Study on the relationship between rice yield and soil nutrients under mixed green manure sowing combined with nitrogen fertilizer reduction in paddy fields

The mixed sowing of Chinese milk vetch and rapeseed during winter is a unique green manure farming mode in southern China that has the potential to replace or partially replace chemical nitrogen fertilizers. However, it is unclear whether a correlation exists between rice yield and soil environmental factors under mixed green manure sowing. This study was conducted on the 3/4 Chinese milk vetch (seed quantity 17.5 kg·hm⁻²) × 1/4 rapeseed (seed quantity 2.5 kg·hm⁻²) farming mode, and five nitrogen application treatments were set as below: 1) double-cropping rice without nitrogen fertilizer application (CK), 2) double-cropping rice with conventional nitrogen fertilizer application (150 kg·hm⁻², N1MR), 3) double-cropping rice with a 20% reduction in nitrogen fertilizer application (120 kg·hm⁻², N2MR), 4) double-cropping rice with a 40% reduction in nitrogen fertilizer application (90 kg·hm⁻², N3MR), and 5) double-cropping rice with a 60% reduction in nitrogen fertilizer application (60 kg·hm⁻², N4MR). Rice yield and soil environmental factors, and the correlations between them under mixed green manure sowing combined with reduced nitrogen fertilizer application were compared and analyzed. The primary results were as follows. 1) The 20% reduction in nitrogen fertilizer application of double-cropping rice under mixed sowing and return of Chinese milk vetch and rapeseed guaranteed stable and increased rice yield. Total rice yield of both seasons under the N2MR treatment was the highest in both years, significantly higher than that under the N4MR and CK treatments in 2022 (P<0.05) by 6.80 and 24.18%, respectively, and considerably higher than that under the CK treatment by 50.22% in 2023 (P<0.05). 2) Multiple t-test analysis of the soil nutrient contents found that mixed sowing of Chinese milk vetch and rapeseed combined with different nitrogen fertilizer reduction levels affected the soil ammonium nitrogen, organic matter, and available potassium contents to various degrees. The organic matter and available potassium contents of each treatment in 2023 were significantly increased by 33.18% and 285.00% (P<0.05), respectively, compared to that in 2022; however, the ammonium nitrogen content was significantly reduced by 73.56% (P<0.05). 3) Soil pH, nitrate nitrogen, and available potassium contents were the primary factors affecting rice yield. Significant positive correlations between the rice yield with 1000-grain weight, and nitrate nitrogen content (P<0.05) were observed, a significantly negative correlation between the rice yield and pH (P<0.05) was observed, and an extremely significant negative correlation between the rice yield and available potassium content (P<0.01) was observed. Overall, mixed 3/4 Chinese milk vetch × 1/4 rapeseed sowing combined with a 20% reduction in nitrogen fertilizer application is a green manure farming mode suitable for expanding in southern rice growing regions, in which soil pH, nitrate nitrogen, and available potassium contents are the primary environmental factors.