Optimizing fertilizer application and straw return to fields to minimize nitrogen and phosphorus runoff losses in double-rice cropping systems

Runoff losses of nitrogen (N) and phosphorus (P) from paddy fields pose considerable eutrophication risks to surrounding water and raise non-point source pollution. Unknown of long-term patterns of losses and mechanisms driving runoff N and P in double-rice cropping systems limited the development of effective mitigation strategies. In this study, typical subtropical double-cropped paddy ecosystems were selected for observation, and total nitrogen (TN) and total phosphorus (TP) concentrations, TN and TP loss loads, grain yields, and soil fertility under no fertilization (TR1), conventional fertilization (TR2, with chemical fertilizer inputs of 150/180 kg N ha−1 and 90/90 kg P2O5 ha−1 for the early/late rice seasons), optimized fertilization (TR3, 105/135 kg N ha−1 and 67.5/36 kg P2O5 ha−1), reduced N and P fertilization (TR4, 52.5/67.5 kg N ha−1 and 33.8/18 kg P2O5 ha−1), increased N and P fertilization (TR5, 180/210 kg N ha−1 and 112.5/112.5 kg P2O5 ha−1), and straw return (TR6, with chemical fertilizer inputs of 78.9/82.8 kg N ha−1 and 60.6/22.2 kg P2O5 ha−1, and 2250/4500 kg ha−1 rice straw returning to fields to maintain equivalent N and P inputs as TR3) were continuously monitored for seven years. The results revealed that (1) the 0–20 days after fertilization was a high-risk period, contributing 41.7–56.6 % of annual TN and 20.2–25.8 % of TP loss loads. (2) TN loss predominantly occurred during the early rice season (58.3–61.3 %), while TP loss is evenly distributed across the winter fallow and two rice seasons. (3) Optimized fertilization significantly decreased TN and TP losses by 11.7 % and 13.7 % compared with conventional fertilization (P < 0.05), with marginal yield reduction (6.03–6.18 %). Straw return further diminished TN and TP losses by 20.1 % and 11.5 %, and enhanced soil organic matter and TN contents by 6.34 % and 4.57 %, respectively. (4) Runoff TN and TP concentrations increased sharply when soil TN and TP exceeded 1.99 and 1.02 g kg−1, respectively. (5) Fertilizer rate primarily impacted TN loss, while soil TP content strongly correlated with TP loss, with runoff volume affecting both. This study emphasizes that P loss in the winter fallow season should not be overlooked, and that combining optimized fertilization with straw return is an eco-friendly, cost-effective measure that lowers N and P losses without crop yields reduction and soil fertility degradation. Understanding N and P losses patterns in subtropical double-cropping paddy fields with various agricultural management measures aids in assessing runoff water reuse potential, to achieve a virtuous cycle of water and nutrients.