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Results from earlier laboratory and field experiments were interrogated for the possibilities of sequestration, or long term accumulation, of carbon from excess greenhouse gases in the atmosphere. In the laboratory study, samples of three (top)soils dominated by kaolinite and illite (together), smectite, and allophane were examined for the adsorption and desorption of dissolved organic carbon (DOC). Adsorption and desorption of DOC were carried out on clay fractions extracted physically and after first native organic matter and then iron oxides were removed chemically. Labeled organic material was added to the soils to assess the priming effect of organic carbon (OC). In the field, changes in OC were measured in sandy soils that had been amended by additions of clay for between 3 and 17 years, both through incorporation of exogenous clay and delving of in situ clay. The laboratory experiments demonstrated that a portion of DOC was held strongly in all soils. The amount of DOC adsorbed depended on clay mineral types, including Fe oxides. Much adsorbed DOC was lost by desorption in water and a substantial amount of native OC was lost on priming with new OC. Addition of clay to soils led to increased OC. Therefore, addition of clay to soil may enhance net sequestration of C. Organic carbon close to mineral surfaces or within micro aggregates is held most strongly. Carbon sequestration may occur in subsoils with unsaturated mineral surfaces. However, incorporation of carbon into macroaggregates from enhanced plant growth might be most effective in removing excess carbon from the atmosphere, albeit over the short-term. Refereed/Peer-reviewed

The interactive effects of the types and contents of soil clay fractions (SCFs) and plant-residue addition rates on soil organic carbon (SOC) stabilisation are largely unknown. We conducted incubation experiments by amending a sandy soil sample with kaolinitic-illitic, smectitic and allophanic SCFs and adding wheat residues to the mineral mixtures to compare their C stabilisation capacity. The rate of carbon (C) decomposition was higher in the kaolinitic-illitic SCF followed by smectitic and allophanic clay minerals. The supply of easily degradable C substrate from decomposing residues markedly influenced the SCFs’ abilities to stabilise SOC. The removal of sesquioxides from the SCFs significantly decreased their C stabilisation capacity, which coincided with a decrease in the dehydrogenase activity of the mineral-residue mixture. The allophanic SCF showed the least microbial activity and the greatest C stabilisation due to having a higher proportion of micropores (75%). The high C stabilisation capacity of allophanic SCF could also be explained by its high specific surface area (119 m 2 g −1 ). The results of this study are helpful to understand the role of various SCFs in stabilising added C originating from external wheat residue addition but warrant further validation under field conditions. Refereed/Peer-reviewed

Using data from the literature, we pose the question of the extent to which a knowledge of soils is contributed by that of its clay fraction. In the past, soil characteristics have not related well to mineralogical analyses but we have redefined clays and also used a computer programme to more closely analyse X-ray diffraction (XRD) profiles. It was found that cation exchange capacities and specific surface areas of soil kaolinites almost always exceed those of standard kaolinites. The surfaces of soil clays are often coated by organic matter, which reduces their capacity to adsorb dissolved organic carbon (DOC). Oxidised iron is closely associated with phyllosilicate clays and enhances their capacity to adsorb DOC. Computer-aided decomposition of XRD traces show that phyllosilicates may contain several phases and react dynamically with potassium-containing solutions. Clays should be regarded as natural products with as yet undiscovered applications, including for cancer treatment. Refereed/Peer-reviewed

Environmental conditions like temperature and moisture could affect the carbon protection capacity of various clay types in soils. Using dominantly kaolinitic-illitic, smectitic and allophanic soils, we conducted systematic incubation experiments over 42 days at different temperatures (4, 22 and 37 °C) and moisture contents (30, 60 and 90% of water holding capacity (available water)). The basal respiration was monitored to study the relative effect of moisture contents and temperature on the carbon protection capacities and mechanisms of the three clay types. The results indicated that carbon decomposition increased with increasing moisture and temperature. A two-component quadratic equation could explain the carbon mineralisation process. The highest C respiration was observed at 37 °C with a 60% moisture level in each of the soil types. Under these conditions, the smectitic soil recorded the highest carbon decomposition followed by the kaolinitic-illitic and allophanic soils. The study of the priming effect using 14C labelled malic acid confirmed the trend of the bulk respiration results. The allophanic soil showed the lowest amount of carbon mineralisation under all experimental conditions. A strong inverse correlation (R2 = 0.90 at p

