Soil_Metagenome_2026
[Background and Aims] Enhancing soil organic carbon (SOC) sequestration in degraded lands is critical for ecosystem restoration and agricultural productivity. While straw return is a common practice, its synergy with humic acid and the underlying mechanisms remain unclear.  [Methods] A field experiment with four treatments—control (CK), humic acid alone (HA), straw alone (SA), and their combination (SHA)—was conducted on a reclaimed coal mining dump in northeast China, using soybean as a test crop. An integrated multi-omics approach was employed, combining soil fractionation, amino sugar and lignin phenol biomarkers, PLFA profiling, metagenomics, and enzyme assays. [Results] The SHA treatment increased total SOC by 22.5% relative to CK, which stemmed from both the direct contribution of humic acid and the synergistic conversion of straw-derived carbon into stable forms, which was evidenced by a disproportionate increase in microbial necromass carbon, primarily driven by fungal necromass carbon (+83.0%). Enhanced genetic potential (e.g., CAZy families GH5 and PL1) and elevated lignocellulolytic enzyme activities were revealed under the combined amendment. Partial least squares path modeling quantitatively validated the enhanced microbial functional capacity as a significant direct driver of the SOC accrual. Notably, the SHA treatment also significantly increased soybean yield by 17.6%. [Conclusion] We conclude that humic acid not only contributes directly to SOC but, critically, directs straw decomposition into a fungi-mediated microbial carbon pump. This process not only builds stable carbon pool but also revitalizes soil biological fertility. These findings provide a mechanistic basis for using the amendments to rapidly restore soil health and carbon stocks in degraded ecosystems.