Mechanisms of enhanced methane emission due to introduction of Spartina anglica and Phragmites australis in a temperate tidal salt marsh

Introductions of exotic plants are widespread ecological threats disturbing the carbon and nutrient cycles in global coastal wetlands. Previous studies have found that the invasion has enhanced methane emission through increased primary production and a changed methanogenic pathway. However, the previous studies did not determine the depth profiles of the changes while the depth in the soil profile to which the disturbance propagates is one of the critical issues to be addressed. In this study, we illustrated the depth profiles of the effect of the introductions of S. anglica and P. australis on the potential methane production, soil biogeochemistry, and soil microbial community which play a substantial role in enhanced methane emission due to the invasions. The in situ field measurements and anaerobic incubation were conducted to determine the mechanistic differences of depth profiles. Both introductions of S. anglica and P. australis increased methane emission through different microbial mechanisms depending on depth profile. Potential methane production was stimulated in shallow and deep soil layer of S. anglica and P. australis-introduced marshes, respectively. S. anglica increased the contribution of methylotrophic methanogenesis in top soil layers while P. australis decreased competitive inhibition by sulfate reducers in all soil layers. These results emphasize the importance of below-ground microbial activity and vertical distribution in carbon cycles in tidal marshes. Further plant invasion studies should focus on depth profiles of microbial processes, as their distribution and activity vary substantially across soil depths.