Poorly crystalline mineral phases protect organic matter in acid subsoil horizons

Abstract

Soil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X-ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon ( 14 C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite-citrate-extractable Fe. The combination of oxalate-extractable Fe and Al explained the greatest amount of variation in stable organic C (R 2 ¼ 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate-soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.

Rapidly growing tropical trees mobilize remarkable amounts of nitrogen, in ways that differ surprisingly among species

“…This soil N MRT is similar to the average global soil C MRT of 32 y (23), a completely independent calculation that lends confidence in our mass-balance estimates. The soil at this study site has a relatively high specific surface area (24,25), and most of the SOM is physically protected by clay minerals rather than being chemically resistant to oxidation (24)(25)(26). The current conceptual framework indicates that this type of organo-mineral complex would have a much longer MRT than SOM stabilized biogeochemically (27); this would explain the high storage capacity for soil C in this site.…”

Comparison of soil organic matter dynamics at five temperate deciduous forests with physical fractionation and radiocarbon measurements

“…Second, turnover times at MI-Fine were similar to those at MI-Coarse despite large differences in texture. Soil mineralogy and the presence of reactive Fe-or Al-oxides have been observed to influence SOM stabilization and soil carbon storage (Heckman et al 2009;Spielvogel et al 2008;Kleber et al 2005), and organo-mineral interactions may vary considerably among our sites.…”

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).…”