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).

Stable isotope analysis reveals differential effects of soil nitrogen and nitrogen dioxide on the water use efficiency in hybrid poplar leaves.

• The effects of nitrogen dioxide (NO2 ) exposure are reported on the physiology, morphology and carbon partitioning of hybrid poplar clone cuttings (Populus ×euramericana) grown under high and low soil nitrogen supply. • Plants were exposed to filtered air or NO2 -enriched air (80-135 nl l-1 ) over 12 wk growth in phytotrons. Stable isotope analysis, combined with CO2 and H2 O gas exchange measurements, biomass analysis and morphological development, was used to assess the integrated long-term effects of NO2 . • NO2 had no toxic effects. A reduced 15 N-isotope ratio indicated incorporation of NO2 while nitrate reductase activity in leaves was stimulated. The two nitrogen sources had differential effects on water use efficiency (WUE): NO2 exposure increased long-term WUE; soil N supply decreased WUE; a result not detectable using growth and short-term gas exchange experiments. Plants benefited from airborne NO2 , increasing CO2 assimilation rate and biomass; both N sources increased shoot production at the expense of root growth. NO2 exposure induced leaf formation with reduced stomatal density and increased leaf area. • NO2 exposure might be beneficial although the reduced root: shoot biomass could have a detrimental effect on nutrient balance and drought resistance.