Gaining a better understanding of carbon (C) dynamics across the terrestrial and aquatic landscape has become a major research initiative in ecosystem ecology. Wetlands store a large portion of the global soil C, but are also highly dynamic ecosystems in terms of hydrology and N cycling, and are one of the most invaded habitats worldwide. The interactions between these factors are likely to determine wetland C cycling, and specifically C accretion rates.  In Great Lakes coastal wetlands, water levels vary over a range of about 1.5 meters, irregularly, but on approximately a 20 to 30 year time scale.

We augmented our Mondrian wetland ecosystem-community model to have ecologically realistic controls of the hydroperiod (the length and degree of wetland flooding) on rates of decomposition, and thus C accretion over decadal time scales in litter, muck, and sediments.  As each layer of plant detritus became inundated by rising flood water, anaerobic conditions built up with the lag time of a few days,  slowing decomposition.  Inundation also slowed N cycling in litter, muck, and sediments.  Because N cycling in Mondrian relates directly to plant productivity and plant community composition (including the success of invasive plants), the effects of hydroperiod were thus full linked to N cycling and plant productivity and community composition in the model.

We simulated the effects of different levels of (1) N loading, (2) hydroperiod, and (3) plant community (natives only vs. invasion scenarios) and their interactions on C accretion outcomes in freshwater coastal wetlands of the Great Lakes region. Results showed that N loading contributed to substantial rates of C accretion by increasing NPP (Net Primary Productivity). By mediating anaerobic conditions and slowing decomposition, hydroperiod also exerted considerable control on C accretion. Invasion success occurred with higher N loading and contributed to higher NPP, while also interacting with hydroperiod via ecosystem-internal N cycling. Invasion success by both Typha glauca and Phragmites australis showed a strong nonlinear relationship with N loading in which an invasion threshold occurred at moderate N inputs. This threshold was in turn influenced by duration of flooding, which reduced invasion success for P. australis but not T. glauca. The greatest simulated C accretion rates occurred in wetlands invaded by P. australis at the highest N loading in constant anaerobic conditions. These model results suggest that while plant invasion may increase C storage in freshwater coastal wetlands, increased plant productivity (both native and invasive) due to increased N loading is the main driver of increased C accretion.

Jason Martina, of Texas A&M university, led the paper reporting these results:

Martina, Jason P., William S. Currie, Deborah E. Goldberg and Kenneth J. Elgersma.  2016. Nitrogen loading leads to increased carbon accretion in both invaded and uninvaded coastal wetlands.  Ecosphere 7(9): e01459.