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To evaluate different mathematical models for describing cumulative release of phosphorus (P), aluminium (Al) and iron (Fe) by citric acid (CA) from soil, an experiment was conducted in packed soil columns with three contrasting soilsviz.Alfisol, Vertisol and Inceptisol. The soils were saturated with CA at 0, 5, 10, 20, 40 and 100 mg L−1, the leachate were collected by liquid displacement at 15, 30, 45, 60 day intervals, and P, Al and Fe releases were analyzed. The release data were fitted to linear, quadratic, exponential, logarithmic, and power form of equations. The results revealed that the highest cumulative release of P, Al and Fe was observed in the 40 mg L−1CA treatment. In the majority of cases, the relationship between CA concentration and cumulative release of ions fitted well in quadratic equation (y = ax2 + bx + c). The Fe and Al releases with time were fitted well in power-form (y = atb), while the cumulative P release in linear equation (y = mx + c). The study also revealed that a substantial amount of released P could serve as potential source of plant nutrient in fixation prone soils. The study demonstrated that CA can be used as a ‘solubilizer’ of soil P, 40 mg L−1dose released the highest P, 0–20 mg L−1dose was observed to be economical and 5 mg L−1was the most promising dose in Alfisol. [ABSTRACT FROM AUTHOR]

This study assessed the effect of land use systems (rice-fallow, bamboo, mandarin and forest) along an altitude gradient (<500, 500–1000 and > 1000 m asl) and soil depth (0–15, 15–30 and 30–45 cm) on C stability in naturally occurring clay-organic complex (NOCOC) and their relationship with soil properties. The C stability in NOCOC (1/k) was determined from desorption rate constant (k) of humus-C by sequential extraction and was correlated with soil properties across the altitude. The C stability in NOCOC decreased (34%) with increasing soil depth from 0–15 to 30–45 cm. Across the altitudes, highest C stability in NOCOC was at > 1000 m asl (8.37 h) which was 12.7 and 9.4% higher than 500–1000 and < 500 m asl, respectively. Irrespective of soil depth and altitude, forest (5.30 h) showed the greatest C stability in NOCOC followed by mandarin (4.64 h), bamboo (4.20 h) and rice-fallow (3.85 h). Measurable soil properties could explain 90–94% variability of C stability in NOCOC across the altitude. Furthermore, C stability in NOCOC increased macroaggregate formation (0.25 mm) and ensured greater physical, chemical and microbial protection of organic C in soil. [ABSTRACT FROM AUTHOR]