FIG. 2.

HCl-soluble Fe²⁺ concentrations in manure slurries after 0, 2, and 6 days in microcosms prepared without gravel (A) and with gravel (B). Microcosms were amended with FeCl₃ to obtain initial concentrations of 0, 500, 1,000, 2,000, and 2,500 mg of Fe³⁺ liter⁻¹. Column heights represent means of four measurements; error bars represent standard deviations from the means. Stars above columns indicate significantly higher mean Fe²⁺ concentrations (P < 0.05) based on paired t tests with means from microcosms without gravel and amended with the same amount of FeCl₃.