Measuring the Interactive Effects of Warming and Wetting On Soil C and N Losses and Microbial Communities Following Forest Harvesting.

Forest harvest is a globally widespread form of land use change that may interact with a changing climate in complex ways. Upon whole-tree harvest, forest soils are susceptible to carbon (C) and nitrogen (N) losses owing to increased decomposition and decreased inputs from vegetation.  General circulation models predict not only an increase in temperature, but also changes in precipitation in the northeastern United States. Increases in soil temperature and moisture, separate from land-use effects, have been shown to increase C and N mineralization.  Therefore, it is plausible that predicted regional climate change could exacerbate post-harvest C and N losses in some areas such as northeastern forests.

A post-harvest climate manipulation experiment was established in Penn State University’s Stone Valley Forest, in Huntingdon County Pennsylvania.  The experimental design included 4 treatments: heated, irrigated, heated+irrigated, and ambient (all with 4 blocked replicates).  Target surface temperature increases for heated treatment plots were set for daytime and nighttime at 1.5 °C and 3.0 °C respectively.  Irrigated plots received precipitation additions of 20% of long-term average, and heated+irrigated plots receive both warming and augmented precipitation. Our objectives are to 1) quantify the changes in the C and N pools and fluxes due to increased temperature and precipitation treatments to a harvested forest and 2) understand how changes in C and N cycling are linked to changes in soil microbial community composition and enzyme activity.

Average annual increase in soil temperature was 2.85 °C at 5 cm depth. After one year of treatment, soil respiration rates were significantly lower in the heated+irrigated plots and higher in heated plots.  Heating decreased soil respiration sensitivity to temperature (Q₁₀). Irrigation alone shows no significant effect on soil respiration. Heating alone has slightly, but significantly, increased the percentage of labile C per g soil organic matter.  These preliminary results suggest that climate change may increase initial soil C loss via respiration.  Further soil analyses and vegetation data will provide a more robust understanding of entire ecosystem C and N loss from harvested forests in a warmer and wetter world.