Straw and straw biochar differently affect fractions of soil organic carbon and microorganisms in farmland soil under different water regimes

Incorporating amendments can change soil organic carbon (SOC) components and biological properties, but the comprehensive utilization of straw and straw biochar in paddy soil remains poorly understood. We studied the changes in the pH, SOC content, labile organic C fractions, and soil microbial communities under alternating wetting and drying (AWD) and continuous flooding (CF). The results showed that the addition of straw and straw biochar increased SOC content, and the relative changes in microbial biomass carbon (MBC), dissolved organic carbon (DOC), and easily oxidizable carbon (EOC) were more sensitive than those in SOC, but soil pH was higher under AWD management than under CF management. Proteobacteria, Bacteroidetes, and Acidobacteria were the three most abundant bacterial phyla, and the three major abundant fungal phyla were Phragmoplastophyta, norank_Eukaryta, and Apicomplexa. A redundancy discriminant analysis (RDA) found that both straw and straw biochar were conducive to bacterial growth, and their combination was more beneficial for fungal growth and development. Aggregated boosted tree (ABT) and Structural equation modeling (SEM) showed that the soil MBC and EOC were the most important factors determining bacterial abundance and diversity, while pH, EOC, and MBC were the main factors influencing the abundance and diversity of fungi. This study explored the changes in SOC fractions and bacterial and fungal communities and provided a theoretical basis for an in-depth understanding of the influence of labile organic C pools on bacterial and fungal communities.