Short-term responses of soil organic carbon stability to vegetation-topography and water-level changes: divergent pathways in coastal wetlands.

Abstract

Soil organic carbon (SOC) pool stability is fundamental to maintaining the long-term carbon sink function of coastal wetlands. Vegetation, topography and water-level changes can influence SOC accumulation and stability by modifying soil properties, carbon inputs, and microbial processes. However, how changes in these factors affect SOC stability and its underlying pathways over the short term remains unclear. Using the unchanged wetland (CK) as a seasonal reference, we investigated the effects of vegetation-topography changes (VT) and water-level changes (WL) on SOC stability across four time points over one year (pre-change, three months, six months, and nine months post-change), and identified the underlying pathways. Seasonal dynamics accounted for much of the variation in SOC and mineral-associated organic carbon (MAOC), and VT and WL altered the magnitude of these changes. The magnitude of SOC change under WL (-16.44%) was smaller than that under CK (-34.41%) and VT (-35.34%), indicating that WL buffered seasonal SOC fluctuations. Concurrently, SOC stability (fMAOC, the ratio of MAOC to SOC) remained stable, with a non-significant decrease under VT and a non-significant increase under WL. Both VT and WL mitigated seasonal particulate organic carbon (POC) decline. VT and WL altered microbial community composition and life-history strategies, with microbial communities shifting toward K-strategy under VT and r-strategy under WL; these shifts were related to labile carbon availability. Structural equation modeling (SEM) further suggested that SOC stability under VT was associated with soil-property changes that were linked to carbon fraction allocation and microbial processes related to MAOC formation. Under WL, SOC stability was more closely associated with increased labile carbon availability and MAOC dynamics under altered water-level and soil conditions. These findings suggest that vegetation-topography and water-level changes can affect SOC fractions and stability through distinct pathways in the short term, while water-level changes may better buffer seasonal SOC fluctuations, providing targeted evidence for optimizing carbon-related management and protecting coastal blue carbon sinks.