This study investigated the effects of surface functional groups, cation exchange capacity (CEC), surface charge, sesquioxides and specific surface area (SSA) of three soil clay fractions (SCFs) (kaolinite–illite, smectite and allophane) on the retention of dissolved organic carbon (DOC) in soils. Physico-chemical properties of the SCFs before and after removing native carbon and/or sesquioxides were characterised, and the DOC adsorption–desorption tests were conducted by a batch method. Native organic carbon (OC)/sesquioxide removal treatments led to a small change in the CEC values of kaolinite–illite, but significant changes in those of smectite and allophane. The net negative surface charge increased in all samples with an increase in pH indicating their variable charge characteristics. The removal of native OC resulted in a slight increase in the net positive charge on soil clay surfaces, while sesquioxide removal increased the negative charge. Changes in the functional groups on the SCF surfaces contributed to the changes in CEC and zeta potential values. There was a strong relationship (R2 = 0.93, p < 0.05) between the Langmuir maximum DOC adsorption capacity (Qmax) and SSA. The Qmax value also showed a moderately strong relationship (R2 = 0.55, p < 0.05) with zeta potential (at pH 7). Qmax was only poorly correlated with CEC and native OC content. Therefore, along with SSA, the surface charge and functional groups of SCFs played the key role in determining the adsorption affinity and hence retention of DOC in soils. Refereed/Peer-reviewed

The 1.41-nm reflection did not expand after Mg saturation and glycerol solvation, but it shifted to 1.38 nm and 1.20 nm when the samples were heated to 350 °C and 550 °C, respectively. The 1.41-nm spacing showed partial collapse after K+ saturation at 80 °C in association with the reinforcement of the 1.0 nm reflection. Collapse of the HIV (001) spacing was related to the release of Al, Mg, and Ca from the interlayer after KCl extraction. The Al was probably present as hydroxy-Al in HIV interlayers before being released. The HRTEM image revealed 1.4-nm lattice fringes interstratified with illite 1.0 nm fringes and two illite lattice fringes merged into one HIV fringe. This suggests that the HIV clays in Jiujiang soils resulted from illite weathering. The shift of a small X-ray diffraction reflection between 1.0 and 1.4 nm for an air-dried and glycerol-solvated sample to 0.997 nm after heating at 550 °C indicated the presence of an irregularly interstratified illite-HIV mineral. The common occurrence of HIV minerals in the Jiujiang red earth sediments suggests a climate with frequent wet and dry cycles prevailed during late-Pleistocene time. To understand the paleoclimatic significance of the red earth sediments in Jiujiang, southern China, hydroxy-interlayered vermiculite (HIV) in the deposits was investigated using X-ray diffraction (XRD), chemical extraction, and high resolution transmission electron microscopy (HRTEM). The XRD results indicated that the clay mineral assemblages of the sediments are mainly illite, kaolinite, HIV, vermiculite, with minor illite-HIV mixed-layer clays. Refereed/Peer-reviewed

Micromorphological observations from 3 different published works have been studied to aid understanding of aggregation and of colloids, both unique to soils. Saprolites in Hong Kong included ‘veins’ of different thicknesses and colours. Optical mineralogy identified them as infill from the neogenesis of clays in rock fractures. The common thicker infills resulted from weathering. Dark infill contained comminuted primary minerals whereas thin pale infill originated hydrothermally. Scanning electron microscopy (SEM) showed that the size, shape, and mineralogy of the kaolin minerals formed in infill depended on the types of cracks in the saprolites and on drying. Energy-dispersive X-ray spectroscopy analyses showed Fe and/or Mn in dark-coloured infill from comminution of primary minerals upon brecciation, or else beside pale infill in tuff, showing seasonal drying in tuff but not in granite. Pale infill gave predominantly large tubular halloysite in granite but large platy kaolinite in tuff, except that hydrothermal kaolin gave small particles. In dark infill, kaolin particles were also small and were kaolinite and halloysite mixtures. The effect of impurity Fe and Mn in constraining kaolin mineral crystallinity in infills simulates some of the effects of impure soil environments. Long-term cultivation of soils in Australia led to environmental scanning electron microscope images of large microaggregates indicating their breakdown and loss. Transmission electron micrographs of ultrathin sections showed that microaggregates of clay size, comprising clay minerals and oxides covering other materials, including organic matter, were predominant in virgin soil but were broken down to fine clay particles that blocked pores in cultivated soils. SEM showed a web of biological origin in long-term irrigated sandy New Zealand soil that surrounded macroaggregates but only became closely attached on drying. The nature of the macroaggregates was affected strongly by their history of drying, even during preparation for analyses. Micromorphology is especially useful for indicating the nature of aggregates in situ in soils.