Annual variation in soil respiration in selected areas of the temperate zone

A 6-year record of soil respiration and CO₂ concentration measurements in various soils with different plant cover is presented. The effect of temperature and soil-moisture content is discussed; for this, ²²²Rn-flux and concentration measurements are used to trace gas transport in the unsaturated soil zone. The CO₂ production in the soil is strongly correlated with the subsoil temperature. Q₁₀-values vary between 1.4 and 2.7, depending on mean annual temperature and precipitation. Soil respiration is reduced not only by extremely dry but also by extremely wet soil conditions. This is probably due to an evolution adaptation of soil microorganism to the long-time mean meteorological situation. The regional mean annual cycle of soil respiration representative for the Rhine Valley area near Heidelberg is deduced from spot measurements in this region. Minimum soil respiration occurs in February; the maximum CO₂ production is in June and July. A steep increase of the respiration curve in spring is followed by a slow decrease from summer to winter. Even during the coldest winter months, soil respiration does not reach zero, but makes up 10% to 15% of the maximum respiration rate. The regional annual mean value of the soil CO₂ production is 6 mmol m^-2 h^-1; the summer mean value is 9 mmol m^-2 h^-1