Characterization of herbaceous encroachment on soil biogeochemical cycling within a coastal marsh

Vegetation transitions occur globally, altering ecosystem processing of organic matter and changing rates of soil biogeochemical cycling. In coastal marshes, more salt- and inundation-tolerant herbaceous species are encroaching on less tolerant species, concomitant with sea level rise. These species shifts could disrupt ecosystem services such as soil organic matter storage and the cycling of carbon (C), nitrogen (N), and phosphorus (P). To determine how these ecosystem processes were affected by encroachment, we characterized biogeochemical properties and functions along a transect of encroaching Distichlis spicata L. Greene (saltgrass) on Spartina bakeri Merr. (cordgrass), two herbaceous species. During both the wet and dry season, nine soil cores were obtained from three community types: saltgrass end member, transition zone, and cordgrass end member. Total soil C, N, and organic matter were greatest within the saltgrass and transition zones. The saltgrass and transition zone soils also supported higher rates of enzyme activity and potentially mineralizable N and P than cordgrass soils during the dry season, and greater potential CO₂ production and microbial biomass C during the wet season. Generally, the transition zone functioned similarly to the saltgrass zone and the encroachment gradient coincided with a 33 cm elevation change. Seasonally, low extractable nutrient availability (nitrate and soluble reactive phosphorus) during the dry season was correlated with overall greater enzyme activity (N-acetyl-β-D-glucosidase, alkaline phosphatase, β-glucosidase, xylosidase, and cellobiosidase) and potentially mineralizable N and phosphorus (P) rates. This study demonstrates that shifts in dominant herbaceous species and accompanying abiotic gradients alters biogeochemical processing of organic matter within coastal marshes.

Keywords: Biogeochemical cycling; Distichlis spicata; Encroachment; Herbaceous marsh; Sea level rise; Spartina bakeri.