Examination of the effects of moisture and temperature on soil organic matter decomposition: Results of a long-term incubation of shallow and deep red spruce forest soil cores

Temperature and moisture are primary environmental drivers of soil organic matter (SOM) decomposition, but in temperate coniferous forests, the response of SOM decomposition to changes in soil climate remains poorly understood. Recent evidence from a red spruce chronosequence indicates increased mineralization of deep soil C pools post-harvest, highlighting the need to understand the physical controls on decomposition processes through depth. The objectives of this research were to determine the influence of soil moisture on heterotrophic soil respiration in shallow and deep soil microcosms and to determine the effect of soil moisture on the temperature sensitivity of soil respiration. This study also evaluated the use of soil microcosms to provide realistic information about soil processes and relationships with temperature and moisture. Minimally disturbed soil cores from shallow (0-25 cm) and deep (25-50 cm) layers were extracted from a 20 year old red spruce stand and were then transferred to a climate chamber where they were incubated for 3 months under constant and diurnal temperature regimes. Soils subjected to different watering treatments representing a range of water contents. Temperature, moisture, and CO2 surface flux were assessed daily for all soils and continuously on a subset of the microcosms. The results from this study indicate that shallow soils dominate the contribution to surface flux (90%) and respond more predictably to moisture than deep soils. An optimum moisture range of 0.15 to 0.60 water-filled pore space was observed for microbial SOM decomposition in shallow cores. For soil moisture conditions experienced by most field sites, flux-temperature relationships alone can be used to reasonably estimate heterotrophic respiration, as in this range moisture does not alter flux, with the exception of rewetting events along the lower part of this optimal range. Outside this range, however, soil moisture determines SOM decomposition rates.