Dissolved soil organic nitrogen and carbon in a Norway spruce stand and anadjacent clear-cut

The aims of this study were to characterize dissolved soil organic N (DON) and C (DOC) in a coniferous stand and an adjacent clear-cut, and to evaluat e the importance of DON in N leaching. The study was carried out in a Norwa y spruce stand and a clear-cutting treatment in the same forest stand. Conc entrations of DON in soil solution were monitored for 5 years after clear-c utting with gravity lysimeters. In the Norway spruce stand DON comprised 62 -83% of the total N in soil solution over the 5-year period. The concentrat ions of DON in the clear-cut were higher than in the forest stand, but the proportion of total N was lower. To characterize dissolved organic matter, soil samples were aerobically incubated for 6 weeks in the laboratory, and the quantity, molecular size distribution and chemical nature of both DON a nd DOC were determined from water extracts made before and after the incuba tion. In the soil samples from the Norway spruce stand, C-rich compounds wi th a high C/N ratio and large molecular size were formed. In contrast, afte r the incubation the major carriers of DON in soil samples from the clearcu t were N-rich organic compounds with a low C/N ratio and a small molecular size. The distribution of different chemical fractions of DOC in soil did n ot differ much whether recovered from the Norway spruce stand or the clear- cut. It was (from highest to lowest concentration): hydrophobic acids>hydro philic acids>phenols>hydrophilic neutrals. A major part of DON was also car ried by these fractions. During incubation the concentration of N-containin g hydrophilic acids increased, especially in the soil from the clearcut. In soil samples from the Norway spruce stand, the rate of net N mineralizatio n was low and no NO3 was formed, whilst the rate of net N mineralization wa s high and net nitrification was intensive in soil from the clear-cut.