Long-term fate of carbon in deeply rooted terrestrial sediment assessed by molecular proxies: sequestration vs. mineralization

Considerable amounts of atmospheric CO₂ are incorporated in plant belowground biomass and thus contribute to soil OM. However, associated with rooting, microorganisms enter the soil and, due to priming effects, might improve C mineralisation. Although these processes are well known for recent topsoils, it remains unclear if and how microorganisms contribute to long-term C dynamics in the subsoil and underlying soil parent material.

This study comprises several state-of-the-art techniques like bacterial DNA and lipid molecular proxies to trace living and fossil microbial biomass in modern and ancient root systems. Throughout a 13 m thick loess-paleosol sequence in SW Germany, which has been penetrated by several generations of roots since the last glacial maximum, both bulk (C_org and C_carb) and molecular changes in the rhizosphere were assessed at different depth intervals.

Phospholipid fatty acids, DNA and intact polar glycerol dialkyl glycerol tetraethers argue for the presence of living microorganisms in the rhizosphere not only of living but also ancient (≥3 ky) roots, which is associated with long-term C dynamics after the lifetime of the root. In the surrounding of living and ancient roots either C enrichment or C depletion was determined, depending on depth and rooting intensity. Especially in areas with high root densities (up to 20.000 root features like biopores, recent and calcified roots m^-2), rhizomicrobial degradation led to decrease of C contents. In depth intervals of lower root feature densities (<<100 m^-2), C accumulation was observed in the rhizosphere and rhizomicrobial degradation was limited.

The penetration of subsoil and underlying sediment by roots does not necessarily lead to additional C stabilization in the long-term, despite locally abundant root features and high portions of incorporated root- and rhizomicrobial-derived OM on a molecular level. At the contrary, priming effects may lead to considerable C loss in densely rooted sediment.