Substituting effect of crop residues for potassium fertilizer in rice-rapeseed rotation system

【Objectives】In order to provide a scientific basis for crop residue management and potassium(K)fertilization, effects of straw returning to field and recommended K fertilizer rate on yields of rice and winter oilseed rape, potassium accumulation amounts of shoots, K use efficiencies, and soil K contents in rice-rapeseed system were studied.【Methods】A long-term(2011-2014)field experiment was carried out in a selected high K level soil in Jianghan plain. There were seven treatments with three replications,1)CK(-K); 2)+K; 3)+S; 4)S+1/4K; 5)S+1/2K; 6)S+3/4K, and 7)S+K, where K and S denoted K fertilizer and crop residues, respectively. The K2O application amount was 90 kg/hm2. 【Results】 1)The application of K fertilizer and straw returning to field could increase the crop yields and K uptakes of shoots at varying degrees for the six growth seasons. Compared with the CK treatment(-K), the straw incorporation with the K fertilizer treatment(S+K)has the best effect, and the yield increased rates are 12.8% and 19.1% for rice and rapeseed and the K uptake increase rates are 35.7% and 79.3%, respectively. 2)The straw returning to field has no significant impacts on the KfREs(K fertilizer recovery efficiency)of rice and winter oilseed rape, but observably decreases the KREs(K recovery efficiency)under the recommended K fertilizer rate. Meanwhile, the straw returning to field also decreases the KfAE(K fertilizer agronomic efficiency)of rice, but enhances the KfAE of winter oilseed rape. The KAE(K agronomic efficiency of straw K with chemical K)of rice is reduced, but that of winter oilseed rape is equivalent compared with the KfAE without the straw incorporation. 3)The correlation analysis among the yield increase rates, K accumulation amounts and K-fertilizer amounts indicates that the recommended rates of K fertilizer are higher than the actual demand for rice and rape growth in consideration of the continuous straw returning to field. Using the fertilizer efficiency model and considering the straw returning to field and nutrient balance, the optimum annual average amounts of K fertilizer(K2O)are 52.0 and 61.9 kg/hm2 for rice and winter oilseed rape, which are 42.2% and 31.2% less than the recommended amounts in this region.【Conclusions】 Under the high soil K level condition, we conclude that the straw returning to field can not only replace partial chemical potassium and get higher yield, but also improve soil available K, maintain the nutrient balance of soil system and assure the virtuous circle of straw potassium resources in rice-rapeseed rotation system.