Deciphering moisture-driven divergence in soil carbon limitation via microbial necromass dynamics across arid versus humid alpine grassland restoration chronosequences

Abstract

Grassland restoration enhances soil organic carbon (SOC) sequestration, a process crucial for global C cycling and climate regulation. However, the influence of aridity gradients on the successional trajectories of SOC fractions during ecological restoration on the Qinghai-Xizang Plateau remains unclear. To address this, we employed a multi-biomarker approach, utilizing amino sugars, lignin phenols, and plant lipids, to quantify microbial necromass and plant-derived C dynamics across arid-to-humid alpine grassland restoration chronosequences. Our findings revealed moisture-dependent C storage dynamics along the restoration chronosequences. In humid regions, SOC exhibited biphasic dynamics: early-stage surges occurred in both plant-derived C (an 872.9% increase, from long-chain fatty acids, suberin, and cutin) and microbial C (an 268.3% increase), followed by late-stage decline. This decline included a 48.7% decrease in subsoil, while overall plant- and microbial-derived C decreased by 67.4% and 59.7%, respectively. In contrast, arid regions maintained stable SOC, which was predominantly composed of microbial necromass (ranging from 33.9 to 52.2% of total SOC). Plant inputs in these regions were primarily limited to lignin phenol enrichment, showing increases of 235.8% in topsoil and 305.7% in subsoil, respectively, compared to unrestored sites. Fungal-derived C was the predominant contributor to microbial SOC formation, with its levels being 2.72 to 8.89 times higher than those of bacteria-derived C. Arid regions exhibited microbial-dominated SOC regulation, whereas humid regions showed stronger plant-soil feedback mediated by pH and C/N ratios. Our findings underscore the importance of microbial-driven C storage in arid regions and highlight the necessity of optimizing soil physicochemical properties in humid regions to enhance C sequestration on the Qinghai-Xizang Plateau.