Nitrogen induced DOC and heavy metals leaching: Effects of nitrogen forms, deposition loads and liming

Distribution, ecological risk, and sediment-influencing mechanisms of heavy metals in surface sediments along the intertidal gradient in typical mangroves in Hainan, China

The relative importance of each sediment physicochemical property to sediment heavy-metal (HM) contents has not yet been quantitatively evaluated. Differences in the HM contents of mangrove surface sediments among the high, middle, and low intertidal zones, and their quantitative relationships to sediment physicochemical properties, were investigated in Dongzhaigang and Qinglan Harbor reserves, Hainan, China. In both reserves, the Cu and Ni concentrations increased significantly from the low to high intertidal zones; the patterns of change in the Mn and Pb contents were opposite in the two reserves. The Cr concentration was significantly lower and the Pb concentration was significantly higher in the dry season than in the wet season. Ecological risks of HM were higher in Dongzhaigang than in Qinglan Harbor. Regression and redundancy (hierarchical partitioning) analyses showed that the sediment total sulfur, nitrogen and potassium contents and pH were key factors affecting the HM contents of mangrove surface sediments.

Interactive effects of ammonium sulfate and lead on alfalfa in rare earth tailings: Physiological responses and toxicity thresholds

Ion-adsorption rare earth ore contains significant levels of leaching agents and heavy metals, leading to substantial co-contamination. This presents significant challenges for ecological rehabilitation, yet there is limited understanding of the toxicity thresholds associated with the co-contamination of ammonium sulfate (AS) and lead (Pb) on pioneer plants. Here, we investigated the toxicity thresholds of various aspects of alfalfa, including growth, ultrastructural changes, metabolism, antioxidant system response, and Pb accumulation. The results indicated that the co-contamination of AS-Pb decreased the dry weight of shoot and root by 26 %-77 % and 18 %-92 %, respectively, leading to irregular root cell morphology and nucleus disintegration. The high concentration and combined exposures to AS and Pb induced oxidative stress on alfalfa, which stimulated the defense of the antioxidative system and resulted in an increase in proline levels and a decrease in soluble sugars. Structural equation modeling analysis and integrated biomarker response elucidated that the soluble sugars, proline, and POD were the key physiological indicators of alfalfa under stresses and indicated that co-exposure induced more severe oxidative stress in alfalfa. The toxicity thresholds under single exposure were 496 (EC5), 566 (EC10), 719 (EC25), 940 (EC50) mg kg-1 for AS and 505 (EC5), 539 (EC10), 605 (EC25), 678 (EC50) mg kg-1 for Pb. This study showed that AS-Pb pollution notably influenced plant growth performance and had negative impacts on the growth processes, metabolite levels, and the antioxidant system in plants. Our findings contribute to a theoretical foundation and research necessity for evaluating ecological risks in mining areas and assessing the suitability of ecological restoration strategies.

Antagonistic effect of mercury and excess nitrogen exposure reveals provenance-specific phytoremediation potential of black locust-rhizobia symbiosis

Interaction of different environmental constrains pose severe threats to plants that cannot be predicted from individual stress exposure. In this context, mercury (Hg), as a typical toxic and hazardous heavy metal, has recently attracted particular attention. Nitrogen (N2)-fixing legumes can be used for phytoremediation of Hg accumulation, whereas N availability could greatly affect its N2-fixation efficiency. However, information on the physiological responses to combined Hg exposure and excess N supply of woody legume species is still lacking. Here, we investigated the interactive effects of rhizobia inoculation, Hg exposure (+Hg), and high N (+N) supply, individually and in combination (+N*Hg), on photosynthesis and biochemical traits in Robinia pseudoacacia L. seedlings of two provenances, one from Northeast (DB) and one from Northwest (GS) China. Our results showed antagonistic effects of combined + N*Hg exposure compared to the individual treatments that were provenance-specific. Compared to individual Hg exposure, combined + N*Hg stress significantly increased foliar photosynthesis (+50.6%) of inoculated DB seedlings and resulted in more negative (-137.4%) δ15N abundance in the roots. Furthermore, combined + N*Hg stress showed 47.7% increase in amino acid N content, 39.4% increase in NR activity, and 14.8% decrease in MDA content in roots of inoculated GS seedlings. Inoculation with rhizobia significantly promoted Hg uptake in both provenances, reduced MDA contents of leaves and roots, enhanced photosynthesis and maintained the nutrient balance of Robinia. Among the two Robinia provenances investigated, DB seedlings formed more nodules, had higher biomass and Hg accumulation than GS seedlings. For example, total Hg concentrations in leaves and roots and total biomass of inoculated DB seedlings were 1.3,1.9 and 3.4 times higher than in inoculated GS seedlings under combined + N*Hg stress, respectively. Therefore, the DB provenance is considered to possess a higher potential for phytoremediation of Hg contamination compared to the GS provenance in environments subjected to N deposition.

