Bacteria–Virus Interactions Are More Crucial in Soil Organic Carbon Storage than Iron Protection in Biochar-Amended Paddy Soils

Iron oxides supposedly provide physicochemical protection for soil organic carbon (SOC) under anoxic conditions. Likewise, biochar can modulate the composition of soil microbial communities. However, how Fe oxides and microbial communities influence the fate of SOC with biochar amendment remains unresolved, especially the effect of the bacteria–virus interaction on SOC dynamics. Here, we performed a four-month pot experiment using rice seedlings with a biochar amendment under waterlogged conditions. Then, soil aggregate sizes were examined to explore the factors influencing the SOC patterns and the underlying mechanisms. We found that biochar altered soil enzyme activities, especially in macroaggregates. Fe oxides and necromass exhibited significant negative relationships with SOC. Bacterial communities were notably associated with viral communities. Here, the keystone ecological cluster (module 1) and keystone taxa in the bacteria–virus network showed significant negative correlations with SOC. However, Fe oxides exhibited substantial positive relationships with module 1. In contrast to the prevailing view, the SOC increase was not primarily driven by Fe oxides but strongly influenced by bacteria–virus interactions and keystone taxa. These findings indicate that biochar governs microbial-mediated SOC accumulation in paddy soil and ascertains the role of viruses in regulating the bacterial community, thus predicting SOC stock.