In the last growing season of a four-year experiment, we investigated the effects of elevated atmospheric CO₂ (370 vs. 570 μmol CO₂ morl^-1) and increased N deposition (0.7 vs. 7 g Nm^-2y^-1) on DOC in an acidic loam and a calcareous sand of model forest ecosystems in large open-top chambers with lysimeters. The added CO₂ was depleted in ¹³C and thus, the net input of 'new' C into DOC could be quantified.
The effects of elevated CO₂ and increased N deposition on concentrations of DOC were negligible, while the differences between the two soils were large. The input of 'new' C into DOC during the 4 years was small. At 5-10 cm depth, the fractions of new C in DOC were between 5 and 8%, whereas the fractions of new C in soil organic C of 0-10 cm depth ranged between 17-22%. The most likely reason for the low input of new C into DOC was that the DOC sampled with suction cups was not in equilibrium with the bulk soil. This was indicated by the greater proportions of new C in water soluble organic C of the bulk soil that were between 25 and 28%. Our results show that soil properties had a greater influence on DOC than the changes in above and below-ground biomass induced by the CO2 enrichment and increased N deposition.