Carbon sequestration in forest soils: effects of soil type, atmospheric CO ₂ enrichment, and N deposition

Summary Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO 2 and to increased N deposition is not completely understood. We studied the effects of CO 2 concentrations at 370 and 570 μmol CO 2 mol −1 air and increased N deposition (7 against 0.7 g N m −2 year −1 ) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open‐top chambers containing an acidic loam and a calcareous sand. The added CO 2 was depleted in 13 C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m −2 ) exceeded that in the calcareous sand (340 ± 40 g C m −2 ) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m −2 in the acidic loam and for 2800 ± 170 g C m −2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico‐chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m −2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO 2 enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO 2 concentrations increased C contents by 50–170 g C m −2 . Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO 2 concentrations had no significant impact on soil C concentrations. Up to 40% of the ‘new’ carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO 2 ‐derived ‘new’ C was incorporated into the labile and easily mineralizable pool in the soil.