Effect of moisture on leaf litter decomposition and its contribution to soil respiration in a temperate forest

The degree to which increased soil respiration rates following wetting is caused by plant ( autotrophic) versus microbial ( heterotrophic) processes, is still largely uninvestigated. Incubation studies suggest microbial processes play a role but it remains unclear whether there is a stimulation of the microbial population as a whole or an increase in the importance of specific substrates that become available with wetting of the soil. We took advantage of an ongoing manipulation of leaf litter C-14 contents at the Oak Ridge Reservation, Oak Ridge, Tennessee, to ( 1) determine the degree to which an increase in soil respiration rates that accompanied wetting of litter and soil, following a short period of drought, could be explained by heterotrophic contributions; and ( 2) investigate the potential causes of increased heterotrophic respiration in incubated litter and 0-5 cm mineral soil. The contribution of leaf litter decomposition increased from 6 +/- 3 mg C m(-2) hr(-1) during a transient drought, to 63 +/- 18 mg C m(-2) hr(-1) immediately after water addition, corresponding to an increase in the contribution to soil respiration from 5 +/- 2% to 37 +/- 8%. The increased relative contribution was sufficient to explain all of the observed increase in soil respiration for this one wetting event in the late growing season. Temperature (13 degrees C versus 25 degrees C) and moisture ( dry versus field capacity) conditions did not change the relative contributions of different decomposition substrates in incubations, suggesting that more slowly cycling C has at least the same sensitivity to decomposition as faster cycling organic C at the temperature and moisture conditions studied.