Palladium-doped and undoped polystyrene nanoplastics in a chronic toxicity test for higher plants: Impact on soil, plants and ammonium oxidizing bacteria.

There is a lack of knowledge about the fate and impact of microplastics (MPs) and nanoplastics (NPs), as well as their potential uptake and impact on plants and microorganisms. The predicted environmental concentrations (PEC) of frequent polymers in soils are low, and therefore, difficult to detect with the available techniques, which explains the knowledge gaps. Therefore, model particles (polystyrene particles (PS-P), 343 nm) and palladium (Pd) nanoparticle-doped polystyrene particles (PS-Pd-PS-P, 442 nm) were synthesized, characterized, and subsequently applied to agricultural soils (Cambisol, Podzol, PS target contents: 25 mg kg-1, 75 mg kg-1, 225 mg kg-1). A combination of different techniques, such as inductively coupled plasma-mass spectrometry (ICP-MS), pyrolysis-gas chromatography-mass spectrometry (Pyr-GC-MS), dynamic light scattering (DLS), and scanning electron microscopy (SEM), were used to characterize the particles in the dispersions, soils and plants. The spiked soils were applied to a chronical plant toxicity test with oat (Avena sativa). The applied particle contents could be recovered from both soils by ICP-MS (Pd, 89% - 99%), and Pyr-GC-MS (PS, 73% - 120%). Moreover, non-aggregated particles in soils and on oat roots were visualized through SEM. The ratio obtained for the Pd contents in oat roots to that in the Cambisol (2.2-2.7) and the Podzol (2.3-2.6) implied that particles accumulated on the root surface or in the roots. No Pd was detected in the oat shoots, which indicated that no translocation occurred from the roots to the shoots. Despite particle accumulation at or in the roots, no clear effects on plant growth were observed. Furthermore, the soil microorganisms (Podzol) and the soil water repellency (Cambisol, Podzol) showed no clear monotone concentration-response relationship after exposure to PS-P and PS-Pd-PS-P. The findings are complex and illustrate the urgent need for further sophisticated experimental studies to elucidate the impacts of NPs on physicochemical soil function, plants, and soil organisms. The model PS-P doped with Pd nanoparticles significantly enhanced the development and validation of methods for investigating MPs and NPs in environmental matrices, highlighting their considerable potential for further studies.

Maize Field Study Reveals Covaried Microbiota and Metabolic Changes in Roots over Plant Growth.

Plant roots are colonized by microorganisms from the surrounding soil that belong to different kingdoms and form a multikingdom microbial community called the root microbiota. Despite their importance for plant growth, the relationship between soil management, the root microbiota, and plant performance remains unknown. Here, we characterize the maize root-associated bacterial, fungal, and oomycetal communities during the vegetative and reproductive growth stages of four maize inbred lines and the pht1;6 phosphate transporter mutant. These plants were grown in two long-term experimental fields under four contrasting soil managements, including phosphate-deficient and -sufficient conditions. We showed that the maize root-associated microbiota is influenced by soil management and changes during host growth stages. We identified stable bacterial and fungal root-associated taxa that persist throughout the host life cycle. These taxa were accompanied by dynamic members that covary with changes in root metabolites. We observed an inverse stable-to-dynamic ratio between root-associated bacterial and fungal communities. We also found a host footprint on the soil biota, characterized by a convergence between soil, rhizosphere, and root bacterial communities during reproductive maize growth. Our study reveals the spatiotemporal dynamics of the maize root-associated microbiota and suggests that the fungal assemblage is less responsive to changes in root metabolites than the bacterial community. IMPORTANCE Plant roots are inhabited by microbial communities called the root microbiota, which supports plant growth and health. We show in a maize field study that the root microbiota consists of stable and dynamic members. The dynamics of the microbial community appear to be driven by changes in the metabolic state of the roots over the life cycle of maize.

Crop genetic diversity uncovers metabolites, elements, and gene networks predicted to be associated with high plant biomass yields in maize.

Rapid population growth and increasing demand for food, feed, and bioenergy in these times of unprecedented climate change require breeding for increased biomass production on the world's croplands. To accelerate breeding programs, knowledge of the relationship between biomass features and underlying gene networks is needed to guide future breeding efforts. To this end, large-scale multiomics datasets were created with genetically diverse maize lines, all grown in long-term organic and conventional cropping systems. Analysis of the datasets, integrated using regression modeling and network analysis revealed key metabolites, elements, gene transcripts, and gene networks, whose contents during vegetative growth substantially influence the build-up of plant biomass in the reproductive phase. We found that S and P content in the source leaf and P content in the root during the vegetative stage contributed the most to predicting plant performance at the reproductive stage. In agreement with the Gene Ontology enrichment analysis, the cis-motifs and identified transcription factors associated with upregulated genes under phosphate deficiency showed great diversity in the molecular response to phosphate deficiency in selected lines. Furthermore, our data demonstrate that genotype-dependent uptake, assimilation, and allocation of essential nutrient elements (especially C and N) during vegetative growth under phosphate starvation plays an important role in determining plant biomass by controlling root traits related to nutrient uptake. These integrative multiomics results revealed key factors underlying maize productivity and open new opportunities for efficient, rapid, and cost-effective plant breeding to increase biomass yield of the cereal crop maize under adverse environmental factors.

