Although research has shown that manure soil subsurface injection reduces nutrient input to the aquatic environment, it is less known if it also reduces antibiotic surface runoff from manure‐applied fields. Surface runoff of four dairy production antibiotics was monitored comparing (i) surface application and subsurface injection of manure and (ii) time gaps between manure application and a subsequent rain event. Liquid dairy manure spiked with pirlimycin, tylosin, chlortetracycline, and sulfamerazine was applied to 1.5‐m × 2‐m test plots at an agronomic N rate via surface application and subsurface injection. On the day of application (Day 0), and 3 and 7 d after manure application, a simulated rainfall (70 mm h−1) was conducted to collect 30 min runoff. Target antibiotics in runoff water and sediment were quantified using ultra‐performance liquid chromatography tandem mass spectrometry. Results demonstrated that runoff was a significant route for transporting antibiotics off manure‐applied fields, amounting to 0.45 to 2.62% of their initial input with manure. However, compared with manure surface application, subsurface injection reduced sulfamerazine, chlortetracycline, pirlimycin, and tylosin losses in runoff by at least 47, 50, 57, and 88%, respectively. Antibiotic distribution between aqueous and solid phases of runoff was largely determined by water solubility and partition capacity of antibiotics to soil particles. Masses in the aqueous phase were 99 ± 0.5, 94 ± 4, 91 ± 7, and 22 ± 15% of pirlimycin, sulfamerazine, tylosin, and chlortetracycline, respectively. Manure application 3 d or longer before a subsequent rain event reduced antibiotic runoff by 9 to 45 times. Therefore, using subsurface injection and avoiding manure application <3 d before rain would be a recommended manure land management best practice. Core Ideas Compared with surface application, subsurface injection reduced antibiotic runoff. About 3% of antibiotics was lost in 30‐min runoff from a manure surface‐applied field. About 1% of antibiotics was lost in 30‐min runoff from manure subsurface‐injected fields. Antibiotic distribution in runoff water and sediment was compound dependent. Manure application at least 3 d before rain reduced antibiotics in surface runoff.
Preferential flow reduces water residence times and allows rapid transport of pollutants such as organic contaminants. Thus, preferential flow is considered to reduce the influence of soil matrix-solute interactions during solute transport. While this claim may be true when rainfall directly follows solute application, forcing rapid chemical and physical disequilibrium, it has been perpetuated as a general feature of solute transport—regardless of the magnitude preferential flow. A small number of studies have alternatively shown that preferential transport of strongly sorbing solutes is reduced when solutes have time to diffuse and equilibrate within the soil matrix. Here we expand this inference by allowing solute sorption equilibrium to occur and exploring how physiochemical properties affect solute transport across a vast range of preferential flow. We applied deuterium-labeled rainfall to field plots containing manure spiked with eight common antibiotics with a range of affinity for the soil after 7 days of equilibration with the soil matrix and quantified preferential flow and solute transport using 48 soil pore water samplers spread along a hillslope. Based on > 700 measurements, our data showed that solute transport to lysimeters was similar—regardless of antibiotic affinity for soil—when preferential flow represented less than 15% of the total water flow. When preferential flow exceeded 15%, however, concentrations were higher for compounds with relatively low affinity for soil. We provide evidence that (1) bypassing water flow can select for compounds that are more easily released from the soil matrix, and (2) this phenomenon becomes more evident as the magnitude of preferential flow increases. We argue that considering the natural spectrum preferential flow as an explanatory variable to gauge the influence of soil matrix-solute interactions may improve parsimonious transport models.
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