Seasonal and spatial variation of methane oxidation in a Danish spruce forest

Methane oxidation in a Danish spruce forest was measured monthly from September 1993 to June 1995 using 30 closed chambers (78 cm2) in permanent positions along a 58 m transect. Average daily CH4 oxidation rates ranged from 27.1 to 57.7 micrograms CH4 m-2 h-1 and were positively correlated with soil temperature (P < 0.001). We found no temporal correlation with other soil variables. Apparent Q10(5-15 degrees C) was 1.35 for CH4 oxidation in situ and 1.36 for intact soil cores incubated in the laboratory. Methane oxidation in situ and in soil samples showed a very consistent pattern along the transect with areas of high or low oxidation rates. The high activity areas coincided with a thick (> 10 cm) layer of humus or humus-enriched soil. In addition. we measured CH4 in soil air and found a positive correlation along the transect between the steepness of the CH4 concentration gradient and CH4 flux at the soil surface (P < 0.001). Vertically, CH4 oxidation in the profile showed a distinct and consistent sub-surface maximum with highest uptake rates at 5-10 cm below the litter layer where soil air CH4 concentration is reduced to about 1 microliter l-1. The maximum was coincident with low average ammonium and nitrate concentrations (< 1 micromoles N g dry wt-1). Addition of ammonium and nitrate (> 1.2 micromoles N g dry wt-1) reduced CH4 oxidation in incubated soil. It is possible that CH4 oxidizing microbes are dismissed from the upper soil layer by high ammonium and nitrate concentrations, but other possibilities could not be excluded. Overall, CH4 flux at the soil surface was controlled by the dynamics of the CH4-oxidizing microbes and to some extent limitation of CH4 diffusion into the soil. In this loamy sand, diffusion limitation is probably operating mainly at high soil water content or at conditions resulting in high CH4-oxidizing activity, e.g. at high temperatures and in areas of high CH4-oxidizing potential.