Irrigation and straw particle size jointly drive soil bacterial community assembly to regulate nutrient cycling in paddy fields

The challenges of water scarcity and imperative of food security necessitate rice production systems that conserve water and enhance yield. Water-saving irrigation and straw return are widely implemented practices. However, the mechanisms of their interaction—specifically, how irrigation and straw physical characteristics influence soil bacterial community structure and function via water-substrate interactions—remain insufficiently understood. The frequent and shallow irrigation (FSI), which maintained anaerobic conditions, was associated with significantly elevated dissolved organic carbon and enriched anaerobic bacterial communities, suggesting accelerated carbon and nitrogen turnover. In contrast, the controlled irrigation (CI), characterized by alternating aerobic and anaerobic phases, was linked to enhanced NO₃ − -N production and increased soil organic carbon accumulation. A significant interaction between straw particle size and irrigation was observed: under CI, powdered straw was more effective in enhancing nitrification and SOC accumulation, likely due to its greater specific surface area, whereas under FSI, segmental straw appeared to provide a more stable carbon source. Temporal succession over two years showed a shift in the core bacterial community from Pseudomonadota to Chloroflexota. Predicted functional gene profiles (via PICRUSt2) indicated that FSI with powdered straw was associated with the upregulation of genes involved in carbon and nitrogen metabolism, while inferred metabolic functions under CI were largely unaffected by straw particle size, collectively indicating that irrigation method predominantly governs soil nutrient cycling and bacterial community assembly. This study substantiates that integrating CI with powdered straw return is an optimal strategy to concurrently achieve water conservation and soil nitrogen accumulation in rice production. • CI promotes soil carbon sequestration and nitrogen accumulation. • Powdered straw enhances CI nitrification; segmental straw aids FSI carbon release. • FSI drives bacteria to anaerobic communities that decompose recalcitrant carbon. • CI with powdered straw optimally enhances carbon storage and nitrogen availability.