Effect of Engineered Nanoparticles of Fe and Zn Oxides on Enzyme Activity and Bacterial Abundance in Soil at Ambient and Elevated Atmospheric CO2

Elevated CO2 altered the nano-ZnO-induced influence on bacterial and fungal composition in tomato (Solanum lycopersicum L.) rhizosphere soils

To investigate whether elevated CO₂ (eCO₂) changes the influence of nanoparticles (NPs) on soil microbial communities and the mechanisms, various nano-ZnO (0, 100, 300, and 500 mg·kg^-1) and CO₂ concentrations (400 and 800 µmol·mol^-1) were applied to tomato plants (Solanum lycopersicum L.) in growth chambers. Plant growth, soil biochemical properties, and rhizosphere soil microbial community composition were analyzed. In 500 mg·kg^-1 nano-ZnO-treated soils, root Zn content was 58% higher, while total dry weight (TDW) was 39.8% lower under eCO₂ than under atmospheric CO₂ (aCO₂). Compared with the control, the interaction of eCO₂ and 300 mg·kg^-1 nano-ZnO decreased and increased bacterial and fungal alpha diversities, respectively, which was caused by the direct effect of nano-ZnO (r =  - 1.47, p < 0.01). Specifically, the bacterial OTUs decreased from 2691 to 2494, while fungal OTUs increased from 266 to 307, when 800-300 was compared with 400-0 treatment. eCO₂ enhanced the influence of nano-ZnO on bacterial community structure, while only eCO₂ significantly shaped fungal composition. In detail, nano-ZnO explained 32.4% of the bacterial variations, while the interaction of CO₂ and nano-ZnO explained 47.9%. Betaproteobacteria, which are involved in C, N, and S cycling, and r-strategists, such as Alpha- and Gammaproteobacteria and Bacteroidetes, significantly decreased under 300 mg·kg^-1 nano-ZnO, confirming reduced root secretions. In contrast, Alpha- and Gammaproteobacteria, Bacteroidetes, Chloroflexi, and Acidobacteria were enriched in 300 mg·kg^-1 nano-ZnO under eCO₂, suggesting greater adaptation to both nano-ZnO and eCO₂. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States 2 (PICRUSt2) analysis demonstrated that bacterial functionality was unchanged under short-term nano-ZnO and eCO₂ exposure. In conclusion, nano-ZnO significantly affected microbial diversities and the bacterial composition, and eCO₂ intensified the damage of nano-ZnO, while the bacterial functionality was not changed in this study.

Engineered zinc oxide nanoparticles: an alternative to conventional zinc sulphate in neutral and alkaline soils for sustainable wheat production

Zinc oxide nanoparticles (ZnONP) were synthesized and characterized using SEM, EDAX, DLS and UV-Vis spectra. Its use as a nanofertilizer as an alternative to conventional zinc sulphate (ZnSO₄.7H₂O) was evaluated in five Zn-deficient soils with a variable pH range (7.2-8.7). For this, the carbon of the soil microbial biomass (SMBC), the bacterial population, the nutrient dynamics and the biometric parameters of the wheat crop were assessed. The varying dosages (0, 100, 200 and 500 mg/L), sizes (30-100 nm), and the spherical shape of ZnONPs were evaluated in comparison to ZnSO₄.7H₂O levels. Results showed the maximum SMBC and bacterial population at 100 mg/L of ZnONPs but a sharp decline at higher concentrations. In addition, soil application of ZnONPs at 5 mg/kg produced a higher root elongation (4.3-8.8%), shoot elongation (3.5-4.0%), total chlorophyll (4.9-5.6%), grain yield (1.7-2.3%) and grain Zn-content (1.6-2.1%) in comparison to the conventional ZnSO₄.7H₂O at 10 mg/L. ZnONPs at 100 mg/L produced a higher soil microbial biomass carbon (3.9-4.6%), bacterial population (7.2-9.0%), germination (22%) and grain Zn-content (17.9-20%) as compared to the conventional ZnSO₄.7H₂O at 0.5%. The higher grain Zn-contents could be attributed to the small size and high surface area of ZnONPs resulting in easy entry into the plant system either through root or foliar by penetrating the pores present in the cell membranes. Conversely, the conventional ZnSO₄.7H₂O, due to its larger size and higher solubility as compared to ZnONPs, has low retention in plant systems, high surface run-off and low fertilizer efficiency. Thus, the authors concluded to apply spherically synthesized ZnONPs (average size-36.7 nm) at 5 mg/kg in the soil application and 100 mg/L in the foliar application for maintaining SMBC and bacterial population, improving total chlorophyll, and grain Zn-contents and overall sustaining wheat production in Zn-deficient neutral and alkaline soils.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02861-1.

