Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (D OC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and D ON from forest floors and their environmental controls as well as the dynam ics of concentrations and fluxes are still poorly understood. We investigat ed the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15-20 cm height were taken with 7 replicates from the forest floor of a mat ure Norway spruce (Picea abies [L.] Karst.) site and established at 10 degr eesC in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d(-1) with a natural throughfall soluti on. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH4, NO3, pH, electrical conductivity and major ions. Dry ing for 10 and 20 days decreased the water content of the Oi horizon from 2 80% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest fl oor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contra ry, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest fl oor percolates.