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.