Effects of rice straw biochar application rates on soil aggregate biogeochemistry and linkages to microbial community structure and enzyme activities

Biochar, a widely adopted soil amendment, has been widely recognized for its potential to improve crop yields and soil nutrients significantly. This enhancement is primarily attributed to the crucial role of soil microorganisms, whose contribution to soil fertility is significant and often underappreciated. However, the effects of varying biochar application rates on soil functional biota, particularly within aggregates that expand soil spatial heterogeneity, remain unclear. Understanding the relationship between nutrient dynamics and microbial community composition in these aggregates is essential for comprehending the intricate connections between soil microbiomes and related biogeochemical cycles. This study utilized long-term experimental soils, including treatments with no fertilizer, chemical fertilizer alone, and chemical fertilizer combined with rice straw biochar at gradient application rates (22.5, 45, 90 t·hm⁻²). The responses of microbial community structure and soil enzyme activities in whole soil and aggregates to different biochar application rates were investigated. Results showed that, compared to NPK treatment, biochar significantly increased bacterial and fungal diversity in macroaggregates. It also notably increased the relative abundance of Proteobacteria and Ascomycota in soil and aggregates, and at the same time reduced the relative abundance of Chloroflexi and Basidiomycota. Furthermore, carbon and phosphorus cycle-related enzyme activities increased significantly with higher biochar application rates. However, the activity of NAG, a nitrogen cycle-related enzyme, decreased as biochar application increased. Mantel analysis revealed that the relationship between microorganisms, enzyme activity, and soil nutrients was closest at a biochar application rate of 45 t·hm⁻². Structural equation modeling demonstrated that macroaggregates exhibited the most complex nutrient accumulation relationships, with bacterial and fungal diversity promoting nutrient accumulation. In conclusion, moderate biochar application induced the most intricate and closely connected microbial networks in macroaggregates, promoting soil nutrient cycling.