Biotic and abiotic factors controlling terpenoid exchange from soil of a mixed temperate forest ecosystem

The contribution of forest floors to the ecosystem budget of volatile terpenoids is still not fully understood. We performed seasonal measurements in a mixed temperate forest to elucidate the effects of tree species (Douglas fir vs. European beech) and abiotic drivers on soil-atmosphere terpenoid exchange. In addition, soil cores were studied under controlled conditions to characterize the effect of ambient air terpenoid concentrations on exchange rates of terpenoids and their enantiomers. Moreover, the role of litter layer and microbial activity on exchange of important terpenoids enantiomers was tested. Soil under Douglas fir emitted monoterpenes at rates up to 3 μg m⁻² h⁻¹, whereas soil under European beech released terpenes at much lower rates and occasionally even took up these volatiles. Exchange followed seasonal patterns with low fluxes during winter, increasing emissions during springtime and reduced exchange rates in summer. Flux rates weakly correlated with soil temperature, soil moisture and ambient terpenoid concentrations. In isolated soil cores emission of terpenoids was enantiomer specific, which was not observed for terpenoid uptake. Increasing ambient concentrations caused a switch from emission to an uptake of terpenoids at compound specific compensation points ranging between 10 and 250 ppt. More detailed analyses indicated that the litter layer and not the mineral soil is the main contributor for soil terpenoid exchange, and that microbial activity plays an important role for terpenoid uptake but not for emission. In conclusion, our results highlight a strong contribution of the forest floor to the ecosystem budget of terpenoids. For temperate forests tree-species and related litter determine terpenoid emission from the forest floor, whereas terpenoid uptake is driven by soil microbial activity. The balance of exchange is modulated by soil temperature and ambient terpenoid concentrations. It remains to be elucidated whether such relationships apply to forests in other biomes.