Vertical distribution of soil organic carbon fractions under different residue management strategies

Returning plant residue to farmland maintains or enhances the fertility and the investigation of how residue management strategies affect soil organic carbon (SOC) and labile organic carbon (LOC) fractions is crucial for addressing concerns related to agricultural sustainability. However, knowledge gaps remain about how these C fractions change in deep soil (>40 cm) under different strategies. A 10‐year field experiment (2012–2021) in Northeast China compared three strategies: residue covered on the surface (RC), residue incorporated into 0–20 cm soil (RI), and residue removed (CK). In 2021, the vertical distribution (0–90 cm) of SOC and five LOC fractions (microbial biomass carbon [MBC], dissolved organic carbon [DOC], particulate organic carbon [POC], easily oxidizable carbon [EOC], and light fraction organic carbon [LFOC]) was analyzed. The results showed that SOC and LOC fractions generally decreased with increasing soil depth, except for the RI treatment in the 0–20 cm layer. The largest treatment differences occurred in the 0–5 cm layer, where RC had significantly higher SOC and LOC concentrations than RI. LOC fractions exhibited similar sensitivity trends in RC (14.82%–105.48%) and RI (13.39%–58.54%) relative to CK. RI resulted in the highest SOC and LOC contents in the 5–20 cm layer, while RC exceeded RI and CK below 20 cm. Below 40 cm, the highest SOC content was mostly observed in RC, but no significant differences among the three treatments were detected in the 20–40 cm layer. POC proportions in the 0–90 cm profile ranged from 30.06% to 5.34% (RC), 26.60% to 4.76% (RI), and 21.75% to 4.78% (CK). Pearson correlation analysis and principal component analysis revealed that SOC changes were primarily driven by POC, EOC, and LFOC, while MBC and DOC played a secondary role due to their high lability. This study highlights that overlooking deep subsoil carbon dynamics masks key opportunities for SOC sequestration and may lead to incomplete conclusions about the impact of residue management on long‐term carbon storage in Mollisols of Northeast China.