Rhizosphere effects of tree species - Large reduction of N2O emission by saplings of ash, but not of beech, in temperate forest soil

Previous research has shown that N2O emissions from forest soils can vary considerably with forest type. It is not yet known whether these variations are mainly reflecting differences between needle-leaved and broad-leaved forests or if there are also flux differences among different broad-leaved tree species. Furthermore, it remains unclear if these differences are merely caused by species-specific leaf litter effects, or whether root-related traits are also influential. We conducted two laboratory experiments with soil from a temperate broad-leaved forest to examine the effects of ash saplings (Fraxinus excelsior L) on N2O emissions at ambient and high nitrate availability (experiment A), and to compare the effects of ash, beech (Fagus sylvatica L.) and ash/beech mixture on N2O emissions (experiment B). In both experiments, a large reduction in N2O efflux was found for the ash treatments as compared to root-free soil (by 94% at ambient nitrate availability and by 98% after the addition of 200 kg N ha(-1) KNO3). The suppressing effect of ash saplings was larger than the stimulating effect of nitrate addition on N2O emissions. Soil planted with beech saplings, tended to reduce N2O emissions as well, but the nonsignificant effect was much smaller than the ash effect. Our study provides evidence that species-specific rhizosphere effects can have a substantial influence on the emission of greenhouse gases from forest soils, which have to be considered in addition to leaf litter effects. (C) 2012 Elsevier Masson SAS. All rights reserved.