Elevated atmospheric CO 2 and increased N deposition effects on dissolved organic carbon—clues from δ13C signature

Abstract

In the last growing season of a 4-yr experiment, the effects of elevated atmospheric CO 2 (370 vs. 570 μmol CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2. 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’.