Revegetation drives the accrual and stabilization of organic carbon in biocrusts and subsoils in the Tengger Desert, north China

Revegetation serves as a critical strategy for desertification reversal and ecosystem restoration in global drylands, which facilitates the development of biological soil crusts (biocrusts) thereby significantly influencing C accumulation and persistence in biocrusts and subsoils. However, the quantitative aspects of SOC accrual and its stabilization mechanisms during biocrust development process following revegetation remain poorly understood. In this study, we investigated the temporal evolution of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions in biocrusts and the underlying soil layers (0–2 and 2–5 cm) along a 68-year revegetation chronosequence in the Tengger Desert. We aimed to elucidate the mechanisms governing the accumulation and stabilization of SOC in biocrust profiles in these fragile ecosystems by analyzing the relationships between SOC fractions and biotic and abiotic factors. Results revealed substantial SOC accumulation across all layers, with biocrusts showing a 91.77-fold increase over the study period compared with the 38.33– and 7.07-fold increases in the 0–2 and 2–5 cm layers, respectively. MAOC concentrations increased significantly from 1.14 to 5.65 g kg⁻¹ in biocrusts, 0.001 to 1.49 g kg⁻¹ in 0–2 cm soil, and 0.002 to 0.33 g kg⁻¹ in 2–5 cm soil. Consequently, the MAOC proportion of total SOC ranged from 19.89 %–37.36 % in biocrusts, 1.46 %–41.99 % in 0–2 cm soil, and 1.59 %–30.96 % in 2–5 cm soil after 68 years. Although POC concentrations also increased, its proportions decreased significantly, and they remained dominant (>58 % of the total SOC) across all layers throughout the chronosequence. Biocrusts accounted for 45.10–70.02 % of the total SOC and 48.11–71.52 % of MAOC stocks in the profile, establishing their pivotal role in SOC accumulation and stabilization in rehabilitated dryland ecosystems. Estimated maximum MAOC concentrations (6.94, 2.55, and 0.96 g kg⁻¹ for the three layers, respectively) and their corresponding proportions (43.50 %, 46.32 %, and 40.05 %) remained substantially below theoretical saturation thresholds, indicating considerable residual sequestration and stabilization potential. Mechanistic analysis showed that revegetation drives SOC accrual and stabilization in biocrust profile through modulating plant-biocrust inputs, soil physicochemical properties, microbial community composition, C-degrading enzyme activities, and MBC. These findings provide crucial insights in C cycling in biocrusts associated with revegetation in arid ecosystems, emphasizing the importance of biocrust development and conservation for long-term SOC sequestration and persistence.