Effect of ammonium and oxygen on methane and nitrous oxide fluxes across sediment-water interface in a eutrophic lake

Abstract

Eutrophication has decreased the O2 content and increased the NH4+ availability in freshwaters. These changes may affect carbon and nitrogen transformation processes and the production of CH4 and N2O, which are important greenhouse gases. We studied release of CH4 and N2O from a eutrophic lake sediment under varying O2 and NH4+ conditions. Intact sediment cores were incubated in a laboratory microcosm with a continuous anoxic or oxic water flows containing 0, 50, 500, 5000, or 15000 μM NH4+. With the anoxic flow, the sediment released CH4, up to 7.9 mmolm-2d-1. With the oxic flow, the CH4 emissions were small indicating limited CH4 production and/or effective CH4 oxidation. Addition of NH 4+ did not affect sediment CH4 release, evidence that the CH4 oxidizing bacteria were not disturbed by the extra NH4+. The release of N2O from the sediment was highest, up to 7.6 μmolm-2d-1, with the oxic flow without NH4+ addition. Oxygen was the key factor regulating the production of NO3-, which enabled denitrification and production of N2O. However, the highest NH 4+ addition increased nitrification and associated O 2 consumption causing a decrease in sediment O2 content and in accumulation of NO3- and N2O, which were effectively reduced to N2 in denitrification. In summary, sediment CH4 and N2O dynamics are regulated more by the availability of O2 than extra NH4+. Anoxia in eutrophic lakes favouring the CH4 production, is the major contributor to the atmospheric consequences of water eutrophication. © 2003 Elsevier Ltd. All rights reserved.