Soil respiration in temperate forests is increased by a shift from coniferous to deciduous trees but not by an increase in temperature

Abstract The rate of litter and soil organic matter decomposition depends on temperature and on leaf carbon content and its availability to decomposers. The effects of these variables on decomposition in temperate forest soils are incompletely understood although future increases are predicted for temperature, the ratio of litter fractions, and the ratio of deciduous to coniferous forests. In this study, a 14-month laboratory incubation experiment of adding deciduous (beech) and coniferous (spruce) litter to soil (Cambisol) was carried out to test the effects of litter fraction, tree species, and temperature on soil respiration (as indicated by CO2 release). The addition of the soluble and insoluble litter fractions as well as the whole tissue caused an increase in soil respiration (relative to the no-addition treatment). The effect of the soluble fraction, however, was less persistent than the effects of whole tissue or insoluble fractions, which suggests generally greater long-term effects of the latter fractions on soil respiration. Fractions derived from the beech litter increased soil respiration more than those derived from the spruce litter. This trend was especially apparent with the soluble litter fraction. A temperature increase of 3 °C caused no difference in soil respiration after the addition of beech litter fractions and even reduced soil respiration after the addition of spruce litter fractions. These results suggest that a future increase in the soluble fraction coupled with a shift in tree species composition from coniferous to deciduous might lead to an increase in soil respiration and thus to an increase in CO2 release from the forest floor. The results, however, also suggest that a 3 °C increase in temperature will not lead to increases in CO2 release from the forest floor, even if deciduous trees become more abundant.