Mineralogical control of organic carbon dynamics in a volcanic ash soil on La Réunion

Abstract

In soil carbon dynamics, the role of physicochemical interactions between organic matter and minerals is not well understood nor quantified. This paper examines the interactions between soil organic matter and poorly crystalline aluminosilicates in a volcanic ash soil on La Re´union in the southern tropics. The soil examined is a profile composed of a surface soil (L-Ao-E-Bh) overlying four buried horizons (horizons 2Bw, 3Bw, 4Bw, 5Bw) that have all developed from successive tephra deposits. Non-destructive spectroscopy (XRD, FTIR and NMR of Si and Al) showed that the mineralogical composition varies from one buried horizon to another. Further, we show that buried horizons characterized by large amounts of crystalline minerals (feldspars, gibbsite) have the least capacity to store organic matter and the fastest carbon turnover. In contrast, buried horizons containing much poorly crystalline material (protoimogolite and proto-imogolite allophane, denoted LP-ITM) store large amounts of organic matter which turns over very slowly. To understand the mechanism of interactions between LP-ITM and organic matter better, we focused on a horizon formed exclusively of LP-ITM. We demonstrate, using Á 14 C and 13 C values, that even though LP-ITM is extraordinarily effective at stabilizing organic matter, C linked to LP-ITM is still in dynamic equilibrium with its environment and cycles slowly. Based on Á 14 C values, we estimated the residence time of organic C as $ 163 000 years for the most stabilized subhorizon, a value that is comparable to that for organic carbon stabilized in Hawaiian volcanic soils. However, this calculation is likely to be biased by the presence of microcharcoal. We characterized the organo-mineral binding between organic matter and LP-ITM by 27 Al NMR, and found that the organic matter is not only chelated to LP-ITM, but it may also limit the polymerization of mineral phases to a stage between proto-imogolite and proto-imogolite allophane. Our results demonstrate the important role of poorly crystalline minerals in the storage of organic C, and show that mineral and organic compounds have to be studied simultaneously to understand the dynamics of organic C in the soil.

Storage and Turnover of Organic Matter in Soil

“…Recent studies suggest that storage and turnover are much more closely related to mineral properties, in particular the poorly crystalline phases, than to texture per se (Torn et al 1997;Masiello et al 2004;Basile-Doelsch et al 2005;Kleber et al 2005). For example, in paired forest soils derived from granitic vs. andesitic parent materials, the latter stored almost 50% more C than did the granitic soils despite similar levels of clay and aggregate stability, climate, and vegetation (Rasmussen et al 2005).…”

The effect of vertically resolved soil biogeochemistry and alternate soil C and N models on C dynamics of CLM4

“…10, we compare model predictions using a single, base set of parameters -0.5 m z τ (e-folding depth for soil turnover), Jackson et al (1996) rooting depth for root C inputs, diffusivities of 1 cm 2 yr −1 by bioturbation and 5 cm 2 yr −1 for cryoturbation, and 0 cm yr −1 advection) -to observations from sites where C and 14 C depth profiles have been measured and reported: Voronazh, Russia (Torn et al, 2002); Thule, Greenland (Horwath et al, 2008); Paragominas, Brazil (Trumbore et al, 1995); Mattole, California (Masiello et al, 2004); La Reunion, South Pacific (Basile-Doelsch et al, 2005); Harvard Forest, Massachusetts (Gaudinski et al, 2000); Gydansky, Western Siberia (Kaiser et al, 2007); and Judgeford, New Zealand; Riverbank, California; and Turlock Lake, California (Baisden and Parfitt, 2007).…”

“…Amelioration of this anomalous N limitation, and its effects on the C storage, is discussed below. (Torn et al, 2002); (b, l) Thule, Greenland (Horwath et al, 2008); (c, m) Paragominas, Brazil (Trumbore et al, 1995); (d, n) Mattole, California (Masiello et al, 2004); (e, o) La Reunion, South Pacific (Basile-Doelsch et al, 2005); (f, p) Harvard Forest, Massachusetts (Gaudinski et al, 2000); (g, q) Gydansky, Western Siberia (Kaiser et al, 2007); and (h, r) Judgeford, New Zealand; (i, s) Riverbank, California; and (j, t) Turlock Lake, California (Baisden and Parfitt, 2007). For all figures, black line is model, and colored lines are observations.…”

The Effect of Dissolved Humic Acids on Aluminosilicate Formation and Associated Carbon Sequestration

“…Their occurrence is contingent upon conditions such as soil pH, the availability of sufficient Si, and the presence of organic material, which can either inhibit the formation of [5], or conversely, stabilize these minerals [6]. The large surface area and high reactivity of these minerals promote complexation with organic matter [6], the essential soil nutrients P and N, and toxic contaminants such as As, Cd, Cu, and Pb [7,8].…”