Biotic and abiotic factors controlling soil respiration rates in Picea abies stands

The response of soil respiration to varying environmental factors was studied in four Picea abies stands (47-, 87-, 111- and 146-year old) during the 1998 growing season. While within-site variations of soil CO2 efflux (up to 1.6 μmol CO2 m−2 s−1) were larger than their diurnal variability (<0.25 μmol CO2 m−2 s−1), spatial variations within a site were smaller than seasonal changes in soil respiration rates (up to 4.4 μmol CO2 m−2 s−1). Highest within-site variability of soil efflux was generally found during the summer months when maximum flux rates of 4–6 μmol CO2 m−2 s−1 were reached (coefficient of variation 40%). Soil temperatures (in the Of and Oh layers, and Ah horizon) showed a pronounced seasonal course, in contrast to soil moisture. An exponential equation best described the relationships between soil temperature in the Of layer and soil CO2 efflux (r2 between 0.75 and 0.81). However, an Arrhenius type equation always resulted in lower r2 values (0.52–0.71). The Q10 values ranged between 2.39 (146-year old stand) and 3.22 (87-year old stand), averaging 2.72 for the P. abies stands within the watershed. The removal of litter and organic layers generally affected soil CO2 efflux negatively. In three of the four P. abies stands (47-, 87-, 146-year old stands), soil respiration rates were reduced by 10–20% after removal of the L and Of layer, and by 30–40% after removal of the L and most of the Of and Oh layers. Thus, mineral soil respiration seemed to contribute a major fraction to the total soil CO2 flux (>60%). Trenching shallow fine roots during collar insertion and mechanical inhibition of root in-growth during the following months allowed fine root respiration to be separated from microbial respiration only in times of highest root growth. Microbial respiration seemed to dominate the respiratory CO2 loss from the forest floor (>70%). The comparison of the annual soil CO2 efflux in the 47-year old P. abies stand (about 710 g C m−2 yr−1) with annual litterfall and root net primary productivity estimates supported this conclusion.