In the present study we evaluate the effect of superparamagnetic
iron oxide nanoparticles (SPIONs) carrying usnic acid (UA) as chemical
cargo on the soil microbial community in a dystrophic red latosol
(oxysol). Herein, 500 ppm UA or SPIONs-framework carrying UA were
diluted in sterile ultrapure deionized water and applied by hand sprayer
on the top of the soil. The experiment was conducted in a growth chamber
at 25 °C, with a relative humidity of 80% and a 16 h/8 h light–dark
cycle (600 lx light intensity) for 30 days. Sterile ultrapure deionized
water was used as the negative control; uncapped and oleic acid (OA)
capped SPIONs were also tested to assess their potential effects.
Magnetic nanostructures were synthesized by a coprecipitation method
and characterized by scanning and transmission electron microscopy
(SEM and TEM), X-ray diffraction (XRD), Fourier-transform infrared
spectroscopy (FTIR), zeta potential, hydrodynamic diameter, magnetic
measurements, and release kinetics of chemical cargo. Uncapped and
OA-capped SPIONs did not significantly affect soil microbial community.
Our results showed an impairment in the soil microbial community exposed
to free UA, leading to a general decrease in negative effects on soil-based
parameters when bioactive was loaded into the nanoscale magnetic carrier.
Besides, compared to control, the free UA caused a significant decrease
in microbial biomass C (39%), on the activity of acid protease (59%),
and acid phosphatase (23%) enzymes, respectively. Free UA also reduced
eukaryotic 18S rRNA gene abundance, suggesting a major impact on fungi.
Our findings indicate that SPIONs as bioherbicide nanocarriers can
reduce the negative impacts on soil. Therefore, nanoenabled biocides
may improve agricultural productivity, which is important for food
security due to the need of increasing food production.