Transcriptome Reveals the Rice Response to Elevated Free Air CO2 Concentration and TiO2 Nanoparticles

Increasing CO₂ levels are speculated to change the effects of engineered nanomaterials in soil and on plant growth. How plants will respond to a combination of elevated CO₂ and nanomaterials stress has rarely been investigated, and the underlying mechanism remains largely unknown. Here, we conducted a field experiment to investigate the rice (Oryza sativa L. cv. IIyou) response to TiO₂ nanoparticles (nano-TiO₂, 0 and 200 mg kg^-1) using a free-air CO₂ enrichment system with different CO₂ levels (ambient ∼370 μmol mol^-1 and elevated ∼570 μmol mol^-1). The results showed that elevated CO₂ or nano-TiO₂ alone did not significantly affect rice chlorophyll content and antioxidant enzyme activities. However, in the presence of nano-TiO₂, elevated CO₂ significantly enhanced the rice height, shoot biomass, and panicle biomass (by 9.4%, 12.8%, and 15.8%, respectively). Furthermore, the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed that genes involved in photosynthesis were up-regulated while most genes associated with secondary metabolite biosynthesis were down-regulated in combination-treated rice. This indicated that elevated CO₂ and nano-TiO₂ might stimulate rice growth by adjusting resource allocation between photosynthesis and metabolism. This study provides novel insights into rice responses to increasing contamination under climate change.

Elevated CO2 levels modify TiO2 nanoparticle effects on rice and soil microbial communities

Evidence suggests that CO₂ modifies the behavior of nanomaterials. Thus, in a few decades, plants might be exposed to additional stress if atmospheric levels of CO₂ and the environmental burden of nanomaterials increase at the current pace. Here, we used a full-size free-air CO₂ enrichment (FACE) system in farm fields to investigate the effect of elevated CO₂ levels on phytotoxicity and microbial toxicity of nTiO₂ (0, 50, and 200mgkg^-1) in a paddy soil system. Results show that nTiO₂ did not induce visible signs of toxicity in rice plants cultivated at the ambient CO₂ level (370μmolmol^-1), but under the high CO₂ concentration (570μmolmol^-1) nTiO₂ significantly reduced rice biomass by 17.9% and 22.1% at 50mgkg^-1 and 200mgkg^-1, respectively, and grain yield by 20.8% and 44.1% at 50mgkg^-1 and 200mgkg^-1, respectively. In addition, at the high CO₂ concentration, nTiO₂ at 200mgkg^-1 increased accumulation of Ca, Mg, Mn, P, Zn, and Ti by 22.5%, 16.8%, 29.1%, 7.4%, 15.7% and 8.6%, respectively, but reduced fat and total sugar by 11.2% and 25.5%, respectively, in grains. Such conditions also changed the functional composition of soil microbial communities, alerting specific phyla of bacteria and the diversity and richness of protista. Overall, this study suggests that increases in CO₂ levels would modify the effects of nTiO₂ on the nutritional quality of crops and function of soil microbial communities, with unknown implications for future economics and human health.

Design Defines the Effects of Nanoceria at a Low Dose on Soil Microbiota and the Potentiation of Impacts by the Canola Plant

Soils act as nanoceria sinks via agricultural spreading and surface waters. Canola plants were grown for one month in soil spiked with nanoceria (1 mg·kg(-1)). To define the role of nanomaterials design on environmental impacts, we studied nanoceria with different sizes (3.5 or 31 nm) and coating (citrate). We measured microbial activities involved in C, N, and P cycling in the rhizosphere and unplanted soil. Bacterial community structure was analyzed in unplanted soil, rhizosphere, and plant roots by 454-pyrosequencing of the 16S rRNA gene. This revealed an impact gradient dependent on nanomaterials design, ranging from decreased microbial enzymatic activities in planted soil to alterations in bacterial community structure in roots. Particle size/aggregation was a key parameter in modulating nanoceria effects on root communities. Citrate coating lowered the impact on microbial enzymatic activities but triggered variability in the bacterial community structure near the plant root. Some nanoceria favored taxa whose closest relatives are hydrocarbon-degrading bacteria and disadvantaged taxa frequently associated in consortia with disease-suppressive activity toward plant pathogens. This work provides a basis to determine outcomes of nanoceria in soil, at a dose close to predicted environmental concentrations, and to design them to minimize these impacts.

Abiotic soil changes induced by engineered nanomaterials: A critical review

A large number of research papers on the fate of engineered nanomaterials (ENMs) in the soil-water system have appeared in recent years, focusing on ENM transport, persistence and toxicological impact. It is clear from these publications that soil is a major sink for ENMs, and that only a small portion degrades or is mobilized further into groundwater. However, to date, very few studies have examined the impact of ENMs on the natural soil-subsurface matrix and its properties. Moreover, it is now well accepted that chemical contaminants are capable of changing soil properties either by inducing direct chemical or physical changes, or through indirect changes by, e.g., influencing biological activity that in turn modifies soil properties. Here, we review studies on the deposition, retention, and accumulation of ENMs in soil, indicative of the extent to which soil acts as a major sink of ENMs. We then examine evidence of how these retained particles lead to modification of surface properties, which are manifested by changes in the sorption capacity of soil for other (organic and inorganic) solutes, and by surface charges and composition different than the natural surfaces. Finally, we demonstrate how this results in physical and hydrological changes to soil properties, including hydraulic conductivity, swelling capacity and wettability. The overall picture revealed in this critical review sheds light on a perspective that has received little attention thus far. These aspects of soil change, due to exposure and subsequent accumulation of ENMs, may ultimately prove to be one of the most important impacts of ENM releases to the environment.