Secondary aluminium, iron and silica phases across a volcanic soil climosequence, Galápagos Islands

Abstract

We applied selective dissolution extractions (Na‐pyrophosphate, acid oxalate, cold NaOH and citrate‐dithionite) to study Al, Fe and Si fractionation in 38 volcanic pedons located on the windward slopes of Santa Cruz (Galápagos Islands) that form a climosequence. The aim was to evaluate the distribution of secondary Fe, Al and Si phases in relation to environmental variables. Two main trends were observed: a geographical pattern in relation to altitude and a vertical differentiation within soil profiles. Soils from the drier coastal area had the smallest amounts of secondary Al, Fe and Si, whereas values were larger in soils at higher altitude, under a more humid climate. Organo–metallic complexes dominate the organic‐rich topsoils in these wetter areas as a result of larger availability of free Al, more humified organic matter with a greater capacity for metal adsorption and larger organic matter contributions from vegetation. This has led to increased accumulation of organic carbon in soils at higher altitudes. At lower elevations, inorganic secondary materials are more frequent, with short‐range‐order minerals prevailing in topsoils, whereas crystalline minerals dominate subsoils. Crystalline minerals are also frequent in some soils at higher altitudes in relation to acid pH. Vertical differentiation within the soil profile is also responsive to altitude: differences between topsoils and subsoils are generally more marked in soils at higher locations, thus wetter areas. A greater degree of weathering seems to be the main factor controlling both altitudinal trends and horizonation through its effect on pH, organic matter content and humification, ultimately controlled by climatic variation. Highlights We analysed soils developed on volcanic material along the windward slope of Santa Cruz Island. Organo–metallic complexes are abundant in acidic, organic‐rich epipedons at higher elevation. Short‐range‐order minerals are more abundant in non‐acidic subsoils and tend to increase with altitude Crystalline minerals predominate at lower elevation and in acidic subsoils at higher altitude.

Geochemistry and microbiology of tropical serpentine soils in the Santa Elena Ophiolite, a landscape-biogeographical approach

“…An analogous case was reported in Italian alpine serpentine soils found above 2000 m altitude, where pedogenesis on serpentinite was very slow or absent when associated with factors such as low plant coverage and steep slopes that favored high erosion rates; resulting in soils with higher amounts of exchangeable Ni compared to most developed soils under coniferous forest [ 66 ]. In other non-serpentine soils, like in Galapagos Islands, weathering processes associated with climate were shown to increase with altitude [ 67 ] and contributed to larger amounts of secondary phases of Fe, Al and Si at higher elevations [ 68 ]. This highlights the necessity to further study the influence of the altitude-associated characteristics (vegetation coverage and climate exposure) in the serpentine soils of SEP despite being the parent rock the main soil-forming agent of these mineral soils [ 13 ].…”

Addition of sorptive mineral phases to soils decreases short‐term organic matter decomposition by reducing microbial access to substrates

“…We hypothesised that the addition of the mineral phases derived from an allophanic clay soil (Parfitt, 1990) would increase the adsorption of SOM onto the surfaces of the mineral clay microstructure to form complexes with aluminium (Kaiser & Guggenberger, 2007; Lützow et al, 2006; Schneider et al, 2010). Although our experimental approach did not allow direct determination of the mechanisms leading to the increase in SOM stabilisation, we interpreted the fraction of aluminium dissolved in sodium pyrophosphate as a quantitative indicator of organo‐mineral complexes that leads to SOM stabilisation (Eimil‐Fraga et al, 2015; Masiello et al, 2004; Parfitt & Childs, 1988; Taboada et al, 2019). However, the use of pyrophosphate‐extractable aluminium as an indicator of the amounts of organo‐mineral complexes has been questioned because small amounts of aluminium can be extracted directly from allophane or aluminium hydroxides (Kaiser & Zech, 1996).…”

“…Concurrently, we measured pH, and hot ( C h ) and cold ( C c ) water extractable C as indicators of SOM pools available for microbial decomposition (Ghani et al, 2003; Landgraf et al, 2006). We determined newly formed organo‐mineral complexes with the added Al‐rich mineral phase by measuring the amounts of pyrophosphate‐extractable aluminium, an indicator of organically bound aluminium (Eimil‐Fraga et al, 2015; Parfitt & Childs, 1988; Taboada et al, 2019). To provide further evidence of the effects of addition of the sorptive mineral phase on changes in the source of the decomposed substrates, we measured the natural abundance δ 13 C signature of respired CO 2 (not requiring the addition of 13 C labelled substrates).…”

Pedogenic bayerite in andic material of Isla Santa Cruz, Galápagos archipelago

“…The site of the studied shallow profile is located about 2 km south-east of Santa Rosa, on Isla Santa Cruz of the Galápagos archipelago, at an altitude of about 380 m. The profile was sampled, as profile n° 42 (XLII), in august 1962 by Georges Stoops, in the course of an extensive pedological and geological field campaign, organized by Ghent University, that covered several islands. Data for soil samples that were collected on Santa Cruz during that expedition have been presented in various early studies (e.g., Laruelle, 1966;Laruelle and Stoops, 1967;Stoops, 1972;Eswaran et al, 1973;Morràs, 1974Morràs, , 1976Morràs, , 1977 and also in some more recent publications (e.g., Taboada et al, 2016Taboada et al, , 2019Stoops et al 2022), all without including results for profile n° 42.…”