Rhizogenic Fe–C redox cycling: a hypothetical biogeochemical mechanism that drives crustal weathering in upland soils

Abstract

Field-scale observations of two upland soils derived from contrasting granite and basalt bedrocks are presented to hypothesize that redox activity of rhizospheres exerts substantial effects on mineral dissolution and colloidal translocation in many upland soils. Rhizospheres are redox-active microsites and in the absence of O 2 , oxidation of rhizodeposits can be coupled by reduction of redoxactive species such as Fe, a biogenic reduction that leads to Fe translocation and oxidation, accompanied by substantial proton flux. Not only do rhizogenic Fe-C redox cycles demonstrate a process by which the rhizosphere affects an environment well outside the near-root zone, but these redox processes are also hypothesized to be potent weathering systems, such that rhizogenic redox-reactions complement acid-and ligand-promoted reactions as major biogeochemical processes that control crustal weathering. The potential significance of Fe-C redox cycling is underscored by the deep and extensive rooting and mottling of upland subsoils across a wide range of plant communities, lithologies, and soil-moisture and temperature regimes.

Are oxygen limitations under recognized regulators of organic carbon turnover in upland soils?

“…Aside from a few exceptions (Andersen et al 1998;van der Lee et al 1999;von Fischer and Hedin 2002;Fimmen et al 2008), the spatial and temporal dynamics of anaerobic microsites in upland soils, the factors controlling their formation and persistence, and their impact on soil C cycling have not been studied in great detail.…”

“…If oxygen supply (via diffusion) in the soil matrix is slower than its consumption (via microbial respiration), the interior of structural units (aggregates or peds) becomes oxygen depleted relative to the exterior (macropores) (as seen in Sexstone et al 1985), leading to gradients in redox potential (Zausig et al 1993). If these gradients persist, frequently promoted by seasonal water saturation (Jacobs et al 2002) or a continuous supply of root C (Richter et al 2007;Fimmen et al 2008;Hong et al 2010), they lead to the differentiation of redoximorphic features.…”

“…Roots can directly reduce soil oxygen concentrations via root respiration (Bidel et al 2000) or indirectly by stimulating heterotrophic respiration in the rhizosphere (Keiluweit et al 2015;Hojberg and Sorensen 1993) and releasing organic reductants (Fimmen et al 2008;Richter et al 2007). A detailed description of these processes is beyond the scope of this effort, but the question of how root-mediated modifications of soil oxygen concentrations affect soil C cycling in upland soils is still unanswered and not considered in current model frameworks.…”

Tropical forest soil microbial communities couple iron and carbon biogeochemistry

“…In addition, manganese (Mn) reduction may also contribute to decomposition because Mn(IV) is a strong anaerobic oxidant, and Mn reduction may support Fe(III)-reducing bacteria (many of which reduce Mn) when redox conditions do not favor Fe(III) reduction (Lovley et al 2004). In Luquillo soils, Fe-oxides are far more abundant than Mn-oxides (Table 1) surfaces (King and Garey 1999, Nevin and Lovley 2002, Fimmen et al 2008). …”

Depth and character of rock weathering across a basaltic‐hosted climosequence on Hawai‘i

“…Therefore, Fe reduction can influence colloidal mobilization physically via dissolution of particles or chemically through changes in pH and corresponding particle surface charges (Thompson et al, 2006a). Fimmen et al (2008) and Grybos et al (2009) have suggested that organic matter is likely to disperse during Fe-C redox cycles, although they do not provide direct evidence of organic colloids.…”