Abstract
Abstract. We investigate the edaphic, mineralogical and climatic controls of soil organic carbon (SOC) concentration utilising data from 147 primary forest soils (0–30 cm depth) sampled in eight different countries across the Amazon Basin. Sampled across 14 different World Reference Base soil groups, our data suggest that stabilisation mechanism varies with pedogenetic level. Specifically, although SOC concentrations in Ferralsols and Acrisols were best explained by simple variations in clay content – this presumably being due to their relatively uniform kaolinitic mineralogy – this was not the case for less weathered soils such as Alisols, Cambisols and Plinthosols for which interactions between Al species, soil pH and litter quality are argued to be much more important. Although for more strongly weathered soils the majority of SOC is located within the aggregate fraction, for the less weathered soils most of the SOC is located within the silt and clay fractions. It thus seems that for highly weathered soils SOC storage is mostly influenced by surface area variations arising from clay content, with physical protection inside aggregates rendering an additional level of protection against decomposition. On the other hand, most of the SOC in less weathered soils is associated with the precipitation of aluminium–carbon complexes within the fine soil fraction, with this mechanism enhanced by the presence of high levels of aromatic, carboxyl-rich organic matter compounds. Also examined as part of this study were a relatively small number of arenic soils (viz. Arenosols and Podzols) for which there was a small but significant influence of clay and silt content variations on SOM storage, with fractionation studies showing that particulate organic matter may account for up to 0.60 of arenic soil SOC. In contrast to what were in all cases strong influences of soil and/or litter quality properties, after accounting for these effects neither wood productivity, above-ground biomass nor precipitation/temperature variations were found to exert any significant influence on SOC stocks. These results have important implications for our understanding of how Amazon forest soils are likely to respond to ongoing and future climate changes.
Highlights
The soil organic carbon (SOC) pool is a function of the amount and quality of organic material entering the soil and its subsequent rate of mineralisation, which can be controlled by the various stabilisation processes that protect SOC from decomposition (Bruun et al, 2010)
The distribution of the sampled sites across the Amazon Basin is shown in Figure 1, with the soils sampled divided a priori into three “clusters” according to their World Resource Base Reference Soil Group (RSG) classification (IUSS, 2014): (1) the typically more strongly weathered Acrisol and Ferralsol soil types dominated by low-activity clays (LACs); (2) other less weathered soils types typically dominated by high-activity clays (HACs); (3) exceptionally sandy soils (Arenosols and Podzols) which we here refer to as “arenic” soil types
The contrasting chemistry of the three soil groups is shown in Fig. 2, where soil effective cation exchange capacity, IE, is plotted as a function of soil clay fraction, clay (0 to 0.3 m depth), with different symbols for each RSG and with the contrasting IE vs. clay domains indicated by different background colours
Summary
The soil organic carbon (SOC) pool is a function of the amount and quality of organic material entering the soil and its subsequent rate of mineralisation, which can be controlled by the various stabilisation processes that protect SOC from decomposition (Bruun et al, 2010). Specific surface area is itself dependent on clay mineralogy, with low-activity clays (LACs) being 1 : 1 aluminosilicates such as kaolinite (hereafter referred to as 1 : 1 clays) with low SSA and low cation exchange capacity (IE). This contrasts with high-activity clays (HACs) which are 2 : 1 alumino-silicates such as smectites and illites (hereafter referred to as 2 : 1 clays) with a much larger IE and SSA (Basile-Doelsch et al, 2005; Lützow et al, 2006). The SSA of clay and oxide mixtures, their chemical nature, and the types of charge predominant in organic matter may all play an important role in the C stabilisation process (Saidy et al, 2012)
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