OOS 26-7 - The effects of plant community composition and redox on the fates of nitrate in a coastal wetland

Wednesday, August 4, 2010: 10:10 AM
310-311, David L Lawrence Convention Center
Whendee Silver, Environmental Science, Policy, and Management, University of California, Berkeley, CA, Bibit Traut, Biology Department, City College of San Francisco, San Francisco, CA and Wendy Yang, Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA
Background/Question/Methods

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.

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