Enhancing carbon sequestration in woodland ecosystems through new planting is a recognized measure to mitigate anthropogenic emission of CO2. However, species specific tree effects on biomass allocation (above and belowground), leaf decomposition, storage and stability of soil organic matter (SOM) under single species and mixtures are largely unclear. We investigated the ecosystem C pools and processes in response to species traits in single stands and the interactive effects in mixed stands of birch (Betula pendula), alder (A/nus glutinosa) and beech (Fagus sylvatica). To estimate standing aboveground biomass, species specific allometric models were developed, and DBH and basal diameter were found as the best predictors of plant woody biomass. Significantly higher aboveground woody biomass was observed in the single stand of birch than alder and beech. In estimates of belowground biomass and turnover, fine root(< 2 mm) biomass was higher in alder whilst the root turnover rate was higher in birch. At the stand level, clear additive mixture effects on above ground biomass
was observed, however at the tree level, birch tended to lower biomass in mixture
presumably due to suppression by the faster growing alder. Significantly lower biomass of beech was observed in mixture compare to monoculture. A similar pattern was observed with fine root biomass production.
In investigations of leaf litter decomposition, in laboratory incubation experiment
significantly higher level of water soluble phenolics was found in the soil solution with birch and beech leaf litter. An in situ litterbag experiment also showed a faster relative decay rate of alder leaf litter compare to birch and beech. This is suggested to be due to higher litter quality of alder, and the higher secondary metabolites in birch and beech. The absolute decay rates and the mass loss revealed a two phase decay pattern. Clear mixture effects were observed with a slower decay during initial stage and higher decay rate at latter stage, suggesting possible antagonistic effects of species specific compounds in the
mixed litter.
After 4 years of afforestation, 7.3 and 8.0 kg m· 2 C stocks in the top metre of soils planted with trees and grassland were estimated, respectively. Up to 40% of total the SOC stock was found in subsoil layers (30-100 cm) suggesting significant contribution of deep soil C pools to sequestration. No clear effect of tree species or mixture on C pool size was observed. Over all, our studies revealed th~t in addition to species specific effects, C storage in soil is largely controlled by soil conditions. Fractionation of SOM into easy degradable labile and recalcitrant pools revealed that species identity and composition did
not affect relative proportions of these fractions in the top 2 soils layers, however; in deep layers differences were highly statistically significant. The absolute recalcitrant pool in top meter soil increased following the order: grassland < beech < alder < birch < mixed soil.
Of the total storage, 30-51 % C was recalcitrant. Litter quality particularly root litter and subsequent decomposition- translocation interactions might be the cause of the high labile C in the deep soil layers.
Overall C dynamics in different plant species showed that birch stands have the
highest aboveground woody biomass. In addition, higher root turnover rate and slower leaf litter decomposition were found in birch stands than alder, suggesting favourable traits for long term storage of C in soil. However, the antagonistic effects on leaf litter decomposition, relatively higher fine root production and turnover, together with the highest recalcitrant SOC pool suggests that, tree mixtures might be the best option in plantations.