Effects of Grazing and Biosolids Application on Zinc Concentration in Soil Solution from a Bahiagrass Pasture in North Florida

Abstract

Pasture-applied waste products, including excreta from grazing animals, contain not only organic residues and nutrients but also a considerable amount of heavy metals (HM) including Zn and Cu, which are essential micronutrients but can be potential sources of water pollution when in excess. The objectives of this study were to assess Zn and Cu concentrations in soil solution in a pasture under two grazing beef-cattle groups, with (G2) and without (G3) medicated feed, and to determine potential aftereffects on soil solution Zn and Cu of a single biosolids application six years earlier at the agronomic rate (B1) and at twice that rate (B2) compared to a mineral-fertilized control (C1). This study was conducted at the University of Florida Santa Fe Beef Research Center, situated near Alachua in north Florida on Ultisols of the Millhopper series. The experimental design was a completely randomized block with suction cup lysimeters assigned to the treatments and installed at the depths of 61, 122, and 183 cm. Water samples from the lysimeters, the three wells on the farm, the nearby stream, and rain events were collected from November 2002 through August 2003 and were analyzed for pH, EC, Zn, and Cu concentrations. The results indicated that the mean Zn concentrations in the soil solution were low and ranged from 0.070 to 0.112 mg L-1 with no significant difference between the grazing treatments. The detected Cu concentrations were very low. For the biosolids aftereffect analysis, however, significantly higher Zn concentrations and downward migrations were observed in the soil solution of B2 plots, where the mean Zn concentration was 0.148 mg L-1 at 183 cm depth. The highest Zn concentrations were detected at the beginning of the monitoring period and decreased rapidly to mean values below 0.03 mg L-1. Despite the considerable range of soil solution Zn concentrations, a tendency for higher Zn concentration in soil solution in comparison to the rain, surface, and ground waters was identified. The results have shown that grazing, particularly medicated feed supplements, had no significant effect on the mean soil solution Zn concentrations. However, the Zn concentration was significantly (p < 0.05) affected by the biosolids application six years earlier, and an increase in soil solution Zn was observed with depth in the profile.