Gordon Jock Churchman

: 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 microaggregates 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.

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.

Unfortunately, in the original publication of the article, Prof. Yong Sik Ok’s affiliation was incorrectly published. The author’s affiliation is as follows.

Defining soil clay minerals as all inorganic particles < 2 µm would enhance their predictive power for soil functions. The future task is to unravel the prominent functional role of clay associations in the “ultimate particles” in soils–microaggregates. High energy techniques and decomposition of XRD patterns offer the prospect of understanding associations of silicates, oxides and organic matter within microaggregates.

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 (Q_max) and SSA. The Q_max value also showed a moderately strong relationship (R² = 0.55, p < 0.05) with zeta potential (at pH 7). Q_max 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.

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.