Responses of N fluxes and pools to elevated atmospheric CO2 in model forest ecosystems with acidic and calcareous soils

The objectives of this study were to estimate how soil type, elevated N dep osition (0.7 vs. 7 g N m(-2)y(-1)) and tree species influence the potential effects of elevated CO2 (370 vs. 570 mu mol CO2 mol(-1)) on N pools and fl uxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in l arge open-top chambers. In the fourth year of treatment, we measured N conc entrations in the soil solution at different depths, estimated N accumulati on by ion exchange resin (IER) bags, and quantified N export in drainage wa ter, denitrification, and net N uptake by trees. Under elevated CO2, concen trations of N in the soil solution were significantly reduced. In the nutri ent-rich calcareous sand, CO2 enrichment decreased N concentrations in the soil solution at all depths (-45 to -100%). In the nutrient-poor acidic loa m, the negative CO2 effect was restricted to the uppermost 5 cm of the soil . Increasing the N deposition stimulated the negative impact of CO2 enrichm ent on soil solution N in the acidic loam at 5 cm depth from -20% at low N inputs to -70% at high N inputs. In the nutrient-rich calcareous sand, N ad ditions did not influence the CO2 effect on soil solution N. Accumulation o f N by IER bags, which were installed under individual trees, was decreased at high CO2 levels under spruce in both soil types. Under beech, this decr ease occurred only in the calcareous sand. N accumulation by IER bags was n egatively correlated with current-years foliage biomass, suggesting that th e reduction of soil N availability indices was related to a CO2-induced gro wth enhancement. However, the net N uptake by trees was not significantly i ncreased by elevated CO2. Thus, we suppose that the reduced N concentration s in the soil solution at elevated CO2 concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influen ced by atmospheric CO2 concentrations. CO2 enrichment decreased nitrate lea ching in drainage by 65%, which suggests that rising atmospheric CO2 potent ially increases the N retention capacity of forest ecosystems.