Root-induced tree species effects on the source/sink strength for greenhouse gases (CH4, N2O and CO2) of a temperate deciduous forest soil

Abstract

Through their leaf litter and throughfall water, tree species can have a pronounced influence on soil chemistry. However, there is little knowledge of species-specific root effects on greenhouse gas fluxes between forest soils and the atmosphere. By growing saplings of beech (Fagus sylvatica) and ash (Fraxinus excelsior) in monoculture or mixture at defined atmospheric and soil conditions in rhizotrons, we tested four hypotheses related to potential root-induced tree species effects on the uptake of CH4 and the emission of N2O and CO2 from the soil. This design excluded putative effects of leaf litter mineralisation on trace gas fluxes. Gas fluxes were measured biweekly using the closed chamber technique; the CO2 derived from root respiration was estimated, and the concentration of organic acids in the rhizosphere solution was analysed. Rhizotrons planted with ash took up significantly more CH4 and emitted less N2O than control rhizotrons without plants. CH4 and N2O fluxes from beech rhizotrons did not differ from the root-free control but were significantly smaller (CH4) or higher (N2O) than the fluxes from the ash treatment. While root respiration of ash was higher than of beech, root-induced soil respiration was higher in the rhizosphere of beech roots. The concentration of organic acids tended to be higher in the rhizosphere of beech and also the composition was different from that of ash. We conclude that tree species identity may substantially alter the soil source/sink strength for greenhouse gases through root-related processes.