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The deep rhizosphere: an underestimated component of long-term organic matter dynamics

Martina Gocke1, Stephan Peth2, Arnaud Huguet3, Sylvie Derennne3, Christelle Anquetil3, Marie-France Dignanc4, Cornelia Rumpel4, Gerard Bardoux4, Guido Wiesenberg5
1 Department of Agroecosystem Research, University of Bayreuth, Germany
2 Department of Soil Science, University of Kassel, Germany
3 METIS, CNRS/UPMC UMR 7619, Paris, France
4 BioEMCo, CNRS, UPMC UMR 7618, Thiverval-Grignon, France
5 Department of Geography, University of Zurich, Switzerland

O 4.4 in Below ground turnover of C and nutrients in forest soils

14.07.2014, 12:15-12:35, H17

Under predicted rising atmospheric CO2 concentration, the potential of C sequestration is discussed for soils1. One of the main factors affecting stability and long-term storage of soil OM is the chemical composition of incorporated organic remains. It is increasingly accepted that roots contribute significant portions to topsoil OM, whereas their role for C cycling is less known for depths >> 1 m, i.e. the deep subsoil2 and underlying soil parent material like terrestrial sediments.

To trace root-related features and organic remains, transects were sampled from ancient (3–10 ky) and recent calcified roots (rhizoliths) via surrounding sediment towards sediment free of visible root remains3, at two sites. At the Nussloch loess-paleosol sequence (SW Germany), transects were collected as intact cores and scanned by X-ray microtomography for visualization of rhizoliths and rhizosphere. Afterwards, cores were cut into concentric slices and, similar to rhizolith and sediment samples from the sandy deep subsoil at Sopron (NW Hungary), analyzed for suberin molecular markers.

3D scanning of Nussloch rhizoliths and surrounding loess showed large channels of former root growth, whereas the root tissue was commonly degraded. Additionally, microtomography enabled assessment of abundant fine calcified roots as well as biopores remaining from fine roots. The total pore volume that was previously filled with root tissue accounted for up to 6.4% in depth intervals with abundant rhizoliths (≥100 m-2), and less than 0.5% in depth intervals with scarce rhizoliths (≤20 m-2). Suberin biomarkers were found in both, recent and ancient root systems, demonstrating their suitability to identify root-derived OM in terrestrial sediments with ages of several tens of ky. Varying relative portions of the respective suberin markers enabled the attribution of Sopron rhizoliths to oak origin, and assessment of the rhizosphere, which extended up to several cm. This confirms recent studies which demonstrated the possible postsedimentary incorporation of considerable amounts of root and rhizomicrobial remains in loess, based on biomarkers deriving either from plants and microorganisms (alkanes, fatty acids)3 or solely from microorganisms (GDGTs)4. These results show that root-derived OM may play an important role in terrestrial sediments, and sedimentary OM does not necessarily reflect solely the aboveground biomass of synsedimentary vegetation. Higher recalcitrance of root remains on one hand, and enhanced microbial activity in the rhizosphere on the other hand, may thus have considerable consequences for C loss/sequestration in deep subsoil in the long-term. Roots do not necessarily contribute to C stabilization in centennial or millenial time scales.

References

1Ciais et al. 2013. In: Stocker et al., Climate change 2013: The physical science basis. 2Rumpel & Kögel-Knabner 2011. Plant Soil 338:143-158. 3Gocke et al. 2014. Catena 112:72-85. 4Huguet et al. 2012. Org Geochem 43:12-19.



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last modified 2014-06-19