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Carbon storage and sequestration in subsoil horizons: knowledge, gaps and potentials

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Carbon (C) sequestered in subsoils generally contributes to more than half of the total stocks within a soil profile. In contrast to topsoil, organic matter (OM) stored in subsoil horizons is characterised by high mean residence times. While the mechanisms and controls of OM stabilisation in topsoils are fairly well understood, processes and dynamics of subsoil OM stabilisation appear to be controlled by other factors. We summarize the available data on C quantities, chemical composition as well as decomposition and stabilisation dynamics in subsoils and discuss the relevance of these processes for longterm carbon storage. Moreover, the importance of spatial dist ribution of SOM and its degraders is addressed. The objective of this chapter was to discuss the controls of carbon sequestration in subsoil horizons in order to explore the possibility to increase soil carbon stocks by carbonising subsoil horizons. Subsoil C is characterised by much higher solubility after destruction of the mineral phase compared to topsoil C suggesting that much of it consists of small molecules stabilised by interaction with the mineral phase. The chemical composition of OM stabilised by mineral interactions in subsoil horizons is different from those of topsoils. Precursors of this C may be root-derived and microbially processed organic matter as well as organic matter transported in dissolved form or by bioturbation. The reports about the environmental controls of subsoil OM degradation are conflicting, most likely due to site-specificity. The lower dynamics of temperature and soil moisture in subsoils may thus enhance or reduce OM mineralization, while low nutrient availability is a more common limiting factor. Spatial distribution of OM may determine the likelihood of its stabilisation at long time scales, which may be most related to absence of energy-rich material needed for decomposition. Therefore a suitable strategy for increasing C stocks in deep horizons may be the addition of highly stable OM such as biochar or highly aliphatic material.
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hal-02805118 , version 1 (06-06-2020)



Abad Chabbi, Cornelia Rumpel, Bernd Marschner. Carbon storage and sequestration in subsoil horizons: knowledge, gaps and potentials. Recarbonization of the biosphere : Ecosystems and the global carbon cycle, Springer, 559 p., 2012, 978-94-007-4158-4 978-94-007-4159-1. ⟨10.1007/978-94-007-4159-1_20⟩. ⟨hal-02805118⟩
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