Soil compaction reduces microbial necromass and alters its relationship with soil organic carbon

Soil mechanical compaction is a critical perturbation factor governing carbon (C) cycling in global agroecosystems, yet its impacts on soil microbial dynamics and underlying mechanisms remain insufficiently understood. This study employed a field-based in situ simulation experiment with three compaction gradients: no artificial compaction (C1), moderate compaction (C2), and severe compaction (C3), to investigate the responses of microbial necromass carbon (MNC), biomass, and community structure across the 0–60 cm soil profile. Compared with the C1, the C2 and C3 significantly reduced aggregate stability in the 0–40 cm soil layer. Additionally, the C3 significantly reduced soil organic carbon (SOC) content by 4.99% and increased bulk density by 10.51% in the 0–20 cm soil layer. The C3 treatment also significantly reduced fungal necromass carbon (FMNC) and bacterial necromass carbon (BMNC) estimated from amino-sugar biomarkers, as well as PLFA-derived fungal and bacterial biomass, in the 0–40 cm soil layer across different stages by an average of 16.95%, 25.00%, 49.37%, and 40.56%, respectively. Soil compaction significantly altered bacterial and fungal community compositions, with >0.25 mm aggregates (R0.25) and geometric mean diameter (GMD) identified as the strongest explanatory variables for bacterial and fungal community variation, respectively. The decrease in mean weight diameter (MWD) caused by soil compaction was indirectly associated with a reduction in FMNC and a decline in SOC content. The random forest model further indicated that FMNC exhibited the highest predictive importance (15.45%) for changes in SOC content. In summary, this study demonstrates that, under soil compaction conditions, FMNC exhibits a stronger statistical association with SOC than BMNC; these findings provide fundamental insights into understanding microbial-mediated carbon pathways in mechanically compacted agricultural soils.