DIAGENESIS OF ORGANIC-MATTER IN A WETLAND RECEIVING HYPERETROPHIC LAKE WATER .2. ROLE OF INORGANIC ELECTRON-ACCEPTORS IN NUTRIENT RELEASE

Constructed marshes are currently being used as a low-cost alternative for treatment of nutrient-enriched waters. These marshes may function as net sinks for nutrients, especially for particulate organic forms of N and P. However, decomposition of organic matter and nutrient release may influence the ability of the marsh to function for this purpose. One of the main factors affecting decomposition is the availability of inorganic electron acceptors (e.g., O2, NO3-, and SO42-). The role of electron acceptor consumption on N and P regeneration and release was investigated using batch incubation experiments with recently deposited organic matter (floc sediment) and peat soils collected from the constructed marsh. In electron acceptor-amended soil cores, electron acceptor consumption proceeded rapidly in the order O2 > NO3- > SO42-. Mean oxygen reduction rate (OR) was 1.6 g O2 m-2 d-1 (2025 g O2 m-3 d-1), with corresponding values for NO3- and SO42- of 0.23 g N m-2 d-1 (60 g N m-3 d-1) and 0.086 g S m-2 d-1 (5.4 g S m-3 d-1), respectively. If electron acceptor consumption was coupled to decomposition of organic matter in floc sediment with a C/N/P ratio of 190:14:1, aerobic catabolism accounted for 92% of NH4+, and soluble P regenerated in the soil, with anaerobic activity (NO3- and SO42- -reduction) accounting for the remaining 8%. In the constructed marsh receiving allochthonous inputs of labile organic matter, however, anaerobic decomposition was expected to be the dominant mechanism for nutrient regeneration. Under SO42- reducing conditions, net rates of organic N and P mineralization were 3.3 to 14 mg N L-1 sediment d-1 and 0.5 to 0.6 mg P L-1, respectively, and were highly correlated to production of dissolved inorganic C plus CH4-C. Once released to the soil porewater, nutrients were available for transport to the water column by diffusion and advection (e.g., gas ebullition), thus impacting water quality.