Synergistic effects of biochar and microbial inoculants on rice productivity and soil fertility are mediated by a nitrogen-dependent microbial pathway

Excessive nitrogen (N) fertilizer application threatens soil health and sustainable rice production. We hypothesized that the combined application of biochar and microbial inoculants (B+M) could maintain rice yield under reduced N input through microbiome-mediated mechanisms, and that this effect may depend on an N threshold. To test this hypothesis, pot and field experiments were conducted using the rice cultivar Nanjing 9108 with four treatments: conventional N (CK), and three B+M treatments with 20% N reduction (N80), 40% N reduction (N60), and 0% N reduction (N100). The results indicated that compared to CK, the N100 and N80 treatments increased rice yield by 10.4-11.7% and 4.1-4.9%, and improved Nitrogen Partial Factor Productivity (NPFP) by 7.8-11.7% and 28.6-30.0%, respectively. In contrast, the N60 reduced yield by 3.9-4.6%. In pot experiments, 16S rRNA gene sequencing revealed that N100 and N80 enriched bacterial phyla (e.g., Proteobacteria, Actinobacteria, Chloroflexi, and Bacteroidota) and enhanced microbial functional genes linked to metabolism, genetic information processing, cellular processes, and environmental information processing, thereby increasing soil nutrient availability. Structural equation modeling demonstrated that the soil bacterial community was the fundamental driver of yield enhancement (model fit: χ²/df = 1.13, RMSEA = 0.042, CFI = 0.961). However, under 40% N reduction, the abundance of key phyla declined and microbial functional potential weakened, leading to reduced soil nutrient availability and yield. These findings reveal an N dependent microbial-mediated pathway governing the synergistic effects of biochar and microbial inoculants, identifying 20% N reduction with B+M as an optimal strategy for sustainable rice intensification.