PROCEEDINGS: Index of Abstracts

EXPERIMENTAL SOIL WARMING EFFECTS ON C, N, AND MAJOR ELEMENT CYCLING IN A LOW ELEVATION SPRUCE-FIR FOREST SOIL

Lindsey E. Rustad, Ivan J. Fernandez, and Stephanie Arnold

Department of Applied Ecology and Environmental Sciences, University of Maine, Orono, ME 04469.

The effect of global warming on north temperate and boreal forest soils has been the subject of much recent debate. These soils serve as major reservoirs for C, N, and other nutrients necessary for forest growth and productivity. Given the uncertainties in estimates of organic matter turnover rates and storage, it is unclear whether these soils will serve as short or longer-term net sources or sinks for C and N if mean air and soil temperatures increase over time. In light of these information needs, a thermal manipulation study was initiated in 1991 at the Howland Integrated Forest Study (HIFS) site to investigate the effect of a 5^oC increase in soil temperature on C and N dynamics in a low elevation spruce-fir forest soil. Elevated soil temperatures have been successfully maintained in replicated 15x15 m plots for two field seasons (1993 and 1994) using heat resistance cables buried 2-3 cm from the soil surface at 20 cm spacings. Replicated unheated plots with cables installed ("cabled control") and with no cable installation ("control") serve as the controls. Results to date indicate significantly increased rates of litter decay, fine root production, and CO₂ evolution in the heated plots relative to the controls as well as decreased concentrations of base cations and Mn in buried mineral soil bags. Soil moisture showed a slight but significant decrease in the O horizon in response to the thermal manipulations and no change in the upper B horizon. Although no statistically significant effect of the thermal manipulation has been observed on N mineralization rates during the first two years of this study, the cumulative amount of NH₄^-N mineralized over this period was greater in the heated plots relative to the control plots. No net nitrification has been observed at this site to date. Taken together, results from this thermal manipulation study indicate that modest changes in temperature can significantly alter C, N, and major nutrient dynamics at this site.