It's all downhill from here, antibiotic runoff that is

 By Angelica Leos

A figure showing the patterns observed of total antibiotic concentration at different sampling sites on the hillslope. The black dots are those with manure and the white dots show the control (no manure treatment). Figure taken from Zhao et al. 2020

Large amounts of antibiotics enter the soil environment through the application of animal manure in agricultural fields to improve nutrient levels. These antibiotics typically enter water sources via runoff or they are drained away from the soil by rainwater. A study by Zhao and colleagues explores the patterns of these antibiotics in the soil in response to rainfall events on a hillside scale. The researchers used a hillslope in the Zhangxi watershed where pig manure was applied in one area and another hillside was used without any manure application. The hillslopes were divided into three areas: cropland, peach orchard, and forestland. For each area, five subsamples of soil were collected and three samples of the surface were collected for each of the plots. The study found that after rainfall events, antibiotics were rich in areas where manure was applied and antibiotic concentration in the soil increased from the top to the bottom of the hills. The researchers also discovered that the soil aggregate, organic matter, and clay content played important roles in the drainage of antibiotics by rainwater. This research impacts how farmers should adapt their manure applications with respect to the type of terrain they are using in order to prevent high concentrations in their crops. 

Original article:

Zhao, Fangkai, et al. "Effects of Land Use and Rainfall on Sequestration of Veterinary Antibiotics in Soils at the Hillslope Scale." Environmental Pollution, vol. 260, 2020, doi:10.1016/j.envpol.2020.114112