Abstract. Dissolved organic matter (DOM) is part of the biogeochemical cycles of carbon
and nutrients, carries pollutants and drives soil formation. The DOM
concentration and properties along the water flow path through forest
ecosystems depend on its sampling location and transformation processes. To
improve our understanding of the effects of forest management, especially
tree species selection and management intensity, on DOM concentrations and
properties of samples from different ecosystem fluxes, we studied
throughfall, stemflow, litter leachate and mineral soil solution at 26 forest
sites in the three regions of the German Biodiversity Exploratories. We
covered forest stands with three management categories (coniferous, deciduous
age class and unmanaged beech forests). In water samples from these forests,
we monitored DOC concentrations over 4 years and characterized the quality
of DOM with UV-vis absorption, fluorescence spectroscopy combined with
parallel factor analysis (PARAFAC) and Fourier transform ion cyclotron
resonance mass spectrometry (FT-ICR-MS). Additionally, we performed
incubation-based biodegradation assays. Multivariate statistics revealed
strong significant effects of ecosystem fluxes and smaller effects of main
tree species on DOM quality. Coniferous forests differed from deciduous
forests by showing larger DOC concentrations, more lignin- and protein-like
molecules, and fewer tannin-like molecules in throughfall, stemflow, and
litter leachate. Cluster analysis of FT-ICR-MS data indicated that DOM
compositions, which varied in aboveground samples depending on tree species,
become aligned in mineral soil. This alignment of DOM composition along the
water flow path in mineral soil is likely caused by microbial production and
consumption of DOM in combination with its interaction with the solid phase,
producing a characteristic pattern of organic compounds in forest mineral
soils. We found similarly pronounced effects of ecosystem fluxes on the
biodegradability of DOM, but surprisingly no differences between deciduous
and coniferous forests. Forest management intensity, mainly determined by
biomass extraction, contribution of species, which are not site-adapted, and
deadwood mass, did not influence DOC concentrations, DOM composition and
properties significantly.
The impact of four decades of annual nitrogen addition on dissolved organic matter in a boreal forest soil