Predicting Zinc Bioavailability by Isotopic Labelling In Soils Amended with Organic Materials.

Application of organic materials is a common practice to increase the quality of soils, but the effect on metal availability remains elusive.
The isotopically exchangeable pool of metals, also called the ‘E' value, may conceptually best represent the fraction of metals that is in dynamic equilibrium with metals in the solution phase.

A non contaminated, agricultural soil was amended with peat, sludge or humic acids (4%). Duplicate samples of 2.5 g soil were added 25 ml of a 0.01M CaCl₂ solution, and the suspensions were spiked with 0.25–0.5 ml carrier-free ⁶⁵Zn (10 000 Bq ml^-1, specific activity between 20 and 87GBq g^-1). The amount of Zn added through the radio-active spike was negligibly small. The suspensions were shaken end-over-end for 24 hours. After phase separation by centrifuging at 3000 g for 20 minutes, two 5-ml samples were taken from each tube. The activity of ⁶⁵Zn (1000–1200 keV) were measured (Minaxi, 5530 auto Gamma) on one sample. The other 5-ml sample was acidified to pH 1 with HNO3, prior to analysis of stable Zn with ICP-OES. The %E value is the proportion of radio-labile metal to aqua regia-soluble metal.

Soil amendment with peat, sludge and humic acids increased the %E value 6, 4 and 2-fold respectively, indicating that organic materials enhanced Zn bioavailability. Variation in soil pH was neglectable and did not explain the increase in Zn availability. Despite the low affinity of Zn for organic matter, the increase in soluble organic matter in the soil solution upon soil amendment increases the rate of Zn complexed with soluble organic matter, which was confirmed by the speciation analysis performed with WHAM-VI. Rate of metal soluble complexes in solution largely determines nutrient uptake, which can be controlled by organic materials application.