Tree species effects on stocks and stability of soil carbon: Links to mycorrhizal association and soil biota composition and functioning

Tree species with leaf litter traits driving slow rates of leaf litter decomposition have traditionally been associated with accumulation of higher soil organic carbon (SOC) stocks than tree species with fast litter decomposition rates. This hypothesis has mainly been based on observations of thick C-rich forest floors under tree species associated with ectomycorrhizae (ECM). However, a recent hypothesis suggested that tree species with foliar litter traits conducive to fast decomposition will lead to more pronounced microbial transformation and stabilization of litter C. The latter tree species are often associated with arbuscular mycorrhizae (AM) and may enhance deeper incorporation of C by more active soil fauna communities and by higher belowground rates of litter input. The Danish multi-site common garden tree species experiment includes ECM and AM tree species that differ widely in traits such as foliar litter chemistry. The experiment has been studied over the last 15 years to document and explain soil C stocks supported by emerging studies of soil fauna and soil microbial community composition and functioning.

The six common European tree species formed distinct groups in soil carbon characteristics as well as in soil biota community composition and functioning that partly reflected their mycorrhizal association. Forest floor C stocks were consistent with the traditional perception of slowly decomposing leaf litter in ECM species being conducive to high C stocks. However, an intriguing pattern of higher C stocks in the mineral soil in AM tree species with high litter quality and characteristic soil biota functioning supported the recent microbial stabilization hypothesis and suggested deeper incorporation of C in more stable forms.

Based on new results on microbial, macro- and mesofauna communities and their functioning, and on repeated soil sampling, this talk will revisit the common garden experiments for a synthesis of processes and patterns in organic matter formation that may explain observed patterns in quantity and quality of SOC.