Budgets and chemistry of dissolved organic carbon in forest soils: effects of anthropogenic soil acidification

Abstract

Budgets and chemistry of dissolved organic carbon (DOC) are compared at three spruce stands receiving different rates of acid deposition. Generally, an increasing input of mineral acids is associated with increased soil acidification and an increased degree of forest decline phenomena. In order to assess the effects of anthropogenic soil acidification on DOC control, we combined measurements of DOC concentrations and fluxes with an analytical approach in terms of DOC fractionation, wet-chemical degradation, and pyrolysis-field ionization mass spectrometry. Mean annual DOC concentration in the mineral soil input varies around 27.0 mg 1 −1 at the less damaged stands, and reaches 59.4 mg 1 −1 at the severely damaged stand. We suggest a biotic control of DOC release in the forest floor, which is in turn influenced by environmental conditions. Enhanced anthropogenic N deposition and crown thinning increase the DOC release in the forest floor due to a stimulation of microbial organic matter mineralization. Furthermore, N hampers the ligninase production which results in the accumulation of partly water-soluble soft-rot lignin-degredation products. Sorption of DOC by sesquioxides in the B horizons reduces the mean annual DOC concentration to values between 2.63 mg 1 −1 in the least acidified soil and 31.15 mg 1 −1 in the most acidified soil. In this soil DOC retention is impeded by soil indigenous factors such as covering of sesquioxides with organic carbon. Moreover, anthropogenic soil acidification results in an increasing DOC output of the mineral soil due to (i) elevated DOC release in the forest floor, (ii) competition of DOC with sulfate for sorption sites, and (iii) loss of Al and Fe oxides/hydroxides due to buffer processes. The less acidified mineral soils are characterized by a pronounced preferential retention of polymeric hydrophobic acids. In contrast, the soil receiving the highest rates of acid deposition shows only a minor qualitative alteration of DOC in the profile.