Microbial drivers of soil organic carbon formation from aboveground residue decomposition in desert ecosystems

Abstract

Soil microorganisms regulate soil organic carbon (SOC) formation through a dual pathway during plant residue decomposition: producing extracellular enzymes to decompose plant residues, and contributing to SOC via microbial biomass and necromass. However, in desert ecosystems that are highly sensitive to climate change and nutrient-poor, the specific mechanisms by which microbial extracellular enzymes and biomass contribute to maintaining SOC pools remain unclear. Therefore, we pulse-labeled the desert plant Alhagi sparsifolia with 13CO₂ at the southern edge of the Taklimakan Desert, collected its aboveground residue, and conducted an in-situ decomposition experiment. Over the following 1–400 days, we tracked the enriched 13C content in SOC and microbial biomass (phospholipid fatty acid, PLFA) at 0–20 cm soil depth. Results showed that the mass loss rate of aboveground residue gradually increased throughout incubation. Under barren conditions, microorganisms decompose SOC to acquire nutrients, resulting in a 20–91 % decrease in 13C in SOC during the initial decomposition phase (1–200d), followed by stabilization in the 200–400d. Similarly, the 13C in microbial biomass also showed an initial decrease followed by stabilization, indicating that after rapidly utilizing the easily decomposable components in the early stage, microorganisms entered a phase of slower metabolism. Among microbial biomass components relative to SOC, Gram-negative (G−) bacteria showed the highest ratio, ranging from 15 % to 40 %. Changes in the 13C content of soil microbial biomass were positively correlated with changes in the 13C in SOC, further confirming that microbial assimilation is a key factor driving SOC dynamics. With prolonged incubation, the ratios of Gram-positive (G+) bacteria and fungi to SOC gradually increased, reflecting a shift in microbial community structure toward groups that are more tolerant of barren conditions and better at utilizing complex carbon sources. During the initial decomposition stage (1-100d), soil extracellular enzyme activity increased by 25 %–170 %, which confirms the enhanced microbial activity to decompose SOC. In summary, microorganisms rapidly assimilate exogenous carbon into biomass, contributing to SOC formation during initial decomposition. Concurrently, to cope with the nutrient-poor environment, microorganism decompose SOC to acquire essential nutrients, resulting in SOC loss. In later stages, microbial activity decreases, leading to stabilization of SOC dynamics. This reveals the critical regulatory role of microbial metabolic strategies in the C cycle of arid regions and provides data support for the study of soil C dynamics in desert ecosystems.