Saltwater intrusion in coastal landscapes has increased globally as a result of climate change driven sea level rise and extreme events. In Coastal North Carolina, a legacy of agricultural land use left behind soil nutrient loads that pose a threat to inland and estuarine water quality. In order to understand the effect of salinization on ecosystem processes in nutrient laden soils, we conducted a large scale salt and nutrient factorial addition experiment in a forested freshwater wetland. We ask: how are salt and nutrient additions impacting ecosystem processes in a coastal plain forested wetland? We measured above ground responses including tree growth, leaf carbon and nitrogen content, and leaf reflectance using a handheld continuous wavelength spectrophotometer. Leaf reflectance was also measured using RGB, multispectral and thermal cameras flown over the experimental site using small drones. We also measured below ground responses to the experimental treatments including root biomass and root growth, decomposition rates, greenhouse gas emissions using in situ gas collars, and microbial activity and carbon mineralization using laboratory soil incubation techniques. We aim to elucidate the early responses to salt and nutrient treatments with particular emphasis on carbon cycling and storage which is an important function of global wetlands.
Results/Conclusions
After nearly three years of salt addition treatments, we elevated the soil salinity 3-10 times above the salinity of the control treatment plots. Soil nutrient elevation was not statistically significant, perhaps in part because background nutrient loads from agricultural legacies were very high. Despite our experimental salinization and fertilization, we have seen no significant change in above ground tree growth, leaf reflectance or leaf C to N ratios. Microbial responses to salinization and fertilization were inconsistent across treatment blocks. Salt suppressed in situ decomposition and enhanced methane emissions under flooded field conditions but had no effect under drier conditions. Carbon mineralization assays did not show consistent patterns across sites and years, however, post sampling salinization assays suggest that microbial communities from the salt exposure treatments have adapted to increased salinity. Our results emphasize the importance of conducting long-term field experiments to test predictions arising from highly controlled lab or greenhouse settings.