Organically bound nutrients in dissolved organic matter fractions in seepage and pore water of weakly developed forest soils

Previous field and laboratory studies showed that organically bound nutrien ts can contribute largely to the export of N, P, and S from soil into aquat ic systems. One possible determinant for the losses of dissolved organic nu trients leaving the soil environment could be their distribution between di ssolved organic matter (DOM) fractions of different mobility in soil. To el ucidate the potential influence of DOM fractions under varying flow conditi ons on the vertical translocation of organically bound nutrients, we determ ined the concentrations and fluxes of dissolved organic C (DOC) and nutrien ts (DON, DOP, DOS) in soil water under a Scots pine (Pinus sylvestris L.) a nd a European beech (Fagus sylvatica L.) forest. We sampled seepage water f rom the organic forest floor layer and the mineral subsoil using zero-tensi on lysimeters and soil pore water using tension lysimeters and suction cups . DOM in soil water was fractionated into hydrophilic and hydrophobic compo unds by XAD-8 at pH 2. We found that the organic forest floor layers were l arge sources for DOC, DON, DOP, and DOS. The dissolved organic nutrients we re mainly concentrated in the hydrophilic DOM fraction which proved to be m ore mobile in mineral soil pore water than the hydrophobic one. Consequentl y, the concentrations and fluxes of dissolved organic nutrients decreased l ess with depth than those of DOC. Concentrations as well as fluxes in subso il pore water of DOC and dissolved organic nutrients in the studied weakly developed soils were high as compared with literature data on deeply develo ped forest soils. Under conditions of rapid water flow through the strongly structured mineral soil at the beech site, almost no retention of DOM took place and thus the influence of the distribution of organically bound nutr ients between the DOM fractions on the export of DON, DOP, and DOS was negl igible.