The CLIMEX soil-heating experiment: soil response after 2 years of treatment

Most model predictions concerning the response of boreal forest ecosystems to climate change are inferred from small-scale experiments on artificial, simplified systems. Whole-ecosystem experiments designed to validate these models are scarce. We experimentally manipulated a small forested catchment in southern Norway by increasing soil temperature (+3 degrees C in summer to +5 degrees C in winter) using heating cables installed at 1 cm depth in the litter layer. Especially nitrification in the 0 to 10-cm soil layer inc reased as a result of the climate manipulation. Betula litter, produced aft er exposing trees for 2 years to ambient and elevated CO2 in greenhouses, w as incubated for 1 year in the manipulated catchment. Exposure to elevated CO2 did not affect the C/N ratio or decomposition of the Betula litter, but lignin content decreased by 10%. We found no effect of elevated temperatur e on litter decomposition, probably due to desiccation of the litter. The h eating cables caused a permanent increase in soil temperature in this soil layer, but when soils were dry, the temperature difference between control and heated plots decreased with increasing distance from the cables. When s oils were wet, no gradients in temperature increase occurred.