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.
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