Coastal wetlands intercept significant amounts of nitrogen (N) from our watersheds, especially when surrounding land cover is dominated by agriculture and urban development. Through denitrification, plant assimilation, and N accumulation in litter and sediments, wetlands can remove excess N from surface water and mitigate eutrophication of connected aquatic ecosystems. However, since excess N can also change plant community composition, especially when threatened by opportunistic invasive species, disentangling the effects of plant communities composition when quantifying wetland N removal along a N loading gradient can be problematic. Here we investigate in silico how N removal via plant uptake and microbial denitrification are affected by community composition, hydroperiod, water residence time and N-loading rates in temperate freshwater coastal wetlands using MONDRIAN, a dynamic process-based ecosystem simulation model. Using a factorial design, we investigate ecosystem N retention and denitrification with and without Phragmites australis invasion by simulating 5 hydroperiods, 3 water residence times, and 4 N-loading scenarios (N=120 treatment combinations). Our goal is to find optimal N removal scenarios across native and Phragmites dominated communities along hydrologic and N loading gradients in Great Lakes coastal wetlands while recognizing potential tradeoffs between other ecosystem functions.
Results/Conclusions
We found that hydroperiod, water residence time, and N-loading all interact to influence N retention and denitrification in Great Lakes coastal wetlands. Interestingly, we also found that community composition had little effect on N removal, despite greater potential NPP in highly productive invaded communities. Drier wetlands (e.g. saturated or temporarily flooded soils) had a limited capacity for N removal compared to wetter wetlands in our simulations. As plant litter and organic matter pools were exposed to oxygen, denitrification stopped, decomposition was accelerated and mineralized N was exported downstream. Additionally, longer water residence time increased denitrification potential while also lowering N loading thresholds for invasion success in those communities. A longer residence time for water and dissolved inorganic N compounds results in N accumulation in wetlands, giving microbes and plants, including opportunists like Phragmites, a larger temporal window for N transformation and uptake, respectively. These simulations help elucidate complex interactions of community composition, N loading and hydrology on N removal. Importantly, our findings demonstrate a potential tradeoff between N removal and Phragmites invasion and also provide practitioners that have resources to alter local hydrology options to increase N removal at the cost of greater invasion risk.