Nutrients in overlying water affect the environmental behavior of heavy metals in coastal sediments

Excessive nutrients in aquatic ecosystems are the main driving factors for eutrophication and water quality deterioration. However, the influence of nutrients in overlying water on sediment heavy metals is not well understood. In this study, the effects of nitrate nitrogen (NO3-N) addition and phosphate addition in the overlying water on the environmental behaviors of chromium (Cr), copper (Cu), and cadmium (Cd) in coastal river sediments were investigated. Fresh estuary sediments and synthetic saltwater were used in microcosm studies conducted for 13 d. To determine the biological effect, unsterilized and sterilized treatments were considered. The results showed that the diffusion of Cr and Cu was inhibited in the unsterilized treatments with increased NO3-N. However, under the NO3-N sterilized treatments, Cr and Cu concentrations in the overlying water increased. This was mostly related to changes in the microbial regulation of dissolved organic carbon and pH in the unsterilized treatments. Further, in the unsterilized treatments, NO3-N addition considerably increased the concentrations of the acid-soluble (Cr, Cu, and Cd increased by 5%-8%, 29%-41%, and 31%-42%, respectively) and oxidizable (Cr, Cu, and Cd increased by 10%, 5%, and 14%, respectively) fractions. Additionally, compared with that in the unsterilized treatments, Cu and Cd concentrations in P-3 treatments decreased by 7% and 63%, respectively. By producing stable metal ions, microorganisms reduced the amount of unstable heavy metals in the sediment and heavy metal concentration in the overlying water, by considerably enhancing the binding ability of phosphate and heavy metal ions. This study provides a theoretical basis for investigating the coupling mechanisms between heavy metals and nutrients.

Role of mycorrhizas and root exudates in plant uptake of soil nutrients (calcium, iron, magnesium, and potassium): has the puzzle been completely solved?

Anthropogenic global change is driving an increase in the frequency and intensity of drought and flood events, along with associated imbalances and limitation of several soil nutrients. In the context of an increasing human population, these impacts represent a global-scale challenge for biodiversity conservation and sustainable crop production to ensure food security. Plants have evolved strategies to enhance uptake of soil nutrients under environmental stress conditions; for example, symbioses with fungi (mycorrhization) in the rhizosphere and the release of exudates from roots. Although crop cultivation is managed for the effects of limited availability of nitrogen (N) and phosphorus (P), there is increasing evidence for limitation of plant growth and fitness because of the low availability of other soil nutrients such as the metals potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe), which may become increasingly limiting for plant productivity under global change. The roles of mycorrhizas and plant exudates on N and P uptake have been studied intensively; however, our understanding of the effects on metal nutrients is less clear and still inconsistent. Here, we review the literature on the role of mycorrhizas and root exudates in plant uptake of key nutrients (N, P, K, Ca, Mg, and Fe) in the context of potential nutrient deficiencies in crop and non-crop terrestrial ecosystems, and identify knowledge gaps for future research to improve nutrient-uptake capacity in food crop plants.

Effects of 12-Year Nitrogen Addition and Mowing on Plant-Soil Micronutrients in a Typical Steppe

Changes in soil micronutrient availability may have adverse consequences on grassland productivity, yet it’s still largely unclear how concurrent human practices, such as fertilization and mowing, affect micronutrient cycling in the plant-soil systems. Here, we measured six essential micronutrient (Fe, Mn, Cu, Zn, Co and Mo) contents in both plant pool (separated as aboveground plant parts, litter, and belowground roots) at the community level and soil pool (0−10 cm depth) after 12-year consecutive nitrogen (N) addition (0, 2, 10, and 50 g N m−2 year−1) and mowing in a typical steppe of the Mongolian Plateau. The results show that (i) medium-N (10 g m−2 year−1) and high-N (50 g m−2 year−1) addition rates significantly increased contents of soil-available Fe (+310.0%, averaging across the two N addition rates), Mn (+149.2%), Co (+123.6%) and Mo (+73.9%) irrespective of mowing treatment, whereas these addition treatments usually decreased contents of soil total Fe (−8.9%), Mn (−21.6%), Cu (−15.9%), Zn (−19.5%), Co (−16.4%) and Mo (−34.7%). (ii) Contents of Fe in aboveground plant parts, litter, and roots significantly decreased, whereas plant Mn increased with N addition. Contents of above ground plant Cu, Zn, Co, and Mo significantly decreased at high-N addition rate, whereas contents of micronutrients in roots and litters, except for Fe, generally increased with N addition. Moreover, the total amount of micronutrients in the plant pool (contents × biomass) significantly increased at the medium-N addition rate but decreased at the high-N addition rate. All N addition rates significantly enlarged the pool of litter micronutrients, and roots could hold more micronutrients under N addition, especially combined with mowing treatment. Importantly, although mowing could regulate the effects of N addition on variables (i) and (ii), the effects were weaker overall than those of N addition. (iii) Changes in root micronutrients, except for Mn, could explain corresponding changes in plant micronutrients (R2: 0.19−0.56, all p < 0.01), and significant linear correlations were also observed between soil-available Fe and Fe in plant and roots. Aboveground plant Mn was significantly correlated with soil-available Mn, while Co and Mo in roots were also significantly correlated with soil-available Co and Mo. These results indicate that soil micronutrient supply capacity may decrease due to a decrease in total micronutrient contents after long-term N addition and mowing. They also suggest that different magnitude responses of soil micronutrients in plants (i.e., litters, roots) and soil should be considered when comprehensively examining nutrient cycling in grassland ecosystems.

