Effects of the antimicrobial agent sulfamethazine on metolachlor persistence and sorption in soil
Introduction
For decades, antimicrobial agents have been widely used in human and veterinary medicine to treat and prevent infectious bacterial diseases. Approximately half of these antimicrobial agents are used worldwide in human medicine; the remainder is used in livestock production (Kümmerer, 2003). In a number of situations where food-producing animals are intensively reared, some antimicrobials are added to food in order to control disease and improve food conversion efficiency. A significant fraction of the antimicrobials fed to animals is excreted unaltered in feces and urine (Levy, 1992). Research indicates that these animal wastes, when applied to fields, represent a potential for the spread of antimicrobials in the environment via non-point source pollution (Kolpin et al., 2002, Christian et al., 2003, Kumar et al., 2004). Furthermore, antimicrobials present in soil and natural water can increase the proliferation of antimicrobial-resistant microorganisms (Rooklidge, 2004).
The most widely used veterinary antimicrobials in the European Union include tetracyclines, macrolides, penicillins, aminoglycosides and sulfonamides (Haller et al., 2002). Sulfonamides are synthetic antimicrobial agents with a broad spectrum against most gram-positive and gram-negative bacteria. According to Thiele-Bruhn et al. (2004), as much as 90% of the sulfonamides are excreted after consumption. Sulfamethazine (sulfadimidine) is a member of the antimicrobials class of sulfonamides. Recent field monitoring conducted in Germany revealed up to 15 μg kg−1 of sulfamethazine in soil seven months after manure application on fields (Christian et al., 2003). Other investigations have also demonstrated the potential for sulfonamides to contaminate subsoil (Boxall et al., 2002).
The environmental fate of antimicrobials in the soil ecosystem is conventionally estimated considering their persistence and sorption in soil. Although such basic information is required to predict the fate of antimicrobial agents in soil, no specific data regarding potential interactions between antimicrobials and soil-applied pesticides are available.
Most modern soil-applied pesticides are transformed in soil by means of microbial degradation (Soulas and Lagacherie, 2001). Since antimicrobial agents are designed to have an adverse effect on microorganisms, their potential effect on the persistence of soil-applied pesticides should be considered. Depending on environmental conditions and other factors (i.e. antimicrobial concentration and bioavailability), the effect of antimicrobial agents on microbial processes involved in the degradation of pesticides is not easily deducible. On the other hand, the study of potential effects of veterinary antimicrobial agents on pesticide degradation in soil would represent a different approach to assess their impact on soil ecology and soil conservation.
Metolachlor is a soil-applied herbicide widely used for pre-emergence and post-emergence weed control in a variety of crops including corn and soybean. Degradation of metolachlor in soil is a microbial process (Miller et al., 1997, Accinelli et al., 2001). Field and laboratory investigations showed that degradation of metolachlor is to some extent related to soil microbial biomass and activity (Staddon et al., 2001, Accinelli et al., 2003). Under some circumstances, degradation and sorption do not prevent transport of metolachlor to surface and subsurface water (Aga and Thurman, 2001, Accinelli et al., 2002, Leu et al., 2004).
The objective of the present study was to determine whether or not the addition of sulfamethazine to soil affects the persistence and sorption of metolachlor in soil.
Section snippets
Soil and soil sample preparation
The upper 0–10 cm of a sandy loam soil (Udertic Ustochrepts, fine, mixed, mesic) with 650 g kg−1 sand, 144 g kg−1 clay, 206 g kg−1 silt, 13 g kg−1 organic C, pH (1:2.5 soil/water mixture) of 6.5, was collected from an uncropped area at the Experimental Farm of the University of Bologna (Italy). The soil had not been previously treated with pesticides, manure or slurry. In the laboratory, the soil was air dried, ground and passed through a 2-mm pore size sieve. For the metolachlor degradation study and
Metolachlor degradation
Fig. 1 shows the degradation of metolachlor in the control soil. The employed soil showed a high capacity for metolachlor degradation. Absence of a lag phase confirmed that metolachlor degradation was a mainly co-metabolic process. After the 50-day incubation period, only 11% of the applied parent compound remained in the soil. As indicated in Table 1, metolachlor degradation was correctly described by the first-order kinetic model (r2 = 0.98), with an estimated half-life (t1/2) of 16 days. This
Conclusions
Within the experimental conditions of the present laboratory study, concentrations of the antimicrobial agent sulfamethazine in the μg kg−1 range did not significantly influence the degradation and sorption of metolachlor in a sandy loam soil. Measurement of enumerable soil bacteria supported these findings. Sulfamethazine concentrations were established on the basis of observed concentrations in soil samples collected from fields that received an application of contaminated manure. Samples were
Acknowledgments
The authors are grateful to the German Academic Exchange Service (DAAD) and to the Programma Vigoni (CRUI) for supporting travelling expenses and to Dr. T. Christian for scientific assistance.
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