphorus, and Olsen phosphorus on phosphatase activity, which exhibited direct positive influence on P availability. In summary, maize–soybean rotation is an effective cropping regime for promoting P accumulation and availability in this region.