Temperature-induced diurnal redox potential in soil.

With the capabilities to measure redox potentials (EH) at a high temporal resolution, scientists have observed diurnal EH that occur in a distinct periodicity in soils and sediments. These patterns have been disregarded for a long time because minor fluctuations of the EH in the tens of mV range are difficult to interpret. Various explanations have been proposed for the origin of diel EH but a cohesive assessment of the temperature-dependency for field- and laboratory-based investigations is missing at present. In this study, we investigated spatiotemporal diel EH of previous long-term (up to 10 years) field- and lab-based monitoring data collected at high-temporal (every hour) and spatial (up to 6 depths) resolution. In addition, we set up a redox experiment where we manipulated the soil temperature (ST) by diel temperature cycles to assess the EH response. Diel fluctuations were absent for laboratory experiments with ΔEH of a few mV (daily EH-max - daily EH-min), but we found pronounced fluctuations up to ∼100 mV for field investigations. The spatiotemporal pattern in EH fluctuations was amplified in the topsoil during the summer months concomitant with ST. We showed for the first time that changes in ST during an incubation experiment altered the EH by -3.3 mV °C-1 and inferred that the diel EH were driven by the thermal conditions of the soil itself. This is particularly important when EH is measured close to the soil surface and underlines that minor fluctuations of the EH with a recurring periodicity should be carefully checked for its dependency with the soil and reference electrode temperature. Redox measurements should not be considered a routine determination and cautious handling of EH data by physical sound corrections is urgently needed in order to link ΔEH to daily biogeochemical cycling in soils.

Soil Organic Matter and Phosphate Sorption on Natural and Synthetic Fe Oxides under in Situ Conditions.

Iron (Fe) oxides in soils are strong sorbents for environmentally important compounds like soil organic matter (SOM) or phosphate, while sorption under field conditions is still poorly understood. We installed polyvinyl chloride plastic bars which have been coated either with synthetic Fe or manganese (Mn) oxides for 30 days in a redoximorphic soil. A previous study revealed the formation of newly formed ("natural") Fe oxides along the Mn oxide coatings. This enables us to differentiate between sorption occurring onto the surfaces of synthetic versus natural Fe oxides. After removal of the bars, they were analyzed by nanoscale secondary ion mass spectrometry (NanoSIMS) to study the distribution of Fe (56Fe16O-), SOM (12C14N-), and phosphorus (31P16O2-) at the microscale. Image analysis of individual Fe oxide particles revealed a close association of Fe, SOM, and P resulting in coverage values up to 71%. Furthermore, ion ratios between sorbent (56Fe16O-) and sorbate (12C14N- and 31P16O2-) were smaller along the natural oxides when compared with those for synthetic Fe oxides. We conclude that both natural and synthetic Fe oxides rapidly sequester SOM and P (i.e., within 30 days) but that newly, natural formed Fe oxides sorbe more SOM and P than synthetic Fe oxides.

Manganese-oxide-coated redox bars as an indicator of reducing conditions in soils.

Identification of reducing conditions in soils is of concern not only for pedogenesis but also for nutrient and pollutant dynamics. We manufactured manganese (Mn)-oxide-coated polyvinyl chloride bars and proved their suitability for the identification of reducing soil conditions. Birnessite was synthesized and coated onto white polyvinyl chloride bars. The dark brown coatings were homogenous and durable. As revealed by microcosm devices with adjusted redox potentials (E), under oxidizing conditions (E ∼450 mV at pH 7) there was no Mn-oxide removal. Reductive dissolution of Mn-oxides, which is expressed by the removal of the coatings, started under weakly reducing conditions (E ∼175 mV) and was more intensive under moderately reducing conditions (∼80 mV). According to thermodynamics, the removal of Mn-oxide coatings (225 mm d) exceeded the removal of iron (Fe)-oxide coatings (118 mm d) in soil column experiments. This was confirmed in a soil with a shallow and strongly fluctuating water table where both types of redox bars were inserted. Consequently, it was possible to identify reducing conditions in soils using Mn-oxide-coated bars. We recommend this methodology for short-term monitoring because tri- and tetravalent Mn is the preferred electron acceptor compared with trivalent Fe, and this additionally offers the possibility of distinguishing between weakly and moderately reducing conditions. If dissolved Fe is abundant in soils, the possibility of nonenzymatic reduction of Mn has to be taken into account.