Seasonal variation in the soil respiration rate in coniferous forest soils

In order to evaluate the seasonal variation in the soil respiration rate, we established three study plots in three different forest site types. Soil samples were taken weekly from the study plots during one growing season. Seasonal variation in soil chemical properties was small, the only clear trends being in the NH4–N and NO3–N concentrations. The soil respiration rates decreased during spring and summer, the minimum values occurring at the end of August. The soil respiration rates again increased during the autumn, but did not reach values close to those in the spring. Adjusting the moisture content of the samples to 60% of water holding capacity (WHC), reduced the variation in the soil respiration rate. When the soil respiration rate was measured at a constant temperature (14 °C), the respiration rate was regulated by moisture and pH. When the moisture content was kept constant (WHC 60%), the respiration rate was mainly controlled by the amount of organic matter (N and Corg) and pH. When the respiration rate was calculated on the basis of the N concentration, the variation was mainly explained by pH. The possible effects of environmental pollution, e.g. anthropogenic soil acidification, on the respiration rate can be identified by keeping the moisture content and temperature constant, and calculating the results on an organic matter basis (N and Corg).