Studies of the soil atmosphere and related physical site characteristics in mineral forest soils

The composition of the soil atmosphere in mineral soils was studied for 3 years, by regular sampling from gas equilibrium chambers followed by gas chromatographic analysis. In sandy loam till soils with different groundwater levels, the mean growing season concentrations of O² decreased and CO² increased as site wetness increased. Similarly, O² decreased and CO² increased with increasing soil depth on freely drained sites. On sites where the subsoil was often waterlogged, concentration gradients between 20 and 50 cm below the mineral soil surface were often reversed, i.e. O² increased and CO² decreased with increasing depth. Periods of rapidly falling O² and rising CO² concentrations were the most conspicuous characteristic of the annual pattern in the mor and the upper mineral soil layers. The main cause is suggested to be impeded gas exchange, due to near–saturation water content. Such periods often started after snowmelt in May and ended after the disappearance of soil frost in June. The porosity of the frozen topsoil in spring, which is affected by the climate in early winter and spring, is proposed as a particularly important ecological factor in these soils. A silt soil and the subsoils of tills with high groundwater levels showed growing season trends, with minimum O² and maximum CO² contents in the late autumn. Concentrations changed temporarily, towards those of the free atmosphere, in the till subsoils during summer droughts. Clearfelling caused a rise of the watertable. O² concentrations decreased and CO² concentrations increased in the humus layer, while the changes in the mineral soil were less clear. The soil atmosphere, represented by the mean growing season concentrations and the minimum O²/maximum CO² concentrations during each growing season, were significantly different on all main sites, and also differed between clearfelled and forested plots.