Investigating influence of nitrogen dynamics and hydroperiod on GHG emissions in Great Lakes coastal wetlands using a simulation model

Abstract

Wetlands impact global warming by regulating the exchange of greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) with the atmosphere. Few studies have investigated the interactive effects of different environmental factors in wetlands, such as water residence time and nutrient inflows, on GHG emissions. Here we investigated GHG emission in Great Lakes coastal wetlands across various hydrology, temperature, and N inflow regimes using a process-based simulation model MONDRIAN. We found the emission of CH4, N2O and sequestration of C (i.e. negative net ecosystem exchange, NEE) all increased with increasing water residence time and N inflow in our modeling results, primarily driven by increased plant productivity and N uptake, which indicated greater C and N cycling rates in the model. The summed global warming potential (GWP) (i.e. sum GWP of CH4, N2O, and NEE) of wetlands on 20-year and 100-year time horizons were both primarily driven by CH4 emissions. Under most conditions, NEE reduced by removing atmosphere C in our results, meaning modeled wetlands were net sinks of carbon as wetland plants assimilated atmospheric CO2 and plant litter became accreted in underlying anaerobic soil. Negative effects of NEE on GWP partially offset the GWP of CH4 emissions. GWP of N2O was negligible because the amount 2 of N2O emitted from these simulated wetlands was very small. Our results suggested that under a wide range of conditions, the summed GWP from Great Lakes coastal wetlands may be strongly controlled by the tradeoffs among CH4 emission and CO2 sequestration, both of which were driven by elevated levels of N inflow in our simulations. Water level scenarios also had an effect on GHG exchanges by moderating the transitions between aerobic and anaerobic conditions. Higher temperature promoted higher GWP but under the modest range of temperature increases we simulated, reflecting those expected in this region by midcentury, temperature effects were minimal compared with those of other factors. These results highlight the previously understated role of nutrients in modulating GWP in coastal wetlands and point out the importance of water residence time in wetlands N cycling.