Effects of cultivation modes and straw returning on soil phosphorus composition and soybean yield in the Songnen Plain of China

Northeast Plain is one of the most important soybean-producing areas in China. Phosphorus can be used to optimize the fertilization for the growth of soybean seedlings, root development, and crop resistance to natural disasters, such as drought. The depth of cultivated land can also affect the soil nutrient content. Compared with no tillage treatment, shallow and deep tillage can improve the soil structure and environment of the tillage layer for the soil nutrient content. A combination of exogenous media and cultivation has been widely used to improve the soil structure and physicochemical properties in recent years. Therefore, it is of great significance to understand the variations in the soil phosphorus that are caused by exogenous media and cultivation modes in black soil areas. Previous research of phosphorus form was focused mainly on individual fertilization or the regulation of individual cultivation modes. It is very necessary to explore the interaction and impact of maize straw returning to the field, and then apply the different cultivation modes to the soil phosphorus. In this study, a series of field experiments were carried out to monitor the effects of different tillage modes and straw returning on the soil phosphorus components in various stages. The results indicate that: 1) Straw returning significantly enhanced the stability of the soil environment and the fixation capacity of soil phosphorus. The total phosphorus (TP) content of the soil increased by 31% after deep tillage (DP) combined with 9750kg/hm² of straw returning to the field. (2) Straw also improved the soil environment and phosphatase content. Compared with the shallow tillage (SP), the DP combined with 9750 kg/hm² of straw increased the content of acid and alkaline phosphatase to 12% and 32.5%, respectively. The application of 9750 kg/hm² of straw for the DP resulted in the lowest content of soil phytase in the critical period, when the soybean plants needed phosphorus; Compared with the CK, the phytase content decreased by 11.3%-19.4%. (3) The Langmuir model better represented the phosphorus adsorption data, compared with the Freundlich. The maximum adsorption of phosphorus was evaluated as well. The maximum phosphorus adsorption capacity, phosphorus adsorption affinity constant, and maximum buffer capacity of straw treatment were 2.4%-8.3%, 8.3%-13.9%, and 2.2%-26.3% lower than those of CK treatment, respectively. (4) Both straw application and DP treatment improved the soil phosphorus availability. According to Hedley's phosphorus grading, the DP combined with straw returning promoted the soil phosphorus components under different environments. The content of unstable phosphorus in the soil was always higher than that of the CK group. The application of straw significantly increased the phosphorus content of each component in the soil, indicating the greatest impact on the unstable phosphorus component. The regulation model of DP combined with 9750 kg/hm² straw returning to the field improved the effective phosphorus content in the soil, when applying 9750 kg/hm² maize straw during deep cultivation. The highest soybean yield reached 1425kg/hm², indicating a better improvement in the soybean yield. The finding can provide a scientific basis to improve the utilization efficiency of soil phosphorus. Strong reference and guidance can also be offered to maintain the soil fertility and farmland soil cultivation in black soil areas.