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Soil redistribution and weathering controlling the fate of geochemical and physical carbon stabilization mechanisms in soils of an eroding landscape

S. Doetterl,J. Cornelis,J. Six,S. Bodé,S. Opfergelt,P. Boeckx,K. Oost

Published 2014 in Biogeosciences

ABSTRACT

The role of eroding landscapes in organic carbon stabilization operating as C sinks or sources has been fre- quently discussed, but the underlying mechanisms are not fully understood. Our analysis aims to clarify the effects of soil redistribution on physical and biogeochemical soil or- ganic carbon (SOC) stabilization mechanisms along a hills- lope transect. The observed mineralogical differences seem partly responsible for the effectiveness of geochemical and physical SOC stabilization mechanisms as the mineral envi- ronment along the transect is highly variable and dynamic. The abundance of primary and secondary minerals and the weathering status of the investigated soils differ drastically along this transect. Extractable iron and aluminum compo- nents are generally abundant in aggregates, but show no strong correlation to SOC, indicating their importance for aggregate stability but not for SOC retention. We further show that pyrophosphate extractable soil components, es- pecially manganese, play a role in stabilizing SOC within non-aggregated mineral fractions. The abundance of micro- bial residues and measured 14 C ages for aggregated and non- aggregated SOC fractions demonstrate the importance of the combined effect of geochemical and physical protection to stabilize SOC after burial at the depositional site. Mineral alteration and the breakdown of aggregates limit the protec- tion of C by minerals and within aggregates temporally. The 14 C ages of buried soil indicate that C in aggregated frac- tions seems to be preserved more efficiently while C in non- aggregated fractions is released, allowing a re-sequestration of younger C with this fraction. Old 14 C ages and at the same time high contents of microbial residues in aggregates sug- gest either that microorganisms feed on old carbon to build up microbial biomass or that these environments consisting of considerable amounts of old C are proper habitats for mi- croorganisms and preserve their residues. Due to continuous soil weathering and, hence, weakening of protection mecha- nisms, a potential C sink through soil burial is finally tempo- rally limited.

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