Greenhouse gas production in a pond sediment: Effects of temperature, nitrate, acetate and season

In this paper we investigate the impact of nitrate (NO3−) concentration and temperature on the production of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). We studied sediment collected during spring, summer and autumn from a constructed pond in South Sweden. Homogenised sediment samples were dark incubated in vitro under N2 atmosphere at 13 °C and 20 °C after addition of five NO3− concentrations, between 0 and 16 mg NO3−–N per litre. We found higher net production of N2O and CO2 at the higher temperature. Moreover, increased NO3− concentrations had strong positive impact on the N2O concentration, but no effect on CH4 and CO2 production. The lack of response in CO2 is suggested to be due to the use of alternative oxidants as electron acceptors. Interaction between NO3− and temperature suggests a further increase of N2O net production when both NO3− and temperature are high. Our interpretation of the CH4 data is that at high concentrations of NO3− temperature is of less importance for CH4 production. We also found that at 13 °C CH4 production was substrate limited and that the addition of acetate increased CH4 as well as CO2 production. There was a seasonal effect on gas production potential, with more CH4 and N2O produced in spring than in summer. Re-calculation of the gas concentrations into global warming potential (GWP) units (i.e. CO2, CH4, and N2O transferred to CO2 equivalents) shows that GWP increases with temperature. However, under environmental conditions generally occurring in South Swedish ponds, i.e. low temperature and high NO3− concentration during spring and high temperature and low NO3− concentration during summer, NO3− concentration is of minor importance.