Nitrogen and potassium co-application sustain soil inorganic nitrogen by reshaping microbial communities and predicted nitrogen-cycling functional genes during rapeseed growth

The synergistic effects of nitrogen (N) and potassium (K) fertilizers on soil biological characteristics remain poorly understood. To address this, a six-year continuous field experiment with rapeseed was conducted, incorporating four N and K application treatments: no N and K (CK), N alone (N), K alone (K), and combined N and K (NK). Soil samples collected at three key growth stages of rapeseed were analyzed using 16S rRNA gene high-throughput sequencing. Results showed that compared to N treatment, the NK combination significantly increased soil inorganic N content during the overwintering stage (at 65 days after planting) by 103.1%. This increase was closely associated with shifts in soil bacterial community structure, changes in the predicted abundance of nitrogen-cycling functional genes, and the restoration of bacterial co-occurrence networks. While N application alone strongly altered the soil microbial community and elevated the predicted abundance of nitrification and denitrification genes, K application alone had no significant effect. In contrast, the combined application of N and K not only reduced the predicted abundance of these N-loss-related genes but also enhanced bacterial network modularity, effectively mitigating the negative effects of N-only fertilization. These findings highlight the importance of balanced nutrient management in maintaining the functional stability of soil microbial ecosystems.