NITROUS-OXIDE (N2O) RELEASE BY STREPTOMYC ES-NITROSPOREUS IN A SANDY LOAM SOIL AS AFFECTED BY PO(2), PH AND AMOUNT OF EASILY DECOMPOSABLE ORGANIC-CARBON

In model experiments under defined conditions (80%WHC, 300 mu g nitrat e-N g(-1) dry soil, 30 degrees C) the effect of pO(2), pH and addition of easily decomposable organic matter on nitrous oxide production by Streptomyces nitrosporeus DSM 40023 from a sterilized sandy loam soil was studied. This streptomycete reduces nitrate to N2O but not to N-2. The strain was inoculated into a sterilized sandy loam sail (pH 6), e nriched with nitrate and incubated at approximately 20 and 10% (v/v) O -2 for 26 days. In another series of experiments the pH was increased with NaOH-solution up to pH 7 or the soil was enriched with pulverized leaves (1%) to increase mineralization activity and the demand for el ectron accepters. In the headspace the concentration of O-2, CO2 and N 2O was analysed by GC. The soil concentration of NO3-, NO2- and NH4+ w as measured as well as the pH value. The population density was determ ined by the plate count method. At a decreased oxygen concentration of about 2.5% (v/v) in the headspace, S. nitrosporeus increased the rele ase of N2O. In the experiments with an initial oxygen concentration of approximately 20% (v/v) this threshold of about 2.5%/(v/v) O-2 was ha rdly reached and consequently only little N2O was produced. Apparently , S. nitrosporeus uses O-2 in preference to nitrate, which is characte ristical of nitrate respiration. A pH increase from pH 6 to 7 reduced the lag phase significantly and increased the rate of oxygen consumpti on, CO2 release and N2O production. Maximum nitrous oxide production w as reached after 13 days. The result indicated that streptomycetes Lik e S. nitrosporeus may use nitrate, rate alternativly to O-2 to maintai n energy conservation (ATP synthesis) with the release of N2O. So far, nothing is known about the role of streptomycetes in contributing to N2O production in natural soils.