Effects of straw return on soil nutrients and crop yield in rice-rapeseed rotation

A seven-year field study from 2016 to 2023 was conducted on typical paddy soil in Jiujing city, Jiangxi province, to evaluate the effects of different straw returning methods on soil nutrients and crop yield in a rice-rapeseed rotation system in the middle-lowerYangtze Plain of China. The study included four treatments: no straw returning (CK), rice straw returning during the rapeseed season (T1), rapeseed straw returning during the rice season (T2), and straw returning during both the rape and rice seasons (T3). Measurements of shoot and root dry matter, crop yield, and soil nutrient content were taken at key growth stages during the 2022 rice season and the 2022-2023 rape season. The results indicated that, compared with CK, straw returning significantly increased soil pH and the contents of organic matter, and N, P, K, and other nutrients. The T3 treatment was particularly effective, significantly increasing soil pH by 9.7% and 12.9%, alkali-nitrogen by 32.9% and 25.5%, available phosphorus by 33.5% and 24.3%, available potassium by 56.3% and 58.6%, total nitrogen by 22.9% and 33.0%, and organic matter by 26.6% and 50.8% (P < 0.05) in the rice and rape seasons, respectively. Dry matter accumulation in rice initially increased and then decreased as the growth period extended. The T2 treatment significantly promoted dry matter accumulation, whereas the T3 treatment inhibited it during the early stages of rice growth. Compared with straw removal, straw returning increased rice grain yield, with the largest increase of 13.9% observed in the T2 treatment (P < 0.05). In rapeseed, dry matter accumulation continued to increase with the growth period, and the T2 treatment most significantly promoted this accumulation. Compared with CK, the T2 treatment resulted in the largest increase in rapeseed grain yield, by 20.0% (P < 0.05). Correlation analysis showed that available potassium was significantly positively correlated with the rice seed setting rate (r = 0.84**, P < 0.01). Total nitrogen and available potassium were significantly positively correlated with the number of pods per plant (r = 0.705*, r = 0.623*, P < 0.05) and rape yield (r = 0.623*, r = 0.690*, P < 0.05). These findings suggest that the major reason for the yield increase may be the elevated content of soil total nitrogen and available potassium due to straw returning. Considering soil nutrients, dry matter accumulation, and crop yield, rape straw returning during the rice season (T2) performed best and could be recommended for promotion and application in the middle and lower reaches of the Yangtze River.