Modeling nutrient cycling and retention in wetlands as a simultaneous driver and outcome of ecosystem self-organization

Ecosystems comprise biological communities together with their abiotic environments, not as separate phenomenon but often as tightly integrated. Plant communities are adapted to the abiotic conditions in a locale, but plant communities may also play a large role in creating those conditions as a type of ecosystem-level autocatalysis. This is particularly true of nutrient cycling. As nutrients flow into an ecosystem from external sources, the system retains limiting nutrients through plant uptake and growth and the storage and cycling of nutrients in detritus, including microbial immobilization. This storage and cycling then strongly affects plant growth and plant community assembly. Nutrient cycling in a particular locale is thus both an outcome, and a driver, of ecosystem self-organization. We studied nitrogen cycling in coastal wetlands in the Laurentian Great Lakes, USA. N inflows likely range from < 1 g N/m2 y in pristine areas to greater than 50 g N/m2 y in areas experiencing agricultural fertilizer runoff. We used an individual-based community-ecosystem model, Mondrian, which has sophisticated N cycling processes and biogeochemistry that spans four levels of ecological organization. It includes numerous biotic and abiotic aspects of N cycling and carbon-nitrogen interactions. We found that the hydrologic drivers of water level (which may range across ca. 50 cm seasonally and 1.5 m on a multi-decadal time scale) and water residence time, by flushing N through the wetland, exerted a strong control on ecosystem self-organization through the N cycle, often determining the dominant plant species. We found that water level fluctuations, by making muck and sediments alternately aerobic and anaerobic and thereby driving coupled nitrification-denitrification, provided another mechanism governing ecosystem organization. Our model simulations illustrate how abiotic environmental conditions, biogeochemistry, and plant functional ecology integrate to create an ecosystem-level attractor in N cycling. The attractor may either be stable over long time periods or shift rapidly as abiotic conditions change. This phenomenon is key to understanding N loads that pass through wetlands and for achieving wetland management goals including efforts to control invasive species.