Vegetation restoration shapes soil organic matter chemistry and microbial processes

Abstract

Vegetation restoration is a critical process for the recovery of ecosystem functioning in red soil (Ultisol) erosion areas, yet the mechanisms underlying its effects on soil organic carbon (SOC) stability and nutrient cycling remain poorly understood. By integrating ¹³C-NMR spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial necromass quantification, this study elucidates the dynamic coupling mechanisms among SOC composition, dissolved organic matter (DOM) molecular signatures, and microbial necromass carbon (MNC) across vegetation types and soil depths in red soil erosion areas of southern China. Vegetation restoration significantly decreased mean soil pH from 5.02 to 4.54 in topsoil (0–10 cm) (P < 0.05). The dissolved organic carbon concentration in topsoil increased from 75.1 mg kg⁻¹ in the degraded site (CK) to 217 mg kg⁻¹ in broadleaf forest over 80 years old (BF) (P < 0.05) and that of easily oxidizable carbon concentrations increased from 0.48 to 2.77 g kg⁻¹ (P < 0.05). ¹³C-NMR analysis revealed a decline in the relative abundance of alkyl C in SOC with vegetation restoration, accompanied by an increase in O-alkyl C. DOM molecular characterization indicated that vegetation restoration promoted the accumulation of oxidized compounds (lignin- and tannin-like molecules) and reduced the abundance of reduced-state compounds (lipid- and protein/amino sugar-derived molecules). Thermodynamic analysis revealed that vegetation restoration decreased energy availability of DOM molecules. Vegetation restoration significantly enhanced MNC accumulation over 10-fold in both 0–10 cm and 20–40 cm soil layers compared to severely degraded zones (P < 0.05). Vegetation restoration significantly increased bacterial Shannon diversity and drove a bacterial community-level transition toward K-strategists, evidenced by a significant shift in the K:r ratio from 0.29 to 9.20 in 0–10 cm soil layer (P < 0.05). Moreover, the abundance of saprotroph-symbiotroph functional guilds in fungal communities increased with vegetation restoration. Path analysis confirmed that soil DOM parameters regulate microbial necromass accumulation and multi-nutrient cycling potential by mediating DOM ΔG⁰_Cox and functional diversity. This study elucidates how vegetation restoration enhances SOC stability and ecological function recovery in red soils by reshaping DOM molecular signatures and microbial life-history strategies. These findings provide a framework for understanding and promoting carbon sequestration mechanisms in erosion-prone ecosystems, emphasizing the critical role of fungal necromass and DOM thermodynamics in long-term soil C stabilization.