| Winter planting of green manure (GM) in rice fields in the middle and lower reaches of the Yangtze River is a crucial measure to reduce chemical nitrogen (N) fertilizer use, improve soil fertility, and maintain rice yields. While this practice has been widely tested, the impact of GM on N runoff loss from rice fields has not been systematically studied. In this study, we used a multi-point network observation method to investigate the effect of GM combined with reduced N fertilizer on N runoff loss in paddy fields. This research aims to provide a theoretical basis for accurately evaluating the environmental benefits of GM in paddy fields. Field experiments were conducted at four experimental stations in Nanjing (Jiangsu Province), Jingzhou (Hubei Province), Chizhou (Anhui Province), and Gaoan (Jiangxi Province). The experiment consistently included four treatments at each site: winter fallow with conventional fertilization (FR), winter fallow with conventional fertilization and straw return (FRS), winter milk vetch and rice with 40% N reduction (MR), winter milk vetch and rice with 40% N reduction and straw return (MRS). The N concentration in ponding water of the paddy fields was measured multiple times on the day of fertilization (before fertilization) and within 10 days after fertilization. Combined the rainfall-water table measurement with the water balance model of paddy fields, we analyzed the effects of GM combined with reduced N fertilizer on N runoff loss and its morphological characteristics in the middle and lower reaches of the Yangtze River. The results showed that the peak and average total N (TN) concentrations under GM treatments(MR、MRS) were reduced by 11.1%~57.9%(26.9%) and 17.1%~27.3%(22.3%), respectively, compared to conventional treatments(FR、FRS) The model-simulated rice field ponding water depth and runoff consistency indices ranged from 0.738~0.985 and model simulation efficiencies ranged from 0.737~0.986, indicating the model's reliability in quantifying N loss from different treatments. The TN runoff loss from the FR, FRS, MR, and MRS treatments ranged from 19.94~25.22 kg hm-2, 17.71~22.36 kg hm-2, 14.60~17.92 kg hm-2, and 13.72~16.73 kg hm-2, respectively. Compared to conventional treatments, GM treatments reduced TN runoff loss by 3.5~10.61 kg hm-2 (22.5%~42.1%). The most significant reduction was observed in Chizhou, Anhui Province, where GM had a more pronounced effect on N runoff loss before fertilization. GM mainly reduced the runoff loss of ammonium N (NH??-N) and dissolved organic N (DON), with reductions ranging from 0.2~9.66 kg/hm2 (7.7%~43.4%) and 0.06~3.49 kg hm-2 (3.7%~39.5%), respectively, indicating that GM effectively reduced N runoff loss primarily by decreasing NH??-N and DON. In conclusion, GM combined with a 40% reduction in chemical fertilizers can significantly reduce the content of NH??-N and DON in paddy fields, thereby decreasing the amount of total N (TN) loss and the risk of ponding water pollution. This green production model is worth promoting in the middle and lower reaches of the Yangtze River. |
