Mechanisms of soil organic carbon storage response to Spartina alterniflora invasion and climate change.

Coastal wetlands have been identified as vital global carbon (C) sinks; however, soil C sequestration in these ecosystems is susceptible to impacts of non-native species invasion and climate change worldwide. Although their potential impacts on soil organic C (SOC) storage have been reported in previous literature, the well-established mechanisms that control SOC storage response, especially in relation to soil depths, is still limited. To fill this knowledge gap, we developed a structural equation model (SEM) to identify mechanisms that account for SOC changes in topsoil (0-0.3 m) and subsoil (0.3-0.6 m) on coastal wetland of the East China Sea, where SOC in both depths increased with exotic Spartina alterniflora invasion. In the initial model, we hypothesized that there were a set of direct and indirect effects of the invasion, climate, and soil physicochemical properties on SOC storage. By evaluating the interactions of these factors, we found relatively complex patterns that vary with depth. For topsoil, the invasion had not only direct effects on SOC storage, but also indirect effects through mediating effects of soil water content (SWC) that was linked to fine soil fractions. For subsoil, the invasion was indirectly related to SOC storage through mediating effects of SOC in topsoil, SWC, and salinity. SOC in subsoil was also affected by temperature. Our results highlight that the response of SOC storage to the invasion and climate change results from the interacting effects of climate-plant-soil system.