Methane oxidation by soils of an N limited and N fertilized spruce forest in the Black Forest, Germany

A long-term experiment was performed at two sites in the Black Forest (Germany), in which methane oxidation rates of soils of an unfertilized spruce site and of a spruce site that had been fertilized with 150 kg of N ha−1 (as (NH4)2SO4) were followed seasonally over approximately three years (1994–1996). Throughout the observation period, the soil at both sites functioned exclusively as a sink for atmospheric CH4. Mean CH4 oxidation rates at both sites were almost identical in magnitude (82.2±34.6 μg CH4 m−2 h−1 for the unfertilized site, and 84.2±31.8 μg CH4 m−2 h−1 for the N fertilized site) during the observation period. Results from an additional small-scale N fertilization experiment indicate that high N applications to the soil of this N-limited forest resulted only in a small reduction of CH4 oxidation: less than 30% for less than 72 d. The results indicate that the atmospheric CH4 uptake activity of the soils of forest ecosystems characterized by N limitation has the capacity to recover rapidly from the inhibitory effects of high inorganic N inputs. CH4 oxidation rates at both sites showed no significant diurnal variation. However, there were significant seasonal differences in the magnitude of CH4 oxidation rates at both experimental sites with high rates during summer, relative low rates during winter and intermediate rates during spring and autumn. Correlation analysis revealed that CH4 oxidation rates were positively correlated with soil temperature and negatively with soil moisture. However, at low soil temperatures (<10°C), temperature was a stronger modulator of CH4 oxidation than soil moisture. Process studies on soil samples in the laboratory confirmed a pronounced positive response of CH4 oxidation to changes in temperature, only within a range of 0–10°C. At both experimental sites, the highest CH4 oxidation activity was observed in the Ah layer (0–120 mm soil depth). Exposure of this layer to the atmosphere, as a result of the removal of the organic layer, resulted in a significant increase of CH4 oxidation rates. Apparently the organic layer functions as a diffusive barrier for atmospheric CH4 or O2 to CH4 oxidizing sites.