Methane oxidation, nitrification, and counts of methanotrophic bacteria in soils from a long-term fertilization experiment (”Ewiger Roggenbau” at Halle)

Aerobic soils are important sinks for atmospheric methane. CH4 oxidation, mediated mainly by methanotrophic bacteria, is the responsible process, which is strongly inhibited by ammonium accessible for nitrification. An inhibitory effect immediately after fertilization as well as a long-term effect exists, which results from repeated ammonium applications and which is independent from the actual concentration of NH4+-N in soil. This long-term effect could be caused by a shift in the microbial population of the soil. Thus, with soil samples from long-term fertilization treatments of the field experiment ”︁Ewiger Roggenbau” at Halle (Germany) incubation studies were conducted to investigate the interference between CH4 oxidation and nitrification and to determine the cell numbers of methanotrophic bacteria. Including the treatments PK, NPK, and farmyard manure, which were established in 1878, a close negative correlation between CH4 oxidation and net nitrification was found (r = —0.92). The CH4 oxidation rates, determined with an initial concentration of 10 μl CH4 l—1, varied between 6.7 and 1.1 μg C kg—1 d—1 in the PK and NPK treatment, respectively. After application of NH4Cl a strong inhibition of CH4 oxidation occurred, which was 91%, 88%, 81%, and 63% in the treatments PK, NPK, FYM, and U (unfertilized), respectively. After a lag-phase of 2 to 3 weeks an incubation with high CH4 concentrations (20 Vol.% CH4) could induce CH4 oxidizing activity in the NPK treatments under continuous rye or maize cropping. An increase of up to 40 times in comparison to the control under atmospheric CH4 (2 μl CH4 l—1) was observed. A negative correlation (r = —0.74) existed between the CH4 oxidation rates of the soils without recently applied NH4+ and the numbers of methanotrophic bacteria, determined with the ”︁most probable number” method (MPN). Thus, the MPN technique is not suitable to characterize the physiologically active population of methanotrophic bacteria in soils, which oxidize CH4 in the atmospheric concentration range. The results of this study suggest that in aerobic arable soils methanotrophic bacteria and not nitrifiers are responsible for CH4 oxidation.