Dissolved organic carbon control in acid forest soils of the Fichtelgebirge (Germany) as revealed by distribution patterns and structural composition analyses

Abstract

Dissolved organic carbon (DOC) was investigated in different compartments of three acid forest soils of the Fichtelgebirge (northeastern Bavaria, Germany) developed under Norway spruce stands. Soil acidification increases from Wülfersreuth (Wü) over Oberwarmensteinach (Ow) to Hohe Matzen (HM). In order to assess factors of DOC control, DOC concentrations and fluxes in the compartments were measured. The structural composition of DOC was evaluated by using wet-chemical methods, CPMAS 13C NMR spectroscopy, pyrolysis-field ionization mass spectrometry, and analytical DOC fractionation. DOC is mainly mobilized in the forest floor; it exhibits high spatial and temporal variation in concentrations and fluxes. Mean annual DOC concentrations of the mineral soil input vary between 2.25 mmol C l −1 in the less acid soils Wü and Ow and 4.53 mmol C l −1 in the highly acidified HM soil. Chemical investigations suggest a biotic control of DOC release, which is in turn influenced by environmental factors. Mobilized DOC consists of highly degraded plant material and microbially synthesized products, but still occurs mainly as macromolecules. Enhanced anthropogenic N inputs and crown thinning seem to accelerate the DOC release in the HM forest floor due to stimulated microbial activity. Furthermore, solubility of organic carbon is enhanced by the decline of divalent and polyvalent cation bridging of organic compounds due to exchange processes with protons. Contact of the soil solution with the mineral B horizons drastically reduce the DOC concentrations to 0.14–0.84 mmol C l −1 mainly by abiotic mechanisms (precipitation, sorption by Fe- and Al-oxides). Between 14.1 and 32.5 g C m −2 yr −1 are translocated into the B horizons. DOC in the mineral soil output corresponds well with DOC in the headwater stream. While in the less acidified soils Wü and Ow most of the DOC is retained in the Bs horizon, the highly podzolized HM soil shows a lower DOC retention capacity over the whole solum and still exhibits an unusually high DOC output of 6.6 g C m −2 yr −1. It is suggested that the elevated content of organic carbon surrounding the oxides, and competition of sulfate with components of DOC for sorption sites is partly responsible for the reduced DOC retention in this soil.