Temperature and Moisture Effects on Soil C Fluxes and Microbial Enzyme Activity in a Coastal Freshwater Forested Wetland.

In coastal wetlands, temperature and hydrology are major drivers of soil microbial processes and soil carbon (C) cycling. While coastal wetlands are recognized as important C storehouses our understanding of temperature and hydrologic effects on greenhouse gas fluxes is lacking which has implications for understanding ecosystem response to rising sea levels and temperature. Organic soils were collected from three locations (hummock surface, hollow surface, and a deep soil (10-30 cm)) from a coastal freshwater forested wetland in North Carolina and incubated in the laboratory (84 d) under different temperatures (27.5ºC and 32.5ºC) and moistures (65% and 100% water holding capacity (WHC)). Deep soils had lower organic matter content (71 ± 10 %) than hummocks (94 ± 1.3 %) and hollows (93 ± 2.6 %). Deep soils were also more enriched in ¹³C (-28.8 ‰ ± 0.18) compared to hummocks (-29.4 ‰ ± 0.09) and hollows (-29.8 ‰ ± 0.19), indicating a greater degree of processing of C. Rates of C mineralization, CH₄ production, and enzyme activity (xylosidase and β-glucosidase) were lower in deep soils, compared to surface soils from hummocks and hollow. Overall, hummocks had the greatest amount of CO₂ and CH₄ production. In all sampling locations, saturation of soils (100% WHC) suppressed CO₂ production and stimulated CH₄ production compared to unsaturated soils (65% WHC). δ¹³CO₂ profiles showed a significant enrichment of respired CO₂ in saturated soils, becoming more enriched with time (-30 to -18 ‰) compared to unsaturated soils (-30 to -34 ‰). For saturated soils, δ¹³CH₄ ranged from -50 to -55 ‰ and there was no temperature or time effect. Peroxidase activity, a lignin degrading enzyme, increased dramatically in saturated soils and was further enhanced at the higher temperature. Enriched δ¹³CO₂ in saturated soils may result from the interactive effects of the C isotopic composition of the source (e.g. soil, litter, roots) of respired CO₂, shifts in microbial substrate use, and C isotopic fractionation of reduced CO₂ in the hydrogenotrophic pathway of methanogenesis. These findings suggest that inundation of organic soils in coastal freshwater forested wetlands results in lower C mineralization and higher methanogenesis, with fluxes of both CO₂ and CH₄ enhanced at higher temperatures. As sea levels and temperature continue to rise globally, changes in coastal wetland C cycling will have important feedbacks with the global C cycle and future climate change.