Elevated atmospheric CO2 and increased N deposition effects on dissolved organic carbon—clues from δ13C signature

In the last growing season of a 4-yr experiment, the effects of elevated atmospheric CO2 (370 vs. 570 μmol CO2 mol−1) and increased N deposition (0.7 vs. 7 g N m−2 yr−1) on concentrations, fluxes, and properties of dissolved organic carbon (DOC) were investigated in two different forest soils. Model ecosystems with spruce and beech were established in large open-top chambers with lysimeters containing an acidic loam and calcareous sand. The added CO2 was depleted in 13C and thus, the net input of new C into soil organic matter (SOM) and DOC could be quantified. The effects of elevated CO2 and increased N deposition on concentrations and properties of DOC collected with suction cups at 5–10 cm depth were negligible. Export of DOC by drainage was reduced under elevated CO2 in the calcareous sand (−22%), but as concentrations were unaffected this was a secondary effect of a reduced drainage. The differences in DOC between the two soils were large, indicating that soil properties had a greater influence on DOC than the CO2 and N induced changes in above- and below-ground biomass. This was probably caused by the low input of ‘new’, less than 4-yr-old C into DOC. At 5–10 cm depth, the fraction of new C was 5±1% of total DOC in the acidic loam and 8±1% in the calcareous sand under elevated CO2. This indicates that potential sources of new C such as throughfall, litter leaching, and rhizodeposits made minor direct contributions to the DOC of the soil solution during the four experimental years. The fractions of new C in DOC at 5–10 cm depth were significantly smaller than those in SOM at 0–10 cm depth, which accounted for 22±2% of total SOM in the acidic loam and for 17±2% in the calcareous sand. The fractions of new C in water-soluble organic C (WSOC) of the bulk soil at 0–10 cm (25–29%) were larger than in DOC, showing that the origins of WSOC and DOC were different. The losses of new C through drainage (30 mg C m−2 yr−1) were small in comparison to the net input of new C into SOM (240–400 g C m−2 4 yr−1 at 0–10 cm depth). This strongly suggests that exports of DOC are not a major part of the ‘missing carbon sink’.