reduction in hydraulic conductivity when the soil is treated
with 2% Al 2 O 3 , ZnO, and CuO ENPs, respectively as
compared to untreated clay owing to pore-clogging (Tan
2017). In support of this study, it was found that reduction in
hydraulic conductivity by clogging soil pores when there is
an addition of nanosized material (montmorillonite) to
fine-grained soil. Similar results also observed when metal
oxide ENPs are added to sands (Braun et al. 2015). The
application of nZVI in the soil at concentrations of 1 and
4 g l
−1 showed no consequences on the hydraulic conductivity of natural soil (Reginatto et al. 2020).
6.2 Effect on Biological Properties of Soil
and Phytotoxicity
The increased liberation of ENPs in soil has resulted in
inevitable accumulation in soil and thus becomes a serious
threat to the soil microbial community. Due to the dynamic
nature of ENPs, they produce certain toxicity to soil
microorganisms either directly or indirectly through interacting with other organic compounds. But due to the
dynamic features of ENPs, it is always under discussion and
newer theories have been developed day by day. The
chemicals present in the root exudates in the rhizosphere
greatly affect the physical properties of soil such as pH,
cation exchange capacity (CEC), and salinity which ultimately influence ENPs aggregation and dissolution. Moreover, the rhizospheric microbiome also produces
biomolecules which affect the ENPs fate. For instance,
amino acids such as cysteine have shown to fasten ENPs
aggregation rates but not long-term aggregation to a larger
size (Hsieh 2010).
The impact of ENPs on the plant growth-promoting rhizobacteria (PGPR) like P. aeruginosa, P. putida,
P. fluorescens, B. subtilis, soil nitrifying bacteria, and
phosphate solubilizing bacteria was visible with retarded
growth in culture conditions (Kumar 2018). Metal ENPs are
generally toxic to microorganisms thus these ENPs damages
plant-fungi and plant-bacteria association. Nano-ZnO and
nano-TiO 2 were reported toxic against B. subtilis, E. coli,
and V. fischeri (Li et al. 2011). Metal oxide ENPs of Cu is
found to be toxic against PGPR such as K. pneumoniae, P. aeruginosa, S. paratyphi, and Shigella strains due
to antibacterial property of Cu (Mahapatra et al. 2008). Iron
and copper-based ENPs are observed to react with peroxides
present in the soil, releasing free radicals which have a toxic
effect on microorganisms (Saliba et al. 2006). Javed et al.
(2019) showed that TiO 2 and CuO ENPs reduced the
microbial biomass of the paddy soil due to their chemical
characteristics. Similar outcomes were contemplated for the
impact of ZnO, TiO 2 , CeO 2 , and Fe 3 O 4 ENPs for lowering
bacterial communities in saline/black soil and it was possibly
responsible for the decreased enzymatic activities of invertase, urease, catalase, and phosphatase in the soil (You et al.
2018). Pérez-Hernández et al. (2020) have also summarized
the impact of various ENPs on the soil microbiota and
suggested that they were responsible for the reduction in the
population of mesofauna and microfauna in soil.
Arbuscular mycorrhizal fungi (AMF) are known to show
symbiotic association with plants and thus largely affect
plant growth. However, it has been reported that AMF
diversity decreased after exposure to Fe 3 O 4 NPs at the
concentration of 10 mg kg
−1 (Cao et al. 2017). On the other
hand, AMF remediates the toxicity when ENPs are exposed
to plants. A study revealed the AMF inoculation eliminated
the negative effect of ZnO ENPs on maize by increasing
plant growth and nutrient uptake. Similar effects were
reported in tomato plants when inoculated with AMF, the
plant showed a reduction in Ag uptake up to 12% after
exposure of Ag NPs (Noori et al. 2017). AMF restricts the
uptake of ENPs by plants through the discharge of glycoprotein known as glomalin which acts as a chelator in the
rhizospheric region (Siani et al. 2017).
The exposure of copper oxide nanoparticles (1000 mg/kg)
prevail suppressed immune response followed by the mortality of the earthworm species (Metaphire posthuma) which
was related to phagocytosis, production of cytotoxic molecules, stress enzymes, and loss of total protein of coelomocytes (Gautam et al. 2018). In another study, CuO ENPs were
found to limit the life span of another invertebrate
species Enchytraeus crypticus (Gonçalves et al. 2017). More
susceptibility of ENPs was recorded in the case of juvenile
species of Lumbricus rubellus than adult populations after
exposure with C-60 ENPs (Van Der Ploeg et al. 2013). The
ENPs remain attached to soil colloids, invertebrates internalize the ENPs by ingestion and eventually get transferred to
the gut epithelium. The ENPs toxicity toward soil invertebrates through hindering ribosomal and histone activity, by
disrupting sugar, protein, and lipid metabolism (Novo et al.
2015). Moreover, NMR studies have shown the amino acid
such as leucine, valine, isoleucine, and sugars such as glucose
and maltose are potential bioindicators of ENPs toxicity in
invertebrates (Liang et al. 2017). The negative impact of Ag
ENPs on reproduction ability in E. andrei (Velicogna et al.
2017) and multiwalled carbon nanotubes (MWCNTs) on E.
fetida population were also reported (Zhang et al. 2014) and it
might be due to the inhibition of antioxidant enzymes and
restriction on metabolic pathways.
