Micro–Nano Aerated Irrigation Mitigates the Accumulation of N, P, and COD Concentrations: Effects of Straw Return and Aerated Irrigation on Water Quality Dynamics under the Soaking Period of Paddy Fields

Straw returning to the field enhances crop yield and cultivated land quality while presenting risks of water quality degradation, particularly during the soaking period. In this study, a barrel planting experiment was used, three treatments (CK: no straw returning, ST: straw returning, SO: straw returning + micro–nano aeration irrigation) were set up to study the dynamic changes of surface water, soil solution (0–10, 10–20, and 20–30 cm), and key water quality indices [total nitrogen (TN), ammonia nitrogen ( NH 4 + - N ), nitrate nitrogen ( NO 3 − - N ), total phosphorus (TP), chemical oxygen demand (COD), dissolved oxygen (DO), pH] in the rice soaking period. The results showed that (1) straw return significantly increased the contents of TN, COD, and TP in paddy field water. Micro–nano aerated irrigation effectively improved water quality after straw return, reducing the average concentrations of TN, TP, and COD by 5.14%–13.54%, 2.08%–37.14%, and 6.07%–43.44%, respectively. (2) During the whole soaking period, there was a significant positive correlation between TN, NH 4 + - N and NO 3 − - N ( p < 0.01 ). However, DO showed a significant negative correlation with COD, TN, and TP only in the middle of immersion, indicating that the increase of pollutant concentration in this stage was accompanied by a continuous oxygen consumption process. (3) The structural equation model showed that DO was the key driving factor for regulating water quality. DO had a strong negative direct effect on COD ( λ = − 0.713 , p < 0.001 ) and TN ( λ = − 0.051 , p < 0.001 ) and indirectly inhibited COD accumulation by regulating the morphological transformation between NH 4 + - N and NO 3 − - N . This study uncovered the underlying mechanisms of straw incorporation and aerated irrigation on water quality during the soaking period, identified critical determinants driving water quality variations, and provided a more comprehensive and in-depth theoretical foundation for the precise and effective mitigation of water quality degradation caused by agricultural straw return.