Seasonality, C:N ratio and plant species influence on denitrification and plant nitrogen uptake in treatment wetlands

Synthetic wastewater with an average total dissolved nitrogen (TDN) concentration of 72 mg l−1 (18 NH4-N and 54 NO3-N) and three organic carbon‑nitrogen (C:N) ratios (0, 1.2, 2.4) was batch-loaded to 72 microcosms planted in monocultures of five plant species or left unplanted. Water quality was measured at 0, 1, 3, 6, 9, and 15 days during 15-day incubations at eight temperature settings ranging from 4 to 24 °C. This allowed for comparisons of TDN removal as influenced by plant species, temperature, and C:N ratio. Strong interactions between independent variables influence interpretation. At a C:N ratio of zero, all plant species significantly increased TDN removal compared to the unplanted microcosms. However, unplanted microcosms showed nearly complete TDN removal at the highest C:N ratio and warm temperatures; removal decreased with decreasing temperature and C:N ratio. TDN removal in Deschampsia cespitosa and Typha latifolia was better but responded similarly to unplanted with respect to temperature and C:N ratio. Conversely, microcosms of Phragmites australis and Schoneoplectus acutus had complete and rapid removal of all applied TDN (177 g N m−2 yr−1) under all C:N ratios and temperatures. Complete TDN removal was observed in Carex utriculata for all C:N ratios and temperatures except for the coldest temperature (4 °C). However, increasing the C:N ratio at this temperature increased TDN removal. Higher C:N ratios increase TDN removal, likely by increased denitrification, but simultaneously reduce plant biomass and plant uptake of nitrogen. Overall, removal of nitrate was enhanced with organic carbon addition and warm temperature, but specific plant species promoted complete removal even without organic carbon and in winter.