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

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.