Wheat-Exuded Organic Acids Influence Zinc Release from Calcareous Soils

Rhizosphere drives plant uptake of sparingly soluble soil zinc (Zn). An investigation with three experiments was conducted to study organic acid exudation by two contrasting wheat genotypes (Sehar-06 and Vatan), Zn fractions in 10 different calcareous soils from Punjab, Pakistan, and release of different soil Zn fractions by organic acids. The two genotypes differed significantly in biomass production and Zn accumulation under deficient and optimum Zn levels in nutrient solution. At a deficient Zn level, Sehar-06 released more rnaleic acid in the rhizosphere than Vatan. Ten soils used in the present study had very different physicochemical properties; their total Zn and Zn distribution among different fractions varied significantly. Zinc release behaviour was determined by extracting the soils with 0.005 mol L-1 citric acid or maleic acid. The parabolic diffusion model best described Zn release as a function of time. Parabolic diffusion model fitting indicated more maleic acid-driven than citric acid-driven soil Zn mobility from different fractions. Cumulative Zn release in six consecutive extractions during 24 h ranged from 1.85 to 13.58 mg kg(-1) using maleic acid and from 0.37 to 11.84 mg kg(-1) using citric acid. In the selected calcareous soils, the results of stepwise linear regression indicated significant release of Fe-Mn oxide-bounded soil Zn by maleic acid and its availability to the Zn-efficient genotype. Hence, release of maleic acid by plants roots played an important role in phytoavailability of Zn from calcareous soils.