The effects of global change on the fate of soil organic matter in tidal freshwater wetlands

Tidal wetlands are sentinel ecosystems for environmental change and human-induced degradation of natural systems. They have existed in a state of equilibrium with sea level rise (SLR) over the past 4,000 years by accumulating soil organic matter (SOM) produced by plants and trapping mineral sediment. Accumulation of SOM is controlled by the balance between plant productivity and decomposition. Both processes are susceptible to anthropogenic disturbance, including salinization caused by declining freshwater flows, eutrophication from fertilizer runoff, or accelerated SLR. I utilized field measurements and a stable-isotope tracer approach to examine how anthropogenic disturbances alter SOM through the direct effects of changes on microbial metabolism and the indirect effects mediated through changes in plant communities. Increased salinity and eutrophication accelerate microbial mineralization of organic matter by providing more energetically efficient metabolic pathways, sulfate reduction (salinity) and nitrate reduction (eutrophication). However, the microbial response is dwarfed by changes in plant productivity and biomass allocation. Salinity and eutrophication reduce total root biomass and allocation of carbon to the rooting zone. Incorporating the observed biogeochemical changes into models of marsh geomorphology reveals that increased salinity reduces the contribution of SOM to vertical accretion and reduces marsh resiliency to accelerated SLR. These results highlight the importance of the complex interactions between plant productivity, microbial activity, and geomorphic processes in tidal wetland landscapes. Because tidal wetlands integrate impacts to both freshwater and marine systems, their survival depends on maintaining the integrity in the continuum of landscapes from the headwaters of rivers to the ocean.