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 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, with double-cropping rice without nitrogen fertilizer application (CK), conventional nitrogen fertilizer application (150 kg·hm⁻², N1MR), a 20% reduction in nitrogen fertilizer application (120 kg·hm⁻², N2MR), a 40% reduction in nitrogen fertilizer application (90 kg·hm⁻², N3MR), and a 60% reduction in nitrogen fertilizer application (60 kg·hm⁻², N4MR). Rice yield and soil environmental factors, and the correlation between them under mixed green manure sowing combined with reduced nitrogen fertilizer 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, and the effect of increasing and stabilizing rice yield was higher during the early rice season than during the late rice season. Regarding the total yield of both seasons, the total rice yield 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 content 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 content to various degrees, in which the organic matter and available potassium content of each treatment were significantly increased by 33.18 and 285.00% (P<0.05), respectively, however, the ammonium nitrogen content was significantly reduced by 73.56% (P<0.05). 3) Soil pH, nitrate nitrogen, and available potassium content were the primary factors affecting rice yield. Significant correlations between the rice yield and 1000-grain weight, pH, and nitrate-nitrogen content (P<0.05) were observed, with significantly positive correlations with 1000-grain weight and nitrate-nitrogen content, a significantly negative correlation with pH, and an extremely significant negative correlation with available potassium content. 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 content are the primary environmental factors.