Denitrification potential and carbon quality of four aquatic plants in wetland microcosms

The C quality of four aquatic plants found in wetlands explains the differences in their relative capacity to fuel denitrification. In replicated (n = 4) flow-through microcosm experiments we compared nitrate removal using equal C additions of two emergent plants (Scirpus acutus, Typha latifolia) and two floating plants (Hydrocotyle umbellata, Lemna minor). As expected, plant litter with the highest initial C/N and lignin content (Scirpus acutus), required significantly (p < 0.05) higher dry matter addition to achieve the nitrate removals of the other plants studied. For all plants, losses in C content and significantly, (p < 0.05) higher C/N ratios in leached litter is consistent with preferential hydrolysis of low molecular weight (MW) carbohydrates. Interestingly, plant-specific differences in this denitrification potential were removed when nitrate removal data was compared on an acid-soluble carbohydrate (ASC) basis. As an efficiency measure, high ratios of applied C to NO3-N decreased denitrification potentials, whereas increasing NO3-N loading at equal C additions increased denitrification potentials. Differences in denitrification potential resulting from carbohydrate and lignin content of the plants studied were generally smaller than relative differences in reported productivity. That much of the plant C left the microcosms before oxidation is consistent with overall electron acceptor limitation in wetlands and suggests observable differences in field-scale denitrification performance may still arise with the presence or absence of highly productive reed (e.g., Scirpus sp., Typha sp.) stands.