Abstract
Exposure to sub-inhibitory concentrations of antibiotics has been shown to alter the metabolic activity of micro-organisms, but the impact on soil denitrification and N2O production has rarely been reported. In this study, incubation and column transport experiments were conducted on soils exposed to as many as four antibiotics in the ng·kg−1 range (several orders of magnitude below typical exposure rates) to evaluate the impact of ultralow dose exposure on net nitrate losses and soil N2O flux over time. Under anaerobic incubation conditions, three antibiotics produced statistically significant dose response curves in which denitrification was stimulated at some doses and inhibited at others. Sulfamethoxazole in particular had a stimulatory effect at ultralow doses, an effect also evidenced by a near 17% increase in nitrate removal during column transport. Narasin also showed evidence of stimulating denitrification in anaerobic soils within 3 days of exposure, which is concurrent to a statistically significant increase in N2O flux measured over moist soils exposed to similar doses. The observation that even ultralow levels of residual antibiotics may significantly alter the biogeochemical cycle of nitrogen in soil raises a number of concerns pertaining to agriculture, management of nitrogen pollution, and climate change, and warrants additional investigations.
Highlights
SMX and SDZ are among the most frequently detected antibiotics in groundwater with reported concentrations ranging from 0.08 ng·L−1 to 1.11 μ g·L−1
Analysis of variance (ANOVA) identified statistically significant dose-responses in for 3 of the 4 antibiotics tested; the majority of these were observed in soils treated with SMX
Disturbances to the biogeochemical nitrogen cycle have been reported in soils and sediment exposed to a wide-range of antibiotic compounds
Summary
SMX and SDZ are among the most frequently detected antibiotics in groundwater with reported concentrations ranging from 0.08 ng·L−1 12 to 1.11 μ g·L−1 13. Assuming a partition coefficient (Kd) of 2.0 Lkg-114, the concentration in saturated soils can be estimated between 0.16 ng·kg−1 and 2.22 μ g·kg−1 though this may vary depending upon the antibiotic source (e.g., sewage sludge vs groundwater) and is subject to rapid dissipative losses[15]. The effect of all four selected antibiotics on gross denitrification was measured in terms of nitrate losses from anaerobic pot incubations in which soils were exposed to ng·kg−1 doses. N2O flux experiments, conducted over moist soils, were performed using NAR, which is less mobile[17] and tends to sorb in the upper, temporally moist soil horizons where N2O is lost to the atmosphere
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