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

Abstract Eutrophication has decreased the O 2 content and increased the NH 4 + availability in freshwaters. These changes may affect carbon and nitrogen transformation processes and the production of CH 4 and N 2 O, which are important greenhouse gases. We studied release of CH 4 and N 2 O from a eutrophic lake sediment under varying O 2 and NH 4 + conditions. Intact sediment cores were incubated in a laboratory microcosm with a continuous anoxic or oxic water flows containing 0, 50, 500, 5000, or 15 000 μM NH 4 + . With the anoxic flow, the sediment released CH 4 , up to 7.9 mmol m −2  d −1 . With the oxic flow, the CH 4 emissions were small indicating limited CH 4 production and/or effective CH 4 oxidation. Addition of NH 4 + did not affect sediment CH 4 release, evidence that the CH 4 oxidizing bacteria were not disturbed by the extra NH 4 + . The release of N 2 O from the sediment was highest, up to 7.6 μmol m −2  d −1 , with the oxic flow without NH 4 + addition. Oxygen was the key factor regulating the production of NO 3 − , which enabled denitrification and production of N 2 O. However, the highest NH 4 + addition increased nitrification and associated O 2 consumption causing a decrease in sediment O 2 content and in accumulation of NO 3 − and N 2 O, which were effectively reduced to N 2 in denitrification. In summary, sediment CH 4 and N 2 O dynamics are regulated more by the availability of O 2 than extra NH 4 + . Anoxia in eutrophic lakes favouring the CH 4 production, is the major contributor to the atmospheric consequences of water eutrophication.