A comparative and modeled approach for three biochar materials in simultaneously preventing the migration and reducing the bioaccessibility of heavy metals in soil: Revealing immobilization mechanisms

The effectiveness and feasibility of the three biochar materials for remediation of arsenic (As) and lead (Pb) contaminated soil were explored in this study. Significant reduction of bioaccessibility and migration risks of both heavy metals have been explained mechanistically by incubation, column experiments and numerical simulation. Langmuir equation fitted As and Pb sorption isotherms better in the control and biochar (BC) amended soils, while Freundlich model was more suitable for iron modified biochar (Fe-BC) and sulfur/iron modified biochar (S/Fe-BC) amended soils, indicating that modified biochar promoted chemical adsorption process for As and Pb. For the three biochar materials, S/Fe-BC showed the best effects on reducing the bioavailability of As and Pb, with a decrease of 40.42%-64.21%. The reduction in bioaccessibility by metal portioning into available and non-available fractions was better for illustrating the mechanisms including adsorption, precipitation/coprecipitation and As(III) oxidation behind S/Fe-BC efficacy. Moreover, S/Fe-BC can effectively inhibit the leaching behavior of As and Pb under acid rain, which increased by 99.89% and 90.18%, respectively, compared with the control. The HYDRUS-1D modeling indicated that S/Fe-BC could continuously treat As (100 mg/L) and Pb (1000 mg/L) contaminated water for 16.22 years and 40.86 years, respectively, and ensure the groundwater quality criteria being met. Based on these insights, we believe that our study will provide meaningful information about the potentials of biochar derived materials for soil heavy metals' remediation.

Inhibition of cadmium uptake by wheat with urease-producing bacteria combined with sheep manure under field conditions

In heavy metal-contaminated farmland, microorganisms or organic fertilizers can be used to minimize heavy metal uptake by crops to ensure food safety. However, the mechanisms by which urease-producing and metal-immobilizing bacteria combined with manure inhibit Cd uptake in wheat (Triticum aestivum L.) remain unclear. Herein, the effects of Enterobacter bugandensis TJ6, sheep manure (SM), and TJ6 combined with SM on Cd uptake by wheat and the mechanisms involved were investigated under field conditions. The results showed that strain TJ6 increased the urease activity and the proportion of strains with a high Cd adsorption capacity in SM, thereby enhancing the Cd adsorption capacity of SM in solution. Strain TJ6 combined with SM improved the rhizosphere soil urease activity, NH₄⁺/NO₃^- ratio, and pH, thus reducing the Cd content (75.9%) in wheat grain. In addition, TJ6+SM reduced the bacterial community diversity but shifted the structure of the bacterial community in rhizosphere soil. Interestingly, the relative abundances of urease-producing bacteria and metal-immobilizing bacteria (Enterobacter, Bacillus, Exiguobacterium, Rhizobium, and Serratia) in rhizosphere soil were enriched, which enhanced wheat resistance to Cd toxicity. These results showed that urease-producing and metal-immobilizing bacteria combined with sheep manure can inhibit the uptake of Cd by wheat.

Modeling and visualizing the transport and retention of cationic and oxyanionic metals (Cd and Cr) in saturated soil under various hydrochemical and hydrodynamic conditions

Cationic and oxyanionic metals are widely existed in the aquatic and soil environment with the process of industrialization and they may behave different transport properties in aquifer systems due to the opposite charges. In this study, the comparative transport behaviors of Cd²⁺ and CrO₄^2- in water-saturated soil columns were investigated under a variety of hydrochemical and hydraulic conditions such as pH, ionic strength (IS), and flow rate. The transport mechanisms of Cd(II) and Cr(VI) were explored by fitting the breakthrough curves with a two-site non-equilibrium transport model. Results indicated that high solution pH inhibited the transport of Cd(II) due to the enhanced electrostatic interaction. In contrast, the migration of Cr(VI) was promoted with the least amount of Cr(VI) (1.23 mg) being retained in soil at high pH, ascribing to the stronger electrostatic repulsion between anions and soil surface. Meanwhile, high pH conditions were not favorable for the participation of reduced iron in the reduction process of Cr(VI), resulting in the least amount of Cr(III) detected (22%). The increase in ionic strength decreased the negativity of the potential at the adsorption plane, which enhanced the transport of cationic Cd(II) and the retardation of anionic Cr(VI). In addition, the increase in flow rate facilitated the transport of Cd(II) and Cr(VI), mainly due to the decreasing contacting with porous media and enhanced dispersion effect. These findings demonstrated that the fate and environmental behavior of metal cations and anions differed with the change of hydrochemical and hydrodynamic properties, which should be considered for the risk assessment and remediation of metal contaminated sites.