Michael Krämer's research while affiliated with Universität Konstanz and other places

NO and N2O release rates were measured in an acidic forest soil (pH 4.0) and a slightly alkaline agricultural soil (pH 7.8), which were incubated at different O2 concentrations (2) and at different NO concentrations (40 – 1000 ppbv NO). The system allowed the determination of simultaneously operating NO production rates and NO uptake rate constants, and the calculation of a NO compensation concentration. Both NO production and NO consumption decreased with increasing O2. NO consumption decreased to a smaller extent than NO production, so that the NO compensation concentrations also decreased. However, the NO compensation concentrations were not low enough for the soils to become a net sink for atmospheric NO. The release of N2O increased relative to NO release when the gases were allowed to accumulate instead of being flushed out. The forest soil contained only denitrifying, but not nitrifying bacteria, whereas the agricultural soil contained both. Nevertheless, NO release rates were less sensitive to O2 in the forest soil compared to the agricultural soil.

The methanotrophs Methylomonas angile (type I) and Methylosinus trichosporium (type II) produced nitrite, nitrate and N2O during growth on methane, apparently by heterotrophic nitrification of ammonium. The methanotrophs were also able to consume NO but did not produce it. After incubation of soil from a drained paddy field in the presence of CH4 the numbers of methanotrophs increased from 105 to 107 per gram dry weigth. The thus enriched soil showed increased rates of NO consumption while rates of NO production did not change.