Coastal wetlands act as filters to retain or remove anthropogenic nitrate (NO3-) from upslope sources before it reaches the marine environment. The effectiveness of coastal wetlands as anthropogenic N filters depends largely on soil redox dynamics and plant community structure and function. Under low redox conditions N is removed via denitrification to dinitrogen (N2) and nitrous oxide (N2O), a potent greenhouse gas; dissimilatory NO3- reduction to ammonium (DNRA) contributes to N retention. Wetland plants can alter soil redox conditions, and can stimulate or inhibit N cycling via effects on soil and plant chemical properties. In this study we used 15N additions to explore the effects of plant community structure and soil redox on gross N transformations, DNRA, and N2O fluxes in a coastal marsh ecosystem. Denitrification was measured in the controls using an acetylene block. Treatments included two monocultures (Distichlis spicata (DISP) and Jaumea carnosa (JACA)), a three species mixture using a random draw of the species present (3 SPP), an unmanipulated control (ALL), and a weeding control with biomass removal equivalent to the other treatments (WEED).
Results/Conclusions Results/Conclusions
Oxygen (O2) concentrations in field soils varied significantly among treatments; JACA had the highest average soil O2 (13 ±0.3%) and DISP having the lowest (9 ± 0.4 %). All sites experienced periodic low redox events (<3% O2). Potential gross mineralization rates were high in all salt marsh communities (53±2 µg NH4-N g d-1) and decreased 37 to 51% under low redox conditions. Potential gross nitrification was only 9 to 18% of potential gross mineralization under an ambient (aerobic) headspace, and was significantly higher in the monocultures and 3 SPP treatments than in the control or WEED treatments. DNRA rates averaged 1.3±08 µg N g-1 d-1 under ambient atmospheres and dropped significantly to 0.3±03 µg N g-1 d-1 under an anaerobic headspace. DNRA rates did not differ among treatments and averaged 26 ± 3% of potential gross nitrification under an ambient atmosphere. We measured almost no N2O production from these soils under laboratory conditions, even in the presence of add NO3-. Denitrification averaged 0.78±0.09 µg N g-1 d-1 under an anaerobic headspace following a NO3- pulse, approximately accounting for the decrease in DNRA rates under these conditions. Our results suggest that while wetland plant community structure can alter redox and gross nitrification, N retention via DNRA and loss via denitrification appear to respond primarily to redox conditions.