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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.