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

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, 5,000, or 15000 microM 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 micromol 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.