4 Discussion

4.1. Effects of straw return on soil carbon and nitrogen fractions

Straw return increases soil carbon and nitrogen pools and improves the soil environment (Yu et al., 2020; Ren et al., 2023b). In the present study, straw return to the field increased the stable carbon and nitrogen pools (SOC, TN, and HFON) and active carbon and nitrogen pools (DOC, MBC, DON, MBN, LFOC, and LFON) in the 0–20 cm soil layer (Tables 1 and 2). Compared with those in the control, SOC and TN increased by 6.4%–39.8% and 2.9%–15.1 %, respectively, with increases in the amount of straw returned to the field. This was because straw applied to the soil as exogenous organic matter increases the soil SOC and TN storage; thus, crop residues are retained in the soil as derived carbon and nitrogen pools (Zhao et al., 2019), consistent with the findings by Dai et al. (2021).
The increase in soil active carbon and nitrogen pools as a result of straw return is a consequence of the straw degradation process releasing large amounts of nutrients and soluble substances and being microbially mediated and stimulating microbial growth (Zhang et al., 2020). Zhao et al. (2020a) demonstrated that straw is more readily available to microorganisms for utilisation and decomposition than biochar, increasing the unstable stable soil organic carbon pools (DOC, MBC, and labile carbon). Straw application induces the microbial turnover of dead material in the soil, increasing DON and MBN levels (Liu et al., 2021; Hassan, 2013), which is consistent with the results obtained in the present study.
Light (LFOC and LFON) and heavy (HFOC and HFON) fraction organic carbons and nitrogen are determined considering the carbon and nitrogen fractions after physical density fractionation, which have different compositions and turnover mechanisms (Crow et al., 2007). The results of the present study revealed that the LFOC content increased with an increase in the amount of straw returned, whereas the HFOC content decreased, which is inconsistent with the results of Hao (2023). This may be because the light fraction is mainly derived from undecomposed plant residues, and straw inputs increase the LFOC and LFON contents along with the increase in crop residues in the soil (Wang et al., 2021b; Yan et al., 2020). Although soil HFOC, as a stable carbon pool component, is usually adsorbed to various soil compounds or minerals, straw inputs stimulated HFOC mineralisation; however, its mineralisation rate was much lower than its formation rate; thus, HFOC decreased with increases in the amount of straw returned in the present study (Chen et al., 2019). Hu et al. (2018) revealed that the carbon dynamics of difficult-to-degrade organic carbon are correlated with the nature of organic matter. Organic matter with a higher content of lignin and polyphenols is more stable in carbon turnover. HFOC and HFON contents are correlated with the carbon and nitrogen turnover processes and they exhibited different variations depending on the microbial load, soil depth, and climatic conditions, which might explain why soil-heavy carbon and nitrogen exhibited different responses to straw return in the present study (Li et al., 2022b).