Subsurface Manure Injection Reduces Surface Transport of Antibiotic Resistance Genes but May Create Antibiotic Resistance Hotspots in Soils

Compared to surface application, manure subsurface injection reduces surface runoff of nutrients, antibiotic resistant microorganisms, and emerging contaminants. Less is known regarding the impact of both manure application methods on surface transport of antibiotic resistance genes (ARGs) in manure-amended fields. We applied liquid dairy manure to field plots by surface application and subsurface injection and simulated rainfall on the first or seventh day following application. The ARG richness, relative abundance (normalized to 16s rRNA), and ARG profiles in soil and surface runoff were monitored using shotgun metagenomic sequencing. Within 1 day of manure application, compared to unamended soils, soils treated with manure had 32.5-70.5% greater ARG richness and higher relative abundances of sulfonamide (6.5-129%) and tetracycline (752-3766%) resistance genes (p ≤ 0.05). On day 7, soil ARG profiles in the surface-applied plots were similar to, whereas subsurface injection profiles were different from, that of the unamended soils. Forty-six days after manure application, the soil ARG profiles in manure injection slits were 37% more diverse than that of the unamended plots. The abundance of manure-associated ARGs were lower in surface runoff from manure subsurface injected plots and carried a lower resistome risk score in comparison to surface-applied plots. This study demonstrated, for the first time, that although manure subsurface injection reduces ARGs in the runoff, it can create potential long-term hotspots for elevated ARGs within injection slits.

A spectrum of preferential flow alters solute mobility in soils

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.

Culturable antibiotic-resistant fecal coliform bacteria in soil and surface runoff after liquid dairy manure surface application and subsurface injection

Land application of manure, while beneficial to soil health and plant growth, can lead to an overabundance of nutrients and introduction of emerging contaminants into agricultural fields. Compared with surface application of manure, subsurface injection has been shown to reduce nutrients and antibiotics in surface runoff. However, less is known about the influence of subsurface injection on the transport and persistence of antibiotic-resistant microorganisms. We simulated rainfall to field plots at two sites (one in Virginia and one in Pennsylvania) 1 or 7 d after liquid dairy manure surface and subsurface application (56 Mg ha^-1 ) and monitored the abundance of culturable antibiotic-resistant fecal coliform bacteria (ARFCB) in surface runoff and soils for 45 d. We performed these tests at both sites in spring 2018 and repeated the test at the Virginia site in fall 2019. Manure subsurface injection, compared with surface application, resulted in less ARFCB in surface runoff, and this reduction was greater at Day 1 after application compared with Day 7. The reductions of ARFCB in surface runoff because of manure subsurface injection were 2.5-593 times at the Virginia site in spring 2018 and fall 2019 and 4-5 times at the Pennsylvania site in spring 2018. The ARFCB were only detectable in the 0-to-5-cm soil depth within 14 d of manure surface application but remained detectable in the injection slits of manure subsurface-injected plots even at Day 45. This study demonstrated that subsurface injection can significantly reduce surface runoff of ARFCB from manure-applied fields.

Plants mediate precipitation-driven transport of a neonicotinoid pesticide

Neonicotinoid insecticides provide crop protection via water solubility and systemicity, yet these chemical characteristics, combined with high toxicity to non-target invertebrates (e.g., honeybees), elicit concern of environmental transport. Neonicotinoids have been detected in soil and surface water throughout North America; however, no investigation has defined a direct connection to planted seed dressings. We quantified the physical transport of thiamethoxam (TMX), a neonicotinoid, under field conditions. We planted TMX-coated corn seeds and maintained plots with and without viable crops (n = 3 plots per treatment) to determine plant influence on pesticide transport. TMX concentrations were measured in soil and drainage throughout the growing season. Storm-generated runoff was the dominant transport mechanism (maximum TMX concentration 1.72 ± 0.605 μg L^-1; no viable plants), followed by shallow (<72 cm) lateral drainage (0.570 ± 0.170 μg L^-1; no viable plants), and deep (110 cm) drainage (0.170 ± 0.265 μg L^-1; viable plants). Soil samples confirmed vertical and lateral movement within 23 and 36 days of planting, respectively. Plants facilitated downward migration of TMX in soil but restricted TMX drainage. Altogether, these study results revealed that neonicotinoids can be transported from seed coatings both above and through the soil profile, which may enable migration into surrounding ecosystems.

Transport of a neonicotinoid pesticide, thiamethoxam, from artificial seed coatings

Neonicotinoid insecticides coat the seeds of major crops worldwide; however, the high solubility of these compounds, combined with their toxicity to non-target organisms, makes it critical to decipher the processes by which they are transported through soils and into aquatic environments. Transport and distribution of a neonicotinoid (thiamethoxam, TMX) were investigated by growing TMX-coated corn seeds in coarse-textured and fine-textured soil columns (20 and 60cm lengths). To understand the influence of living plants, corn plants were terminated in half of the columns (no plant treatment) and allowed to grow to the V5 growth stage (33days of growth) in the other half (with plant treatment). TMX was analyzed in leachate 12 times over 33days and in bulk soil after 8, 19, and 33days of corn growth. All 20cm columns leached TMX at levels exceeding the United States Environmental Protection Agency benchmark for aquatic invertebrates (17.5μgL^-1). TMX migrated from seeds to adjacent bulk soil by the eighth day and reached deeper soil sections in later growth stages (e.g., 30-45cm depth by Day 33). Fine-particle soils transported over two orders of magnitude more TMX than coarse-textured soils (e.g., 29.9μg vs 0.17μg, respectively), which was attributed to elevated evapotranspiration (ET) rates in the sandy soil driving a higher net retention of the pesticide and to structural flow occurring in the fine-textured soil. Living plants increased TMX concentrations at depth (i.e., 30-60cm) compared to the no plant treatment, suggesting that corn growth may drive preferential transport of TMX from coated seeds. Altogether, this study showed that neonicotinoid seed coatings can be mobilized through soil leachate in concentrations considered acutely toxic to aquatic life.