In peri-urban critical zones, soil ecosystems are highly affected by increasing urbanization, causing probably an intense interaction between dissolved organic matter (DOM) and heavy metals in soil. Such interaction is critical for understanding the biogeochemical cycles of both organic matter and heavy metals in these zones. However, limited research has reported the correlative distribution of DOM and heavy metals at high seasonal and spatial resolutions in peri-urban critical zones. In this study, 160 soil samples were collected from the farmland and forestland of Zhangxi watershed, in Ningbo, eastern China during spring, summer, fall and winter four seasons. UV–visible absorption and fluorescent spectroscopy were used to explore the optical characteristics of DOM. The results indicated a mixture of exogenous and autogenous sources of DOM in the Zhangxi watershed, while DOM in farmland exhibited a higher degree of aromaticity and humification than that in forestland. Fluorescent results showed that humic acid-like, fulvic acid-like and microbial-derived humic-like fractions were mostly affected by seasons. The distribution of heavy metals was affected mainly by land-use changes and seasons. Correlation analysis between heavy metals and DOM characteristics and components suggested that aromatic and humic substances were more favorable in binding with EDTA extractable Ni, Cu, Zn and Cd. The bioavailable Cd and Pb decreased due to binding with humic fractions, indicating its great effects on the bioavailability of Cd and Pb. Overall, these findings provide an insight into the correlative distributions of DOM and heavy metals in peri-urban areas, thereby highlighting their biogeochemical cycling in the soil environment.

With the rapid development of nanotechnology in agriculture, there is increasing urgency to assess the impacts of nanoparticles (NPs) on the soil environment. This study merged raw high-throughput sequencing (HTS) data sets generated from 365 soil samples to reveal the potential ecological effects of NPs on soil microbial community by means of metadata analysis and machine learning methods. Metadata analysis showed that treatment with nanoparticles did not have a significant impact on the alpha diversity of the microbial community, but significantly altered the beta diversity. Unfortunately, the abundance of several beneficial bacteria, such as Dyella, Methylophilus, Streptomyces, which promote the growth of plants, and improve pathogenic resistance, was reduced under the addition of synthetic nanoparticles. Furthermore, metadata demonstrated that nanoparticles treatment weakened the biosynthesis ability of cofactors, carriers, and vitamins, and enhanced the degradation ability of aromatic compounds, amino acids, etc. This is unfavorable for the performance of soil functions. Besides the soil heterogeneity, machine learning uncovered that a) the exposure time of nanoparticles was the most important factor to reshape the soil microbial community, and b) long-term exposure decreased the diversity of microbial community and the abundance of beneficial bacteria. This study is the first to use a machine learning model and metadata analysis to investigate the relationship between the properties of nanoparticles and the hazards to the soil microbial community from a macro perspective. This guides the rational use of nanoparticles for which the impacts on soil microbiota are minimized.

Heavy metal contamination has been threatening the health of human beings. To decrease the bio-toxicity of heavy metals, a thiol-functionalized nano-silica (SiO₂-SH) was adopted to remediate the soil contaminated by lead (Pb), cadmium (Cd) and copper (Cu). The remediation effect of SiO₂-SH on contaminated soils was investigated by the uptake of the heavy metals into lettuce and pakchoi in pot experiment. The bio-toxicity of the SiO₂-SH was evaluated, and its immobilization mechanisms were proposed by the fraction distribution of Cd, Pb and Cu. It was found that the SiO₂-SH can significantly reduce the uptake of Cd, Pb, Cu into pakchoi by 92.02%, 68.03%, 76.34% and into lettuce by 89.81%, 43.41%, 5.76%, respectively. The chemical species analyses of Cd, Pb, Cu indicate SiO₂-SH can transform the heavy metal in acid soluble states into reducible fraction and oxidizable fraction, thereby inhibiting the extraction of heavy metals into soil solution. The concentrations of microbial biomass carbon, organic matter, and cation exchange capacity of the soil increased while the soil bulk density decreased after remediation. Those changes demonstrate that SiO₂-SH not only has no bio-toxic impact on the soil environment but also improves the soil environment, which proves the prepared SiO₂-SH is environmental-friendly. The SiO₂-SH could be a promising amendment for heavy metal contaminated soils.

