Soil solution and extractable soil nitrogen response to climate change in two boreal forest ecosystems

Several studies show that increases in soil temperature result in higher N mineralization rates in soils. It is, however, unclear if additional N is taken up by the vegetation or accumulates in the soil. To address this question two small, forested catchments in southern Norway were experimentally manipulated by increasing air temperature (+ 3 degrees C in summer to + 5 degrees C in winter) and CO2 concentrations (+ 200 ppmv) in one catchment (CO2T-T) and soil temperature (+ 3 degrees C in summer to + 5 degrees C in winter) using heating cables in a second catchment (T-T). During the first treatment year, the climate treatments caused significant increases in soil extractable NH4 under Vaccinium in CO2T-T. In the second treatment year extractable NH4 in CO2T-T and NO3 in T-T significantly increased. Soil solution NH4 concentrations did not follow patterns in extractable NH4 but changes in soil NO3 pools were reflected by changes in dissolved NO3. The anomalous behavior of soil solution NH4 compared to NO3 was most likely due to the higher NH4 adsorption capacity of the soil. The data from this study showed that after 2 years of treatment soil inorganic N pools increased indicating that increases in mineralization, as observed in previous studies, exceeded plant demand and leaching losses.