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).