The effect of long-term cropping systems on adsorption of metals was studied for soils obtained from two sites, Clarion-Webster Research Center (CWRC site) at Kanawha and Galva-Primghar Research Center (GPRC site) at Sutherland, under long-term rotation experiments in Iowa. Each experiment consisted of three cropping systems: continuous corn (CCCC), corn-soybean-corn-soybean (CSCS), and corn-oats-meadow-meadow (COMM), and treated with (+N) and without (0 N) ammoniacal fertilizer. The amount of metal adsorbed was calculated by subtracting the amount of metal recovered from that initially added. Statistical analysis (ANOVA) showed that cropping systems significantly affected organic C content (P < 0.05) and soil pH (P < 0.01), whereas fertilization with NH4+ significantly affected soil pH and percentage base saturation (P < 0.01) of soils from the GPRC site. No such effects were observed for the soils from the CWRC site. Langmuir adsorption maxima values varied among the metals, but the highest was for Pb; the highest bonding energy coefficient value for soils from both sites was for Cu. Treatments with ammonium-forming fertilizers significantly decreased (P < 0.05) the Cd adsorption maxima of soils under CCCC at both research sites and Cu and Pb adsorption maxima of soils at the GPRC site but not at the CWRC site. In general, CSCS and COMM cropping systems did not significantly affect the metal adsorption maxima of soils obtained from both sites. Cadmium, Cu, and Pb adsorption were significantly correlated (P < 0.05) with pH and percentage base saturation for soils from both sites.
