Deep smashing and uniform incorporation of straw prevent straw burning and improve the return of nutrients to soil

Objectives To increase the practicality of returning straw to soil, the effects of smashing degree and returning method on reducing the straw burning rate and nutrient loss were explored.

Methods Field and simulated experiments of returning rice straw to soil were carried out in Ningxiang, Hunan. The treatments included returning straw to the field in the belt without smashing (T1), returning moderately smashed straw to the field in the belt (T2), and returning deeply smashed straw to the field evenly (T3). The length of the straw returned in T1, T2, and T3 were 39.0 cm, 14.4 cm, and 5.3 cm, respectively. The corresponding uniformity coefficient was 37.7%, 45.4%, and 87.4%, respectively. The treatments in the simulated experiment had the same straw length and returning patterns as T1, T2, and T3 in the field experiment, but recorded as M1, M2, and M3, respectively, and an extra treatment of returning deeply smashed straw (5.3 cm) to the field in the belt (M4). After harvesting early rice, the straw was burned and returned to the field according to the designated treatments. The burning rate of straw and the nutrient content of straw ash were analyzed to calculate nutrient loss and loss rate. The contents of N, P, K, and organic matter in the soil were determined one day after burning and at the maturity of late rice.

Results Under simulated experiment, when the degree of straw smashing reached M4 in the belt, the burning rate was significantly lower than those in M1 and M2, the N content of M4 treatment in the ash after burning (P < 0.05) increased by 76.5% and 73.5% compared with those of M1 and M2 treatments, respectively. Phosphorus content (P < 0.05) decreased by 42.0% and 39.1%; potassium content decreased by 11.1% and 10.7%; and carbon content increased by 37.3% and 36.0%. After burning the straw (excluding rice stubble) on the experimental field, the average losses of C, N, P₂O₅ and K₂O in T2 were 1101.3, 34.1, 2.7, and 13.9 kg/hm²; the average loss rates were 91.1%, 89.8%, 22.4%, and 16.1%, respectively. T3 straws could not be burnt; thus, all the nutrients in the treatment was returned to soil. The average nutrient assimilation of N, P₂O₅ and K₂O in T1 and T2 treatments decreased by 18.8, 3.2, and 8.2 percentage point, respectively, compared to before burning. The nutrient return as a percentage of fertilizer use of N, P₂O₅, and K₂O in T3 increased by 18.7, 3.5, and 9.1 percentage point, respectively, compared with the average value recorded in T1 and T2. After straw burning, soil available N, P, and K in T3 were (P < 0.05) higher than those in T1 and T2 at the maturity stage of late rice. There was no significant (P > 0.05) difference in the active organic matter among the three treatments.

Conclusions Deep smashing and uniform throwing of straw could prevent straw burning, promote the complete return of straw nutrients to soil, reduce the loss of nutrients caused by straw burning, and enhance sustainable straw utilization in double-cropping paddy systems.