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Soil clay minerals significantly influence the accumulation and stabilization of organic carbon (OC). However, the effect of interactions among phyllosilicate clay minerals, native OC and sesquioxides (Fe/Al oxides) on the adsorption-desorption of dissolved organic carbon (DOC) under different background electrolyte types and concentration is poorly understood. A set of batch adsorption-desorption experiments were conducted using pedogenic clays extracted from soils dominated by kaolinite-illite (Kaol-Ill), smectite (Smec) and allophane (Allo). The clay samples were sequentially treated to remove native OC and sesquioxides, and tested for adsorption-desorption of DOC under various solution conditions. All the experiments were conducted at pH 7 using water extractable fraction of OC from wheat residues. DOC adsorption increased with increasing background electrolyte concentration, and the presence of Ca 2+ significantly enhanced the uptake in comparison to Na + due to a possible cationic bridging effect. Under all electrolyte conditions, the maximum DOC adsorption capacity (Q max ) (mg g −1 ) of the soil clay fractions (SCF) maintained the order: Allo N Smec N Kaol-Ill. A similar order was also observed when the adsorption capacities were normalized to the specific surface area (SSA) of the SCFs (mg m −2 ). DOC adsorption showed a positive relationship with SSA, and sesquioxides and allophanic minerals provided the largest contributions to the SSA in the SCF. Removal of sesquioxides from the SCF resulted in a decrease in SSA and thus DOC adsorption, whereas removal of native OC increased the SSA and subsequent DOC adsorption. Because this study used pedogenic SCFs which represented soils formed in different environments instead of processed clays from geological deposits, it provided realistic information about the interaction of DOC with SCF in relation to their native OC and sesquioxide contents. It also revealed the importance of Ca 2+ in enhancing the carbon adsorption capacities of these SCFs.

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A long-term field demonstration was initiated in 1995 to evaluate the effect of organic manures (FYM, poultry manure and pressmud) and mineral fertilizers on changes of pH, electrical conductivity (EC), organic C and nutrient contents under pearl millet -wheat cropping sequence. Continuous application of organic manures alone or in conjunction with NP fertilizer for 10 years decreased the soil pH. However, a reverse trend was observed in case of EC. Organic C content of the soil decreased from its initial value, when only NP fertilizers were applied and increased significantly with the application of organic manures applied alone or with NP fertilizers. The highest organic carbon content of the soil has approached to 0.99% in plot receiving 15 Mg FYM þ150 kg N þ30 kg P 2 O 5 ha 71 . The application of organic manures with or without NP fertilizers could not sustain the original level of N. However, their application increased the available P, K and DTPA extractable Zn, Fe, Mn and Cu content of soil. Application of P, K and micronutrients can be avoided with the application of organic manures. The build-up of organic C and nutrient contents was higher in surface soil as compare to subsurface soil.

A long term issue that has hampered the efficient operation of heavy haul tracks is the migration of fluidized fines from the shallow soft subgrade to the overlying ballast, i.e., mud pumping. This paper presents a series of undrained cyclic triaxial tests, where realistic cyclic loading conditions were simulated at low confining pressure that is typical of shallow subgrade beneath a ballast track. Subgrade soil specimens with a low plasticity index collected from a field site with recent history of mud pumping were tested at frequencies from 1.0 to 5.0 Hz and a cyclic stress ratio (CSR) from 0.1 to 1.0. The experimental results indicate that under adverse loading conditions of critical cyclic stress ratio (CSRc) and frequency, there is upward migration of moisture and the finest particles towards the specimen top, and this causes the uppermost part of the soil specimen to soften and fluidize. Conversely, a smaller value of CSR tends to maintain stability of the specimen despite the increasing number of loading cycles. It is noteworthy that for any given combination of CSR and frequency, the relative compaction has a significant influence on the cyclic behaviour of the soil and its potential for fluidization.

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Results from earlier laboratory and field experiments are 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. Labelled 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 substantial 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. OC close to mineral surfaces or within microaggregates is held most strongly. C sequestration may occur in subsoils with unsaturated mineral surfaces. However, incorporation of carbon into macroaggregates from enhanced plant growth might be most effective to remove excess carbon from the atmosphere, albeit over the short-term.