LAUSR

Abstract Coastal salt marshes consist of a mosaic of vegetated and open water features, which naturally evolve and change over time. However, the rapid expansion of open water areas has been associated with marsh degradation and there is a growing need for detailed studies as coastal wetlands continue to degrade under increasing rates of sea level rise and related stressors. Yet, few studies investigate soil physicochemical and biogeochemical properties within different marsh landscape features, which could provide insight into mechanisms of the formation and expansion of open water areas. The current study compared wetland soil physical and microbial properties observed in vegetated areas with shallow open water areas called pannes, identifying a number of significant differences. Panne soils possessed lower bulk density, total C, N, P, SOC, DOC, and SRP compared with vegetated marsh areas, suggesting a shift in nutrient pools as vegetated areas transition into shallow open water features. Panne features also displayed significantly lower microbial pool sizes and processing rates than vegetated marsh soils, suggesting reduced capacity for nutrient processing in open water areas. Further, extractable NH4-N was highest in the panne soils suggesting that the absence of macrophytes decreased N uptake in open water areas. Also related to the lack of vascular plants, extractable DOC in pannes averaged less than half the concentration found in vegetated marsh areas, despite a smaller difference in soil total C. Results underscore the importance of incorporating heterogeneous landscape soil conditions when evaluating marsh degradation and considering potential restoration activities.