Diethyl phthalate (DEP), as a kind of universally used plasticizer, has aroused considerable public concern owing to its wide detection, environmental stability, and potential health risks. In this work, the highly efficient removal of DEP by ferrate (VI) (Fe(VI)) was systematically explored in soil environment. The effects of the oxidant dosages, soil types, as well as the presence of coexisting cations and anions in tested soil on DEP removal were evaluated. When the dosage of Fe(VI) was 20 mM, complete removal of DEP (50 μg/g) was achieved in the tested soil after 2 min of reaction. Furthermore, the removal rate of DEP was closely related to the soil types, and the degradation rates were decreased obviously in red soil (RS), black soil (BS) and paddy soil (PS), probably due to the acidic condition and high content of organic matters. Moreover, the presence of Ca²⁺, Mg²⁺ and Al³⁺ in soil can inhibit the removal of DEP by Fe(VI), while SO₄²⁻ has an slightly promotion effect. Six oxidation intermediates were detected in the reaction process of DEP, product analysis revealed that the transformation of DEP was mainly through two pathways, including hydrolysis and hydroxylation reactions, which were probably mediated by oxygen atom transfer process of Fe(VI). Based on the frontier electron density theory calculation, two ester groups of DEP were prone to be attacked by Fe(VI), and the hydroxyl addition tended to occur at the para-position of one of the ester groups on the benzene ring. This study provides a novel approach for phthalate esters removal from soil using Fe(VI) oxidation and shows new insights into the oxidation mechanisms.

Biochar has been proven as a soil amendment to improve soil environment. However, mechanistic understanding of biochar on soil physical properties and microbial community remains unclear. In this study, a wood biochar (WB), was incorporated into a highly weathered tropical soil, and after 1 year the in situ changes in soil properties and microbial community were evaluated. A field trial was conducted for application of compost, wood biochar, and polyacrylamide. Microstructure and morphological features of the soils were characterized through 3D X-ray microscopy and polarized microscopy. Soil microbial communities were identified through next-generation sequencing (NGS). After incubation, the number of pores and connection throats between the pores of biochar treated soil increased by 3.8 and 7.2 times, respectively, compared to the control. According to NGS results, most sequences belonged to Anaerolinea thermolimosa, Caldithrix palaeochoryensis, Chthoniobacter flavus, and Cohnella soli. Canonical correlation analysis (CCA) further demonstrated that the microbial community structure was determined by inorganic N (IN), available P (AP), pH, soil organic C (SOC), porosity, bulk density (BD), and aggregate stability. The treatments with co-application of biochar and compost facilitated the dominance of Cal. palaeochoryensis, Cht. flavus, and Coh. soli, all of which promoted organic matter decomposition and ammonia oxidation in the soil. The apparent increases in IN, AP, porosity, and SOC caused by the addition of biochar and compost may be the proponents of changes in soil microbial communities. The co-application of compost and biochar may be a suitable strategy for real world biochar incorporation in highly weathered soil.

Humic acid (HA) plays vital roles in regulating the environmental behaviors of metals and thus their toxicity to biota. However, the inner relation between metal bioavailability to soil organisms and the presence of HA with different molecular weight (Mw) is not well documented. In this study, we separated HAs into four fractions with Mw range of 5-30k Da, and discussed their ability to alleviating the toxicity of Cd and Pb to earthworm. The bioaccumulation capacities (Cₘₐₓ) increased in order of: UF1<UF2<UF3<UF4, which is in line with the variations of bioavailable concentrations of Cd and Pb in soil. Variations of Mw and binding capacities of HA determine the accumulation behavior in soil solution. The unsatisfactory of biotic ligand model fitting and the differences in fractions of the total biotic ligand sites (f) in earthworm bound by Cd and Pb suggested that only free species of Cd could be considered as biological available to earthworm, while the Pb–HAs complexes have potential ability to interact with earthworm membrane. Antioxidant enzymes are effective biomarkers, and HA with lower Mw play more important roles in restricting the toxicity of soil Cd and Pb to earthworm. These results reveal the different mechanism for HA controlling metal bioavailability between Cd and Pb in soil environment.

Propylparaben is widely used as a preservative in pharmaceuticals and personal care products and is ultimately excreted by the human body. Thus, propylparaben reaches sewage and enters the soil environment by sludge fertilization and wastewater irrigation. However, there are few existing studies on the toxicity and risks of such chemicals in terrestrial environments. In this study, a multispecies bioassay for propylparaben was performed and protective concentrations (PCs) were derived based on toxicity values by probabilistic ecological risk assessment. Acute and chronic bioassays were conducted on 11 species in eight taxonomic groups (Magnoliopsida, Liliopsida, Clitellata, Entognatha, Entomobryomorpha, Chromadorea, Chlorophyceae, Trebouxiophyceae). Based on the toxicity values calculated, the PC₉₅ values for acute and chronic SSDs were 13 and 6 mg/kg dry soil, respectively. Toxicity varied among taxa, with soil algae emerging as the most sensitive to propylparaben. This may be attributable to differences in exposure pathways among species. The exposure pathway of propylparaben can be altered by adsorption to soil particles. As parabens are presently under-regulated globally in terms of their environmental effects, our findings can serve as the basis to propose standard values for environmental protection.

