Responses of Leaf Litter Breakdown Rates and Microbial Enzyme Activity to Salinity in North Carolina Wetlands

Abstract

Sea-level rise and human activities are causing the increase of salinity in coastal freshwater wetlands. Increased salinity in some wetlands has been found to accelerate leaf litter decomposition, an important driver of nutrient availability and carbon sequestration. Research at Timberlake Observatory for Wetland Restoration (TOWeR) and two reference wetlands in eastern North Carolina has documented periods of increased salinity associated with drought. Here, I examined breakdown rates of leaf litter from common wetland tree species (Nyssa biflora and Liquidambar styraciflua) in TOWeR and two reference wetlands. I also examined macroinvertebrate abundance and microbial enzyme activity on the litterbags. Leaf breakdown and microbial activity was also measured in a microcosm experiment that exposed microbial inoculums from both reference wetlands to high and low salinities. Microbial extracellular enzyme activity for the acquisition of carbon (beta-glucosidase, BG), nitrogen (N-acetylglucosaminidase, NAG, and leucine aminopeptidase, LAP), phosphorus (acid phosphatase, AP), sulfate (arylsulfatase, AS), and the breakdown of phenol groups (phenol oxidase, PO) was obtained using standard fluorometric (absorbance for PO). Leaf litter breakdown rates, as well as BG, NAG, LAP, PO, and AP activity were expected to increase with higher salinity, while the activity of AS and macroinvertebrate abundance was expected to decrease. During the 29 weeks of the field experiment, salinity incursion was not as prevalent as in previous years; reaching conductivities of 111.59 and 1863.37 [mu] S cm⁻¹ for the sites with the lowest and highest respectively. Despite the lack of a large change in salinity, I observed a tripling of field leaf litter breakdown rate (from 0.001±0.0001 d⁻¹ to 0.0029±0.0001 d⁻¹) and a linear increase of breakdown rates with increasing conductivity across the sites (R²=0.84 p=0.027). Microcosm breakdown rates were higher than field breakdown rates (0.0026±0.007 d⁻¹ to 0.0033±0.0006 d⁻¹) and did not correspond with salinity but did have a strong negative linear relationship with the amount of dissolved organic carbon (DOC) available (R²=0.96 p=0.006). Enzyme activity increased in response to increased salinity in the field and microcosm experiment but responses were not consistent between lab and field and overall were low compared to literature values. Macroinvertebrate presence was low, only being present in 49 out of 315 litter bags, and did not correlate to salinity or increased breakdown rates. Overall, the lack of consistent results between field and microcosm suggest that small changes in salinity are unlikely to lead to major changes in leaf decomposition and microbial enzyme activity.