Soil Terpenoid Content and Emissions Are Shaped by Litter Chemistry and Soil Depth

Abstract Background and Aims This study investigates how soil terpenoid content and emissions vary with soil depth between two sites with contrasting tree species in a mixed temperate forest. We aimed to understand how species-specific litter chemistry and depth-dependent soil conditions jointly shape soil terpenoid content and emissions. Methods Foliar and soil samples were collected from evergreen Douglas fir and deciduous European beech plots under comparable climatic and edaphic conditions. Terpenoid contents in foliage were analyzed, as well as soil terpenoid content and emissions dependent on soil depth. Additionally, soil CO2 emissions, δ13C values, and physicochemical properties were assessed. Results Douglas fir plots showed substantially higher terpenoid content and emissions than beech plots, consistent with foliar chemical differences. Terpenoids were concentrated in surface layers and declined with depth. Emissions were limited to the surface soil in the Douglas fir plot, whereas the beech plot showed little depth variation. CO2 emissions and δ13C values varied with depth, indicating a shift in microbial activity and substrate origin from plant-derived litter in surface soils to more microbially processed sources in deeper layers. Conclusions This study indicates that tree species is the primary driver of soil terpenoid dynamics, as species-specific litter chemistry governs both the magnitude and vertical distribution of soil terpenoid content and emissions. Depth-related patterns appear to be associated with microbial activity, the origin of decomposed substrates, and soil physicochemical properties. These findings underscore the importance of accounting for both vegetation composition and depth-resolved soil processes when assessing VOC budget in forest ecosystems.