ACIDIFICATION EFFECTS ON DISSOLVED ORGANIC-MATTER MOBILITY IN SPRUCE FOREST ECOSYSTEMS

Fluxes and composition of dissolved organic matter (DOM) were compared at three differently acid-loaded podzolic forest soils in the Fichtelgebirge, Bavaria, Germany. DOC fluxes are highest in all compartments at the Hohe Matzen site, which receives the highest amounts of acid deposition. At this site, the pronounced DOM mobilization in the forest floor (31.7 kmol C ha(-1) y(-1)) goes along with a net N-tot and SO42- release, thus indicating humus disintegration. Structural chemical investigations suggest that DOM mobilization in the forest floor is mainly controlled by microbial activity. The high N deposition at Hohe Matzen (2.3 kmol N-tot ha(-1) y(-1)) generally stimulates microbial activity as shown by the elevated mobilization of microbial carbohydrates. From CuO oxidation data and field-ionization mass spectra, iris hypothesized that a second effect of the high N loadings is the retardation of ligninolysis by white-rot fungi and the accumulation of soft-rot degradation products. The latter have been shown in the literature to react with organic N-compounds and to be highly water-soluble. In the mineral soil, DOM is retained mainly by sorption and precipitation processes. Most important for an effective DOM retention are high contents of pedogenic Fe and Al oxides/hydroxides. According to pyrolysis data, high molecular compounds are selectively retained in the B horizons. Blockade of sorption sites by formerly adsorbed DOM and SO42- can reduce sorption capacity for DOM. At the Hohe Matzen site, the Bh horizon grows downwards the profile due to progressive saturation of the sorption sites by DOM and SO42-, and due to depletion of pedogenic Fe and Al by buffering processes. The reduced DOM retention is also expressed by the breakthrough of high molecular compounds at Hohe Matzen.