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Dolomite (CaMg(CO3)2) precipitation is kinetically inhibited at surface temperatures and pressures. Experimental studies have demonstrated that microbial extracellular polymeric substances (EPS) as well as certain clay minerals may catalyse dolomite precipitation. However, the combined association of EPS with clay minerals and dolomite and their occurrence in the natural environment are not well documented. We investigated the mineral and textural associations within groundwater dolocrete profiles from arid northwest Australia. Microbial EPS is a site of nucleation for both dolomite and authigenic clay minerals in this Late Miocene to Pliocene dolocrete. Dolomite crystals are commonly encased in EPS alveolar structures, which have been mineralised by various clay minerals, including montmorillonite, trioctahedral smectite and palygorskite‐sepiolite. Observations of microbial microstructures and their association with minerals resemble textures documented in various lacustrine and marine microbialites, indicating that similar mineralisation processes may have occurred to form these dolocretes. EPS may attract and bind cations that concentrate to form the initial particles for mineral nucleation. The dolomite developed as nanocrystals, likely via a disordered precursor, which coalesced to form larger micritic crystal aggregates and rhombic crystals. Spheroidal dolomite textures, commonly with hollow cores, are also present and may reflect the mineralisation of a biofilm surrounding coccoid bacterial cells. Dolomite formation within an Mg‐clay matrix is also observed, more commonly within a shallow pedogenic horizon. The ability of the negatively charged surfaces of clay and EPS to bind and dewater Mg2+, as well as the slow diffusion of ions through a viscous clay or EPS matrix, may promote the incorporation of Mg2+ into the mineral and overcome the kinetic effects to allow disordered dolomite nucleation and its later growth. The results of this study show that the precipitation of clay and carbonate minerals in alkaline environments may be closely associated and can develop from the same initial amorphous Ca–Mg–Si‐rich matrix within EPS. The abundance of EPS preserved within the profiles is evidence of past microbial activity. Local fluctuations in chemistry, such as small increases in alkalinity, associated with the degradation of EPS or microbial activity, were likely important for both clay and dolomite formation. Groundwater environments may be important and hitherto understudied settings for microbially influenced mineralisation and for low‐temperature dolomite precipitation. [ABSTRACT FROM AUTHOR]

In order to have a better understanding of the mobility and bioavailability of silver in the environment, it is important to investigate the desorption behaviour of silver from some environmentally relevant soil components. Four single soil constituents (charcoal, humic acid, ferrihydrite, goethite) were chosen to investigate the rate of desorption of silver ions. The effect of reaction period between silver and the soil constituents on subsequent desorption was also investigated. A significant proportion of Ag+ sorbed by Fe oxides, humic acid, and charcoal cannot readily be desorbed back into solution. For goethite and humic acid, a longer contact period between Ag+ and the adsorbing phase caused subsequent slower rates of desorption, and a smaller proportion of adsorbed Ag+ was desorbed back into solution. Two-site and log-normal equations, assuming the desorption reaction was first-order, gave excellent fits to experimental data, except for Ag+ desorption from charcoal, in which case the kinetic experimental data fitted to a l-site model better. The parameters obtained from the modelling provide information regarding the possible changes in metal-oxide binding and metal humic acid complexing mechanisms, and these changes can be interpreted as a re-arrangement of Ag ions to sites with slower desorption reaction rates. All the reaction sites on charcoal have very similar desorption rates (rate constants), and these sites are relatively slow to desorb Ag. Fe oxides, humic acid, and charcoal could be very important sinks for Ag. The longer the Ag interacts with soils in environment, the less mobile Ag becomes, and the less toxic and bioavailable it will be to living organisms. Additional keywords: first-order reactions, log-normal distribution, desorption rate constants., Introduction The mobility and the bioavailability, and so the potential toxicity, of a trace metal ion in soils depends on its concentration in soil solution, and the releasing or binding [...]

