4.2. Effects of crop rotation and fertilization on the changes in soil properties
Soil pH plays an important role in crop production, nutrient chemistry, soil organisms, and shaping plant community composition (Sun et al., 2020). Both CU and BBU fertilizations resulted in a significant short-term increase in soil pH under RG and RR rotations (Figs. 4a, S1a), which was caused by the production of OH from the hydrolysis of urea and the limitation of nitrification under paddy soil (Curtin, Peterson, Qiu, & Fraser, 2020). However, fertilization did not significantly increase the soil pH under RW rotation. This might be due to the strong nitrification process in the aerobic environment during the wheat cropping season in which more H+ was generated to counteract the increase in pH from paddy rice season. SOC is critical for soil structure and workability, the ability of soils to store nutrients and water, and for the global C cycle (Sun et al., 2020). In this study, almost all treatments significantly enhanced the SOC, which was dominated by the retention of straw residues (Y. Zhao et al., 2018). The previous study indicated that SOC stock changes in Chinese croplands were positively correlated with N fertilizer input and crop residue C input (Y. Zhao et al., 2018). BBU enhanced the plant growth leading to more inputs of C from straw and roots, which presumably contributed to the higher SOC in BBU fertilizations (Pampolino, Laureles, Gines, & Buresh, 2008). Generally, crop rotation had a direct impact on the dynamic of SOC by altering the input C to the soil and the soil microbial activity (D’Acunto, Andrade, Poggio, & Semmartin, 2018; Osanai, Knox, Nachimuthu, & Wilson, 2021). However, our result showed that there was no significant difference in the changes in SOC among the three cropping rotations, which might be due to the short experiment period. Similarly, soil TN contents were increased after crop rotation and fertilization, which presumably was attributed to the straw turnover. The changes in soil TN contents under RG and RR rotations were higher than that under RW rotation. This difference was caused by the additional N fixed by Chinese milk vetch during RG rotation and a greater amount of residual including rape straw and grain during RR rotation (Alam, Bell, Haque, Islam, & Kader, 2020). Soil POXC, which stands for labile SOC fraction, is useful as an indicator for assessing soil health and C sequestration potential (Lucas & Weil, 2021). Both CU and BBU fertilizations significantly increased the soil POXC contents compared with WN treatment except for RG rotation. The change in soil POXC content was positively correlated with the changes in SOC and TN contents (Fig. S3). Fertilization enhanced the soil C input by improving crop growth and promoted the decomposition of stubble and straw residuals. The decreases of soil AP and available Mg were mainly caused by crop uptake and the increase of available Zn mostly due to the application of Zn fertilizer. Other available trace elements such as Mn, Fe, and Cu had a positive correlation with SOC and TN, indicating that the availability of these elements was related to SOC and TN contents.