Wetland carbon and nitrogen mineralization in response to biotic and abiotic global change

Background/Question/Methods

Plants affect biogeochemical cycling via uptake, root exudation, and decomposition. Tidal wetlands are particularly sensitive to these processes due to their slow decomposition rates owed to anoxic conditions. Current projections are unclear as to how global change conditions (elevated CO2 and nitrogen) may affect how plants influence this cycling. Additionally, many American wetlands are currently experiencing invasions from the European species Phragmites australis. Our goal was to compare the effect on microbial decomposition and thus biogeochemical cycling caused by the invasive Phragmites against the native flora under present-day and global change conditions. Our 2.5 x 1.25 m wetland chambers are placed along the edge of a Phragmites invasion so that half of each chamber contains Phragmites and half native community. They have been factorially exposed to two levels of CO₂ (ambient or ambient +340 PPM) and two levels of N addition (0 or 25 g m^-2 y^-1) for 4 years. Soil samples were taken from each plot; a sample from a depth of 15 cm and from 50 cm from underneath each plant type. These soils were homogenized and incubated under field conditions in the lab while C and N mineralization measurements were taken periodically to assess decomposition. 

Results/Conclusions

Species effects dominated C mineralization dynamics when compared to global change factors. Incubations derived from Phragmites soil had C mineralization rates up to 70% greater than rates from native soil. Within species, elevated carbon and nitrogen increased overall C mineralization by 24%. N mineralization rates did not differ between species; however elevated CO2 and N addition decreased N mineralization rates by 29%. Ongoing experiments will identifying whether controls on wetland soil decomposition are due to priming from rhizosphere oxidation or root exudation.