It has been demonstrated that the high bulk densities of clayey subsoils of Sodosols can result from a process involving shrink/swell cycles and the development of ped coatings containing topsoil material. Our objective was to study this process according to land use and to compute the amount of organic carbon introduced into the subsoil as a consequence of ped coating in the subsoil. In an area located east of Katanning, we carried out an intensive soil survey and selected 2 closely adjacent sites with similar soils but differing in their land use: a cultivated soil and a never cultivated soil in an undisturbed strip of land. Pit faces and horizontal planes of the top of the subsoil were studied. The polygonal network corresponding to the cross-section of the prismatic peds was described. The clay content and bulk density of the prisms was determined. The carbon contents of the coatings and of the sandy material rich in organic carbon that corresponded to remnants of native vegetation was measured. Results showed that the coating process has occurred under native vegetation but is more active when the land has been used for agricultural and pastoral activities, as indicated by thicker sandy-clay coatings on the vertical faces of prisms and the higher bulk densities in the subsoil. The mean bulk density was 1.71 g/[cm.sup.3] at 20-25 cm depth at the undisturbed site and was 1.86 and 1.82 g/[cm.sup.3] at 20-25 and 25-30 cm depth, respectively, at the Cultivated site. The total soil organic carbon stock was estimated to be close to 68.9 and 61.0 Mg/ha at the undisturbed and cultivated sites, respectively. The organic carbon stock in the subsoil was 38.5 and 23.3% of the respective total stocks. One fourth of the carbon stock in the subsoil corresponded to materials rich in organic carbon that originated from roots of native vegetation. The organic carbon stock in the ped coatings was Additional keywords: Grey Clay, prismatic structure, coating, shrinkage., Introduction The bulk density of clay soils can differ widely depending on clay content, the packing of elementary clay particles (Bruand and Tessier 2000), and water content (Fredlund and Rahardjo [...]

Two ancient lateritic soil profiles from Brazil that are now experiencing hydromorphic conditions were investigated by chemical extractions, X-ray diffraction, differential thermal and thermogravimetric analysis, and analytical transmission electron microscopy (ATEM) to identify if the hydromorphic conditions had affected soil minerals. The soils are composed of gibbsite and kaolinite with less quartz, anatase, goethite, pedogenic chlorite, and amorphous alumino-silicate phases. These last 2 constituents occur in the middle and upper horizons of both soil profiles, together with considerable amounts of organic carbon. Analytical TEM showed that the amorphous phases enveloped corroded gibbsite and kaolinite crystals and may indicate the transformation of these minerals to amorphous phases. The amorphous phases have a similar microfabric to that of allophane and ATEM analyses of the amorphous phases gave an Al/Si atom ratio that was always >2, and commonly about 10. These atom ratios are consistent with the bulk chemical results obtained using pyrophosphate, oxalate, and dithionite extractants, but not with the theoretical ratio for allophane. The Al/Si atom ratio of the amorphous phases was related to the Al content of the mineral enveloped by the amorphous phases, i.e. gibbsite or kaolinite. This association supports the interpretation that the amorphous phases formed from the crystalline minerals. The saturated condition of the profiles, together with the high concentration of organic matter in the upper horizons, favours dissolution of the original gibbsite and kaolinite in the laterite and their transformation to amorphous alumino-silicate phases with a high A1 content., Introduction The geology of the Triangulo Mineiro region of Minas Gerais State, Brazil, has been reported by several authors (e.g. Barbosa et al. 1970; Nishiyama 1989). Few studies have focused [...]

This study aims to evaluate the effect of forest ecosystem on soil properties in a natural reforestation area of Thai Royally-Initiated Khao Cha-Ngum deteriorated area. The forest ecosystem has changed tremendously over 20 years. It changed from degraded forests to dry dipterocarp forests (flat ground and south) and mixed deciduous forests (north, east and west). Soil in the west is different from others because tree species in the west aspect area is more diverse than others. It has equal accumulation of organic matter throughout the soil profile; whereas the accumulation of organic matter of others is in A horizon. Mineral composition of the south, east and west soils contained goethite indicating that these soils were more humid than the flat ground and north soils. Soil micromorphological characteristics indicated that all soils had highly activities of soil organisms. Natural reforestation for a long time has a distinct effect on soil formation in relation to the variety of trees because trees affect the soil moisture, quality and quantity of the litter. [ABSTRACT FROM AUTHOR]