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 r esearch project that focuses on the response of forest ecosystems at t he catchment scale to increased CO2 and temperature. I(IM catchment (8 60 m(2)) is completely enclosed by a transparent greenhouse, receives deacidifed ''clean'' rain, and has elevated CO2 (560 ppmv) and elevate d air temperature (3 degrees-5 degrees C above ambient). The uppermost 20% of the catchment is partitioned off, is not subject to changed CO 2 or temperature, and serves as an untreated control. Fluxes of nitrat e 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 t he sealing of the walls to the enclosure. N flux in runoff from three reference catchments and the control section did not change. The net l oss of inorganic N was thus about 20 mmol m(-2) treated soil y(-1). Th ere were no changes in organic N or total organic carbon in runoff. Th e ecosystem switched from a net sink to a net source of inorganic nitr ogen (N). The increased loss of N may be due to accelerated decomposit ion of soil organic matter induced by higher temperature. Due to many decades of N deposition from long-range transported pollutants, the ec osystem prior to treatment was N saturated. If global change induces p ersistent 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 a nd hi deposition.