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 (Germ any), in which methane oxidation rates of soils of an unfertilized spruce s ite and of a spruce site that had been fertilized with 150 kg of N ha(-1) l as (NH4)(2)SO4) were followed seasonally over approximately three years (19 94-1996). Throughout the observation period, the soil at both sites functio ned exclusively as a sink for atmospheric CH4. Mean CH4 oxidation rates at both sites were almost identical in magnitude (82.2 +/- 34.6 mug CH4 m(-2) h(-1) for the unfertilized site, and 84.2 +/- 31.8 mug CH4 m(-2) h(-1) for the N fertilized site) during the observation period. Results from an addit ional small-scale N fertilization experiment indicate that high N applicati ons 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 t hat the atmospheric CH4 uptake activity of the soils of forest ecosystems c haracterized by N limitation has the capacity to recover rapidly from the i nhibitor): effects of high inorganic N inputs. CH4 oxidation rates at both sites showed no significant diurnal variation. However, there were signific ant seasonal differences in the magnitude of CH4 oxidation rates at both ex perimental sites with high rates during summer, relative low rates during w inter and intermediate rates during spring and autumn. Correlation analysis revealed that CH4 oxidation rates were positively correlated with soil tem perature and negatively with soil moisture. However, at low soil temperatur es (<10<degrees>C), temperature was a stronger modulator of CH4 oxidation t han soil moisture. Process studies on soil samples in the laboratory confir med a pronounced positive response of CH, oxidation to changes in temperatu re, only within a range of 0-10 degreesC. At both experimental sites, the h ighest CH4 oxidation activity was observed in the A(h) layer (0-120 mm soil depth). Exposure of this layer to the atmosphere, as a result of the remov al of the organic layer, resulted in a significant increase of CH4 oxidatio n rates. Apparently the organic layer functions as a diffusive barrier for atmospheric CH4 or O-2 to CH4 oxidizing sites. (C) 2001 Elsevier Science Lt d. All rights reserved.