Effects of combined application of straw, biochar and biogas slurry on soil characteristics and crop yield

Excessive use of chemical fertilizers has posed a serious threat to soil degradation and environmental pollution during agricultural production. It is urgent to recycle resources for high soil fertility and crop yield. This study aims to explore the effects of biochar and biogas slurry treatment on the decomposition rate of straw and the nutrient release rate after straw decomposition. A one-year rice-wheat rotation was carried out in the field. 8 treatment groups (T1-T8) were also set. A systematic investigation was made to compare the effects of the single and combined application of straw, biochar, and biogas slurry on the growth and yield, soil microbial biomass, soil nutrient content, soil pH, soil conductivity, and soil aggregates. The results showed that the decomposition rate of the straw returned was about 75%. The decomposition rate and nutrient release rate of deep straw returning in the paddy field were slightly higher than those of the shallow straw. While in the dry field, the abundance of soil microorganisms was significantly improved before and after returning to the field. The abundance of actinomycetes increased to 36.04% at the classification level suitable for the high straw decomposition. While the abundance of Proteobacteria was reduced to 10.24%, this was beneficial to balance the soil trophic flora. The dominant flora of straw decomposition was expanded to accelerate the straw decomposition. The combined application of straw, biochar, and biogas slurry significantly improved the soil quality and crop yield. Among them, the total nitrogen, phosphorus, potassium, and organic carbon content of soil increased respectively. The average content of the total organic carbon in the wheat-paddy field was higher than that in the paddy field. But the range of the paddy field was greater than that of the wheat paddy field. The biochar and biogas slurry were applied to hold the carbon and nitrogen ratio caused by pure straw returning to the paddy field. As such, the overall C/N ratio was within the range of suitable soil for cultivation. The combined application of straw, biochar, and biogas slurry increased the proportion of soil aggregates greater than 0.053 mm. The soil aggregate structure was improved by the combined application of biochar and biogas slurry, which was higher than that of straw returning. There was some increase with the increase in biogas slurry returning. At the same time, the crop yield shared an upward trend with the increase in the amount of straw returning to the field. The yield of rice and wheat in the total straw returning treatment group was higher than that in the half straw returning treatment. The biogas slurry (T8) performed the best in the total nitrogen content of biochar, and twice the amount of nitrogen. The soil fertility, the excellent physical and chemical properties of the soil were maintained to significantly improve the crop yield. The yield of rice reached 8 748.33 kg/hm², which was 17.98% higher than that of the biochar and the nitrogen return treatment group (T6). The yield of wheat was 3 182.20 kg/hm², which was 25.02% higher than that of the T6 treatment group. Moderate application of the straw, biochar, and biogas slurry can be expected to improve the soil quality of farmland for high crop yield.