Synergistic effects of irrigation regime and straw crushing degree on nitrogen and phosphorus runoff and leaching in paddy fields

To clarify the synergistic effects of irrigation regime and straw crushing degree on nitrogen and phosphorus loss and leaching in paddy fields and the DOC-mediated mechanism, based on a barrel experiment in 2023–2024, five treatments were set: conventional flooding without straw (CK), frequent and shallow irrigation + powdered/segmented straw (FSI + P/FSI + S), and controlled irrigation + powdered/segmented straw (CI + P/CI + S). Water dynamics and changes in nitrogen and phosphorus and DOC in surface water and seepage water were monitored. The results showed that controlled irrigation significantly reduced surface drainage, seepage, and TN and TP runoff loss and leaching flux by reducing irrigation frequency and water input; powdered straw was more conducive than segmented straw to enhancing nutrient retention and reducing mobile nitrogen and phosphorus concentrations, and its synergistic emission reduction effect with controlled irrigation was more prominent. DOC was extremely significantly positively correlated with NO 3 −-N and TP leaching fluxes, and was an important carrier for the synergistic migration of carbon, nitrogen, and phosphorus. CI + P had the best comprehensive emission reduction effect, and it achieved optimal emission reduction through the synergistic pathway of water regulation–nutrient retention–DOC mediation, and can serve as a preferred integrated strategy for water conservation, nutrient loss mitigation, and straw resource utilization in paddy fields.