Veterinary antibiotics have been detected as contaminants of emerging concern in soil environment worldwide. Animal manure is frequently applied to agricultural fields to improve soil fertility, which can result in introducing large amount of antibiotics into soil environment. However, few attempts have been made to identify the spatial and temporal dynamics of veterinary antibiotics in soil at the hillslope scale with different land uses. This study was performed to explore the pattern and variability of veterinary antibiotics in the soil in response to rainfall events. Results showed that higher concentrations of veterinary antibiotics were generally found in cropland (292.6 ± 280.1 ng/g) and orchard (228.1 ± 230.5 ng/g) than in forestland (13.5 ± 9.9 ng/g). After rainfall events, antibiotics accumulated in the soil at the positions where manure was applied, especially under high-intensity rainfall conditions. However, the antibiotic concentration in soil slightly increased from the top to the bottom of hills, thus indicating the restricted contribution of runoff to antibiotic transport, especially under low-intensity rainfall conditions. In addition, most antibiotics were sequestered in the surface soil (0–10 cm), and higher antibiotic concentrations were observed in deep soil (20–40 cm) in cropland than orchard. The soil aggregate, organic matter, and clay content played important roles in antibiotic sequestration along the hillslope subject to low-, medium-, and large-amount rainfall events, respectively. This study identified that land use, rainfall conditions, and soil structures jointly affect the spatial and temporal variability of antibiotics in soils on hillslopes.

The human pathogenic bacteria (HPB) in animal feces may disseminate to agricultural soils with their land application as organic fertilizer. However, the knowledge about the impacts of different sources and rates of animal manures on the temporal changes of soil HPB remains limited, which hamper our ability to estimate the potential risks of their land application. Here, we constructed an HPB database including 565 bacterial strains. By blasting the 16 S rRNA gene sequences against the database we explored the occurrence and fate of HPB in soil microcosms treated with two rates of swine, poultry or cattle manures. A total of 30 HPB were detected in all of manure and soil samples. Poultry manure at the high level obviously improved the abundance of soil HPB. The application of swine manure could introduce concomitant HPB into the soils. Of which, Pseudomonas syringae pv. syringae B728a and Escherichia coli APEC O78 may deserve more attention because of their survival for a few days in manured soils and being possible hosts of diverse antibiotic resistance genes (ARGs) as revealed by co-occurrence pattern. Bayesian source tracking analysis showed that the HPB derived from swine manure had a higher contribution to soil pathogenic communities than those from poultry or cattle manures in early days of incubation. Mantel test together with variation partitioning analysis suggested that bacterial community and soil physicochemical properties were the dominant factors determining the profile of HPB and contributed 64.7% of the total variations. Overall, our results provided experimental evidence that application of animal manures could facilitate the potential dissemination of HPB in soil environment, which should arouse sufficient attention in agriculture practice and management to avoid the threat to human health.

The interactive effects of elevated atmospheric CO₂ and nanoparticles (NPs) on the structure and function of soil bacterial community remain unknown. Here we compared the impacts of CeO₂ (nCeO₂) and Cr₂O₃ (nCr₂O₃) nanoparticles on the taxonomic compositions and functional attributes of bacterial communities under elevated CO₂ (eCO₂). The stimulated enzyme activities (dehydrogenase, acid phosphatase and urease), increased microbial biomass carbon (MBC), and higher bacterial alpha-diversity were observed under the combined effects of eCO₂ and NPs compared to the single NP treatment, indicating eCO₂ could mitigate the adverse effect of NPs on soil microorganisms. NPs and eCO₂ are important factors influencing the alpha- and beta-diversity (17% and 18% of variations were explained) as well as functional profile (20% and 26% of variations were explained) of bacterial communities. Rising CO₂ level promoted the resilience of NP-resistant bacterial populations, primarily the members of Alphaproteobacteria, Gammaproteobacteria and Bacteroidia, which are also characterized by the fast carbon use capability. Moreover, the significantly (P < 0.05) higher metabolic quotient (qCO₂), reduced available carbon and overrepresented carbon metabolism genes at eCO₂vs. ambient CO₂ (aCO₂) indicate the acceleration of available carbon turnover in NP-exposed soils. Correlation analysis revealed that mitigation of NPs toxicity by eCO₂ could be attributed to the remarkable decline of bioavailable metals disassociated from NPs and available carbon level, as well as promotion of the rapid carbon-metabolizing microbes. Our study pointed out the positive role of eCO₂ in alleviating the adverse effect of NPs on microbiological soil environment, and results can serve as important basis in establishing guidelines for lowering the ecotoxicity of NPs.