Effects of Salinity on Productivity and Biogeochemical Processes in Tidal Freshwater and Oligohaline Wetlands of South Carolina, USA

Abstract

A principal threat to tidal freshwater and oligohaline wetlands in the Southeastern United States is salinity intrusion from sea level rise, which affects their structure, growth, and function. This study was designed to improve our understanding of nutrient cycling and growth dynamics in relation to increasing salinity within tidal freshwater and oligohaline wetlands near Georgetown, South Carolina. To achieve this goal, net primary productivity (NPP), nutrient dynamics, microbial biomass, and litter decomposition were quantified across a salinity gradient at three forested sites along the Waccamaw and Sampit rivers, and also a tidal oligohaline marsh. The gradient was comprised of an upper (freshwater; <0.1 ppt), middle (moderately salt impacted; 1.8 ppt), and lower (heavily salt impacted; 2.8 ppt) forested site, and an oligohaline marsh (5.4 ppt). We hypothesized that NPP will decrease as salinification increases in the forested sites due to inhibited ability of plants to take up key nutrients, thus decreasing the capacity for C assimilation and maintenance of effective nutrient pools. Additionally, at the forested sites we hypothesized that litter decomposition will slow as salinity influences the microbial community and litter breakdown, possibly facilitating nutrient limitations at the most saline, forested site undergoing active salinity-induced transition to marsh. In contrast, at the marsh we hypothesized that high belowground NPP would drive increased microbial biomass. Response variables for the forested sites included: NPP (both aboveground and belowground NPP for 2011); a 68-week foliar decomposition study; microbial biomass determined from soil samples taken every 6 weeks from December 2010-July 2012; foliar analyses completed in 2009; and two fertilized root ingrowth core studies from the spring and autumn of 2011. Those for the marsh consisted of microbial biomass and root productivity estimates (April 2011-July 2012). Results supported portions of our hypotheses. For example, a significant decrease was seen in total NPP between the upper (2263 g m-2 yr-1) and lower forested sites (634 g m-2 yr-1) as salinities transitioned from 0.1 to 2.8 ppt. The marsh had the greatest mean live root biomass, which included an estimated 3.30 Mg ha-1 of C and 0.07 Mg ha-1 of N. Mass remaining of foliar litter at week 68 did not differ among sites. However, decreased P resorption proficiency at the lower site paired with no difference in soil extractable P between upper and lower forested sites suggest inhibited plant acquisition of P at the lower forested site. As salinity increases, tidal freshwater forested wetlands may have a reduced potential to cycle and utilize nutrients, thus leading to declines in productivity and C storage. However, belowground C storage and N pools possibly rebound after forest transitions to marsh. Information gained from this study is vital to our understanding of how global climate change affects biogeochemical fluxes of coastal wetlands and their ability to store C.