Impacts of Phragmites australis Invasion on Soil Enzyme Activities and Microbial Abundance of Tidal Marshes

The rapid expansion of Phragmites australis in brackish marshes of the East Coast of the USA has drawn much attention, because it may change vegetation diversity and ecosystem functions. In particular, higher primary production of Phragmites than that of other native species such as Spartina patens and Schoenoplectus americanus has been noted, suggesting possible changes in carbon storage potential in salt marshes. To better understand the long-term effect of the invasion of Phragmites on carbon storage, however, information on decomposition rates of soil organic matter is essential. To address this issue, we compared microbial enzyme activities and microbial functional gene abundances (fungi, laccase, denitrifier, and methanogens) in three depths of soils with three different plants in a brackish marsh in Maryland, USA. Laccase and phenol oxidase activities were measured to assess the decomposition potential of recalcitrant carbon while β-glucosidase activity was determined as proxy for cellulose decomposition rate. Microbial activities near the surface (0–15 cm) were the highest in Spartina-community sites followed by Phragmites- and Schoenoplectus-community sites. A comparison of stable isotopic signatures (δ13C and δ15N) of soils and plant leaves suggests that deep organic carbon in the soils mainly originated from Spartina, and only the surface soils may have been influenced by Phragmites litter. In contrast, fungal, laccase, and denitrifier abundances determined by real-time qPCR exhibited no discernible patterns among the surface soils of the three vegetation types. However, the abundance of methanogens was higher in the deep Phragmites-community soil. Therefore, Phragmites invasion will accelerate CH₄ emission by greater CH₄ production in deep soils with abundant methanogens, although enzymatic mechanisms revealed the potential for larger C accumulation by Phragmites invasion in salt marshes in the east coast of the USA.