Disentangling interactions between microbial communities and roots in deep subsoil

Soils, paleosols and terrestrial sediments serve as archives for studying climate change, and represent important terrestrial carbon pools. Archive functioning relies on the chronological integrity of the respective units. Incorporation of younger organic matter (OM) e.g. by plant roots and associated microorganisms into deep subsoil and underlying soil parent material may reduce reliability of paleoenvironmental records and stability of buried OM. Long-term effects of sedimentary characteristics and deep rooting on deep subsoil microbial communities remain largely unknown. We characterized fossil and living microbial communities based on molecular markers in a Central European Late Pleistocene loess-paleosol sequence containing recent and ancient roots with ages of several millenia. The molecular approach, comprising free and phospholipid fatty acids (FAs), core and intact polar glycerol dialkyl glycerol tetraethers (GDGTs), as well as 16S rRNA genes from bacterial DNA, revealed the presence of living microorganisms along the sequence, with bacterial community composition comparable to that of modern topsoils. Up to 88% redundancy between bacterial genetic fingerprint and molecular signature of fossil microorganisms suggested a time-integrated signal of the molecular markers accumulated over a time span potentially lasting from sedimentation over one or more rooting phases until today. Free FAs, core GDGTs and DNA, considered as remains of fossil microorganisms, corresponded with ancient and recent root quantities, whereas phospholipid FAs and intact polar GDGTs, presumably derived from living microorganisms, correlated only with living roots. The biogeochemical and ecological disequilibrium induced by postsedimentary rooting may entail long-term microbial processes like OM mineralization, which may continue even millenia after the lifetime of the root. Deep roots and their fossil remains have been observed in various terrestrial settings, and roots as well as associated microorganisms cause both, OM incorporation and mineralization. Therefore, these findings are crucial for improved understanding of OM dynamics and carbon sequestration potential in deep subsoils.