Composition and reactivity of DOC in forest floor soil solutions in relation to tree species and soil type

Metal coordinating properties of DOC (dissolved organic carbon), and hence its influence on heavy metal release and mineral weathering, depend on the composition and properties of DOC. Tree species produce litter with different chemical composition and degradability, and these differences might influence the composition and reactivity of DOC in soil solutions. Accordingly, analysis of composition and reactivity of DOC in soil solution samples collected by centrifugation from 16 forest soil O horizons from the four tree species beech (Fagus sylvatica L.), oak (Quercus robur L.), grand fir (Abies grandis Lindl.), and Norway spruce (Picea abies (L.) Karst.) on two clayey and two sandy soils were carried out. The composition and properties of DOC were determined by capillary zone electrophoresis, acid-base titration, Cu ion titration, total elemental analysis, IR and UV spectroscopy, and metal release assays. Concentrations of DOC ranged from 20 to 163 mM with pH ranging from 4.6 to 7.3. Norway spruce produced the highest DOC concentration, and the lowest pH. Carbon in low-molecular-weight aliphatic carboxylic acids (LACA) accounted for less than 6% of DOC with formic and acetic acids as the most abundant LACAs. The DOC was cation exchanged and proton saturated to obtain comparable forms of DOC. Titratable carboxylic acid and phenolic groups were in the range 51 to 82 and 20 to 64 mmol.mol(-1) C, respectively, with fewer phenolic groups in grand fir DOC as the only significant difference. Infrared spectra of freeze-dried DOC samples suggest low contents of aromatic C in the DOC especially from grand fir stands. Stability constants, log K of Cu-DOC complexes, determined by Cu ion titration of DOC samples with fitting of the data to a two-site binding model, were in the range 5.63 to 6.21 for the strong binding sites and 3.58 to 4.10 for the weak sites, but with no significant effects of tree species or site. Freeze-dried DOC samples were found to consist of 41 to 45% C, 38 to 49% O, 4.4 to 5.4% H and 1.2 to 2.0% N and C/N ratios in the range 26 to 42. Reactivity of DOC in terms of release rates of Cd, Cu and Fe cations from a soil sample (flow cell experiments) showed no significant differences among DOC samples from different tree species and soil types. Apparently, only minor differences occur in chemical composition and reactivity of equivalent concentrations of DOC in forest floor soil solutions irrespective of origin, i.e. four tree species and two soil types. Soil solution pH and the concentration of DOC produced by various tree species are the critical parameters when distinguishing among tree species in relation to heavy metal release and mineral weathering.