A Zn solubility trial was established using all soil samples (n = 36;
18 topsoils, 0–30 cm and 18 subsoils, 30–60 cm). Soil (5.0 g, b2 mm)
was suspended in 60 mL 0.01 M Ca(NO3)2 in glass flasks in a reciprocal
shaking water bath at 25 °C constant temperature. A gas line with 3%
CO2 partial pressure in airwas introduced in a constant bubbling stream
"into the suspensions to simulate conditions in an active topsoil
(c. 100 × atmospheric CO2 concentrations). Soil supernatant solutions
were extracted three times over an incubation period of 3 days and
assayed for cations, pH, dissolved organic carbon and dissolved inorganic
carbon (DOC and DIC). DOC and DIC in the filtered soil solution
(b0.22 μm) from the un-spiked control suspensions were determined
using a Shimadzu TOC-V CPH/CPN Total Organic Carbon Analyser
(Model TNM-1); other elements, including Zn, were determined by
ICP-MS (as above) to enable speciation modelling.
The free metal ion activity of Zn2+ in Peshawar soils was modelled
using two approaches based on adsorption (Model I) and solubility
(Model II) mechanisms. Model I (Eq. (2)) assumed that a pHdependent
distribution coefficient (Kd) described the partitioning of
Zn between a free ionic form and an adsorbed labile fraction (from the
E-value),