Microbial necromass outweighs lignin in contributing to soil recalcitrant carbon accumulation during vegetation restoration in a subtropical karst region

Vegetation restoration is an important global strategy to enhance ecosystem resistance and promote soil organic carbon (SOC) sequestration. However, under this strategy, the mechanism underlying soil recalcitrant organic carbon (ROC) accumulation, particularly the relative contributions of plant‐ versus microbial‐derived carbon, remains insufficiently explored.

Using a space‐for‐time substitution approach, we selected three land‐use types (cropland, shrubland and secondary forest) to represent a vegetation restoration chronosequence in karst regions of southwest China. We quantified soil lignin and microbial necromass carbon (MNC) by employing biomarker technologies (lignin phenols and amino sugars) and combined these measurements with ROC to assess relative contributions of plant versus microbial residues to stable SOC accumulation.

Across the three land‐use types, ROC accounted for more than 60% of total SOC pool. Vegetation restoration significantly increased soil ROC content by 193%, indicating that this strategy improved SOC stability and benefited long‐term preservation of soil carbon. While both lignin and MNC contents significantly increased during vegetation restoration, MNC was identified as the key predictor of soil ROC accumulation. Soil MNC accumulation in shrublands and secondary forests was strongly associated with increased soil carbon and nitrogen availability and mineral protection (exchangeable calcium+magnesium and free and short‐range ordered iron+aluminium oxides). Concurrently, higher substrate availability and mineral protection also facilitated soil lignin preservation. Importantly, fungal and bacterial necromass contributed almost equally to ROC accumulation. This phenomenon likely stemmed from the improvement in soil pH during vegetation restoration, which alleviated acid stress on bacterial proliferation.

Synthesis and applications : Our study expands the traditional lignin‐centric theoretical framework of carbon accumulation and emphasizes that the unique geochemical environment of karst regions facilitates a synchronous accumulation of bacterial and fungal necromass, jointly promoting stable SOC sequestration. We therefore propose that restoration strategies for karst‐degraded ecosystems should focus on fostering active soil microbial communities while introducing plant species that facilitate soil calcium enrichment to enhance microbial carbon sequestration potential and promote the long‐term preservation of SOC.