Carbon (C) cycling in soils is fundamentally linked to the metabolism of microbial communities. Saltwater intrusion into coastal wetlands with sea level rise (SLR) will increase salinity (stress), and marine-derived nutrient subsidies (phosphorus, P; sulfur S) with uncertain effects on microbial activities and biogeochemical processes. Freshwater coastal wetlands of the Florida Everglades are particularly vulnerable to increases in salinity and P due to reductions in freshwater availability and extreme P-limitation. Our objective was to understand how soil microbial extracellular enzyme activities (EEAs) and ecosystem respiration (ER) differentially respond to marine water salinity and elevated P to affect soil C balance and nutrient acquisition in freshwater sawgrass peat soils. Using a 2 ´ 2 factorial design in experimental wetland mesocosms, we treated sawgrass peat cores (38 ´ 53 ´ 40 cm) with ambient and elevated levels of salinity (+7 ppt) and P (2500 μg L-1). During initial exposure to treatment conditions (52 d), we measured C- and nutrient-based EEAs and ER in surficial soils to assess sensitivity to the subsidy of P and stress of salinity.
Enzyme activities decreased with elevated salinity for acid phosphatase (AKP), arylsulfatase (ARS), beta glucosidase (BG) (2-3 ×lower than ambient; ANOVA, P < 0.05). However, elevated salinity and P combined do not appear to suppress EEAs relative to ambient (P > 0.05). Salinity and P did affect cellulase activities (CEL). Phosphorus potentially mediated suppression of enzyme activity with increased salinity during initial exposure. The C:P stoichiometry of EEAs (BG+CEL:AP) approached global averages of 1:1 in elevated P treatments, suggesting that P subsidies can balance freshwater microbial demands for C and P under elevated and ambient salinity exposure. Soil ER was not altered after 52 d exposure to elevated salinity and P. Early exposure to elevated salinity suppresses microbial enzyme function; however, subsidies of P have the potential to mediate the stressful effect.