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

Mixed sowing of Chinese milk vetch and rapeseed in winter is a unique green manure farming mode in southern China, which has the potential to replace or partially replace nitrogen fertilizer. However, it is not clear whether there is a correlation between rice yield and soil environmental factors under green manure mixed sowing. This study was conducted in 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). The rice yield, soil environmental factors and the correlation between rice yield and soil environmental factors under green manure mixing combined with nitrogen fertilizer reduction were compared and analyzed. The main results were as follows: 1) The 20% reduction in nitrogen fertilizer application of double-cropping rice under mixed sowing and returning of Chinese milk vetch and rapeseed could guarantee stable and increased rice yield, in which the effect of increasing and stabilizing rice yield was higher in the early rice season than in the late rice season. In terms of the total yield of both seasons, the total rice yield of both seasons of N2MR treatment was the highest in both years, significantly higher than that of N4MR and CK treatments in 2022 (P<0.05) by 6.80% and 24.18%, respectively, and considerably higher than that of CK treatment by 50.22% in 2023 (P<0.05). 2) Multiple t-test analysis of soil nutrient contents found that mixed sowing of Chinese milk vetch and rapeseed combined with different levels of nitrogen fertilizer reduction would affect soil ammonium nitrogen, organic matter and available potassium contents to various degrees, in which the organic matter and available potassium contents of each treatment was significantly increased 33.18% and 285.00% (P<0.05), but the ammonium nitrogen content was significantly reduced 73.56% (P<0.05). 3) Soil pH, nitrate nitrogen and available potassium content were the main factors affecting rice yield. There were significant correlations between rice yield and 1000-grain weight, pH and nitrate nitrogen content (P<0.05), among which there were significant positive correlations with 1000-grain weight and nitrate nitrogen content, significant negative correlation with pH and extremely significant negative correlation with available potassium content. Overall, the 3/4 Chinese milk vetch × 1/4 rape mixing combined with 20% reduction in nitrogen fertilizer application was a green manure farming mode suitable for spreading in southern rice areas, in which soil pH, nitrate nitrogen and available potassium contents are the main environmental factors.