Effect of increased carbon dioxide and temperature on runoff chemistry at a forested catchment in southern Norway (CLIMEX project)

CLIMEX (Climate Change Experiment) is an integrated, whole-ecosystem research project that focuses on the response of forest ecosystems at the catchment scale to increased CO2 and temperature. I(IM catchment (860 m(2)) is completely enclosed by a transparent greenhouse, receives deacidifed "clean" rain, and has elevated CO2 (560 ppmv) and elevated air temperature (3 degrees-5 degrees C above ambient). The uppermost 20% of the catchment is partitioned off, is not subject to changed CO2 or temperature, and serves as an untreated control. Fluxes of nitrate and ammonium in runoff from I(IM catchment increased from 2 mmol m(-2) y(-1) each in the 3 years before treatment to 6 and 3 mmol m(-2) y(-1), respectively, in the 3 years after treatment (May 1994-April 1997), despite a 15 mmol m(-2) y(-1) decrease in N dry deposition due to the sealing of the walls to the enclosure. N flux in runoff from three reference catchments and the control section did not change. The net loss of inorganic N was thus about 20 mmol m(-2) treated soil y(-1). There were no changes in organic N or total organic carbon in runoff. The ecosystem switched from a net sink to a net source of inorganic nitrogen (N). The increased loss of N may be due to accelerated decomposition of soil organic matter induced by higher temperature. Due to many decades of N deposition from long-range transported pollutants, the ecosystem prior to treatment was N saturated. If global change induces persistent losses of inorganic N on a regional scale, the result may be a significant increase in nitrate concentrations in fresh waters and N loading to coastal marine ecosystems. In regions with acid sensitive waters, such as southern Norway, the increased nitrate release caused by global change may offset improvements achieved by reduced sulfur and hi deposition.