The phytotoxicity of ENPs mainly depends on its size,
shape, chemical properties, and chemical subcellular sites
where it is accumulated. Depending upon the chemical and
physical nature of plant cell wall, ENPs act as a carrier or
modulator which interacts with cellular processes. ENPs
when interacts physically with plant cells, it mainly clogs the
cellular structures mechanically while chemically it
Engineered Nanoparticles in Agro-ecosystems: Implications …
111
with 2% Al 2 O 3 , ZnO, and CuO ENPs, respectively as
compared to untreated clay owing to pore-clogging (Tan
2017). In support of this study, it was found that reduction in
hydraulic conductivity by clogging soil pores when there is
an addition of nanosized material (montmorillonite) to
fine-grained soil. Similar results also observed when metal
oxide ENPs are added to sands (Braun et al. 2015). The
application of nZVI in the soil at concentrations of 1 and
4 g l
−1 showed no consequences on the hydraulic conductivity of natural soil (Reginatto et al. 2020).
6.2 Effect on Biological Properties of Soil
and Phytotoxicity
The increased liberation of ENPs in soil has resulted in
inevitable accumulation in soil and thus becomes a serious
threat to the soil microbial community. Due to the dynamic
nature of ENPs, they produce certain toxicity to soil
microorganisms either directly or indirectly through interacting with other organic compounds. But due to the
dynamic features of ENPs, it is always under discussion and
newer theories have been developed day by day. The
chemicals present in the root exudates in the rhizosphere
greatly affect the physical properties of soil such as pH,
cation exchange capacity (CEC), and salinity which ultimately influence ENPs aggregation and dissolution. Moreover, the rhizospheric microbiome also produces
biomolecules which affect the ENPs fate. For instance,
amino acids such as cysteine have shown to fasten ENPs
aggregation rates but not long-term aggregation to a larger
size (Hsieh 2010).
The impact of ENPs on the plant growth-promoting rhizobacteria (PGPR) like P. aeruginosa, P. putida,
P. fluorescens, B. subtilis, soil nitrifying bacteria, and
phosphate solubilizing bacteria was visible with retarded
growth in culture conditions (Kumar 2018). Metal ENPs are
generally toxic to microorganisms thus these ENPs damages
plant-fungi and plant-bacteria association. Nano-ZnO and
nano-TiO 2 were reported toxic against B. subtilis, E. coli,
and V. fischeri (Li et al. 2011). Metal oxide ENPs of Cu is
found to be toxic against PGPR such as K. pneumoniae, P. aeruginosa, S. paratyphi, and Shigella strains due
to antibacterial property of Cu (Mahapatra et al. 2008). Iron
and copper-based ENPs are observed to react with peroxides
present in the soil, releasing free radicals which have a toxic
effect on microorganisms (Saliba et al. 2006). Javed et al.
(2019) showed that TiO 2 and CuO ENPs reduced the
microbial biomass of the paddy soil due to their chemical
characteristics. Similar outcomes were contemplated for the
impact of ZnO, TiO 2 , CeO 2 , and Fe 3 O 4 ENPs for lowering
bacterial communities in saline/black soil and it was possibly
responsible for the decreased enzymatic activities of invertase, urease, catalase, and phosphatase in the soil (You et al.
2018). Pérez-Hernández et al. (2020) have also summarized
the impact of various ENPs on the soil microbiota and
suggested that they were responsible for the reduction in the
population of mesofauna and microfauna in soil.
Arbuscular mycorrhizal fungi (AMF) are known to show
symbiotic association with plants and thus largely affect
plant growth. However, it has been reported that AMF
diversity decreased after exposure to Fe 3 O 4 NPs at the
concentration of 10 mg kg
−1 (Cao et al. 2017). On the other
hand, AMF remediates the toxicity when ENPs are exposed
to plants. A study revealed the AMF inoculation eliminated
the negative effect of ZnO ENPs on maize by increasing
plant growth and nutrient uptake. Similar effects were
reported in tomato plants when inoculated with AMF, the
plant showed a reduction in Ag uptake up to 12% after
exposure of Ag NPs (Noori et al. 2017). AMF restricts the
uptake of ENPs by plants through the discharge of glycoprotein known as glomalin which acts as a chelator in the
rhizospheric region (Siani et al. 2017).
The exposure of copper oxide nanoparticles (1000 mg/kg)
prevail suppressed immune response followed by the mortality of the earthworm species (Metaphire posthuma) which
was related to phagocytosis, production of cytotoxic molecules, stress enzymes, and loss of total protein of coelomocytes (Gautam et al. 2018). In another study, CuO ENPs were
found to limit the life span of another invertebrate
species Enchytraeus crypticus (Gonçalves et al. 2017). More
susceptibility of ENPs was recorded in the case of juvenile
species of Lumbricus rubellus than adult populations after
exposure with C-60 ENPs (Van Der Ploeg et al. 2013). The
ENPs remain attached to soil colloids, invertebrates internalize the ENPs by ingestion and eventually get transferred to
the gut epithelium. The ENPs toxicity toward soil invertebrates through hindering ribosomal and histone activity, by
disrupting sugar, protein, and lipid metabolism (Novo et al.
2015). Moreover, NMR studies have shown the amino acid
such as leucine, valine, isoleucine, and sugars such as glucose
and maltose are potential bioindicators of ENPs toxicity in
invertebrates (Liang et al. 2017). The negative impact of Ag
ENPs on reproduction ability in E. andrei (Velicogna et al.
2017) and multiwalled carbon nanotubes (MWCNTs) on E.
fetida population were also reported (Zhang et al. 2014) and it
might be due to the inhibition of antioxidant enzymes and
restriction on metabolic pathways.
The phytotoxicity of ENPs mainly depends on its size,
shape, chemical properties, and chemical subcellular sites
where it is accumulated. Depending upon the chemical and
physical nature of plant cell wall, ENPs act as a carrier or
modulator which interacts with cellular processes. ENPs
when interacts physically with plant cells, it mainly clogs the
cellular structures mechanically while chemically it
Engineered Nanoparticles in Agro-ecosystems: Implications …
111