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Titel |
Quantifying nutrient uptake as driver of rock weathering in forest
ecosystems by magnesium stable isotopes |
VerfasserIn |
David Uhlig, Jan A. Schuessler, Julien Bouchez, Jean L. Dixon, Friedhelm von Blanckenburg |
Konferenz |
EGU General Assembly 2017
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250139056
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Publikation (Nr.) |
EGU/EGU2017-2220.pdf |
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Zusammenfassung |
Plants and soil microbiota play an active role in rock weathering and potentially couple
weathering at depth with erosion at the soil surface. The nature of this coupling
is still unresolved because we lacked means to quantify the passage of chemical
elements from rock through higher plants. In a temperate forested landscape of
the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured
magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota.
We find that Mg is highly bio-utilised: 50-100 % of the Mg released by chemical
weathering is taken up by forest trees. To estimate the tree uptake of other bio-utilised
elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements
and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux
minus secondary mineral formation flux). We find a deficit in the dissolved fluxes
throughout, that we attribute to the nutrient uptake by forest trees. Therefore, both
the Mg isotopes and the flux comparison suggests that a substantial part of the
major element weathering flux is consumed by the tree biomass. This isotopic and
elemental compartment separation is preserved only if the mineral nutrients contained
in biomass are prevented from re-dissolution after litter fall, showing that these
nutrients have been removed as “solid” biomass. The enrichment of 26Mg over
24Mg in tree trunks relative to leaf litter suggests that this removal occurs mainly in
coarse woody debris (CWD). Today, CWD is exported from the ecosystem by tree
logging. Over pre-anthropogenic weathering time scales, a similar removal flux might
have been in operation in the form of natural erosion of CWD. Regardless of the
removal mechanism, our data provides the first direct quantification of biogenic
uptake following weathering. We find that Mg and other bio-elements are taken
up by trees at up to 7 m depth, and surface recycling of all bio-elements but P is
minimal. Thus, in the watersheds of the SSCZO in which weathering is fast and
kinetically-limited, the coupling between erosion and weathering might be established by
bio-elements that are taken up by trees, not recycled and missing in the dissolved river flux
due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the
partitioning of a biogenic weathering flux into eroded plant debris might represent a
significant global contribution to element export after weathering in eroding mountain
catchments that are characterised by a continuous supply of fresh mineral nutrients. |
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