Elevated CO₂ influences nutrient availability in young beech-spruce communities on two soil types

The objectives of this study were to investigate how different soil types and elevated N deposition (0.7 vs 7 g N m^-2a^-1) influence the effects of elevated CO₂ (370 vs 570 µmol CO₂ mol^-1) on soil nutrients and net accumulation of N, P, K, S, Ca, Mg, Fe, Mn, and Zn in spruce (Picea abies) and beech (Fagus sylvatica). Model ecosystems were established in large open-top chambers on two different forest soils: a nutrient-poor acidic loam and a nutrient-rich calcareous sand. The response of net nutrient accumulation to elevated atmospheric CO₂ depended upon soil type (interaction soil × CO₂, P<0.05 for N, P, K, S, Ca, Mg, Zn) and differed between spruce and beech. On the acidic loam, CO₂ enrichment suppressed net accumulation of all nutrients in beech (P<0.05 for P, S, Zn), but stimulated it for spruce (P<0.05 for Fe, Zn) On the nutrient-rich calcareous sand, increased atmospheric CO₂ enhanced nutrient accumulation in both species significantly. Increasing the N deposition did not influence the CO₂ effects on net nutrient accumulation with either soil. Under elevated atmospheric CO₂, the accumulation of N declined relative to other nutrients, as indicated by decreasing ratios of N to other nutrients in tree biomass (all ratios: P<0.001, except the N to S ratio). In both the soil and soil solution, elevated CO₂ did not influence concentrations of base cations and available P. Under CO₂ enrichment, concentrations of exchangeable NH₄⁺ decreased by 22% in the acidic loam and increased by 50% in the calcareous sand (soil × CO₂, P<0.001). NO₃^- concentrations decreased by 10-70% at elevated CO₂ in both soils (P<0.01).