Evidence for the accumulation of toxic metal(loid)s in agricultural soils impacted from long-term application of phosphate fertilizer

The sources of bioavailable toxic metals in sediments regulated their aggregated form, environmental responses and health risk-a case study in Liujiang River Basin, China

Bioavailable toxic metals (BTMs) predominantly stem from anthropogenic emissions, but their emission inventory remain unspecified. Considering the regulation of toxic metals (TMs) forms, variations in BTMs input linked to sources likely result from specific TM forms. To test this, we identified BTMs sources in Liujiang River Basin sediments, focusing on their aggregation and risk based on varying TMs forms. The findings indicated BTMs ratios decreased in the order as follows: Cd > Pb > Zn > Cu > As > Cr > Hg. PMF identified BTMs sourced from industrial, agricultural, and mining emissions with ratio of 43.2 %, 36.7 % and 20.2 %, respectively. The correspondences between the correlation coefficients of TMs forms and the sources proportions suggested reducible forms were predominantly present in industrial BTMs, whereas residual and carbonate-bound forms were less abundant in mining and agricultural BTMs, respectively. Additionally, the correspondence between correlations of environmental parameters and the sources proportions confirmed different responses of industrial, agricultural and mining BTMs for Eh, for OM and Mz and for pH, TDS and EC, respectively. HI values, generally within 1, suggested the absence of non-carcinogenic risks. Conversely, TCR values, frequently exceeded 10-6, highlighted a significant carcinogenic risk, particularly for children. Considering industrial BTMs contributed most to both HI and TCR values, frequent aeration in sediments, particularly for industrial zones, will effectively alleviate the aggregation of BTMs in Liujiang River Basin. This study confirmed TMs forms in discharged wastewater are tightly related to BTMs aggregations, therefore, strengthening their monitoring is essential.

Reintegrating human excreta into the agriculture of rural China: Ecological risks and application strategies

Treating and valorizing human excreta in rural China is challenging because of its high hazardous element content. Moreover, studies on the concentrations and ecological risks of human excreta in different regions are limited. To address this gap, we conducted a cross-sectional study using data from rural China. We assessed the concentrations of six heavy metals-Cd, Cr, Cu, Hg, Pb, Zn-and the metalloid As in the samples and evaluated their contamination levels and potential ecological risks. An improved model for the safe use of human excreta was developed based on the input-output equilibrium of hazardous elements in soils, which allowed for the calculation of safe application limits for each province. The results showed that the Zn concentrations were consistently high, but Cd was the most concerning contaminant. In most provinces, the use of human excreta was found to pose potential ecological risks, with Cd and Hg identified as the primary risk factors. The safe limit for human excreta in each province ranged from 39.77 to 212.02 t∙hm⁻2 over a 100-year control period. These results can guide human excreta usage strategies and provide theoretical support and a reference for the safe application of human excreta in China and other low- and middle-income countries.

Adsorption and ecotoxicology studies with aqueous solution of Cr(VI) ions using adsorbent materials derived from Inga edulis

Adsorbents derived from the bark of Inga edulis were developed for the removal of Cr(VI) ions. Chemical activation with sulfuric acid led to significant changes in the physicochemical properties of the biomass. The adsorbent materials denoted IB (inga biomass) and AIB (activated inga biomass) showed higher removal efficiencies at pH 2.0 and dosages of 1.0 and 0.25 g L-1, respectively. The adsorption kinetics could be fitted using the Elovich model, indicating that the adsorption occurred on heterogeneous surfaces. The maximum adsorption capacities were 46.0 mg g-1 for IB and 356.6 mg g-1 for AIB, with behaviors that could be described by the Langmuir (monolayer) and Freundlich (multilayer) models, respectively. XPS analyses confirmed the reduction of Cr(VI) to Cr(III), due to interactions with oxygenated functional groups. Thermodynamic evaluation indicated that the adsorption was spontaneous, with exothermic character for IB and endothermic character for AIB. Ecotoxicological assays using Daphnia similis showed that a Cr(VI) concentration of 4.34 μg L-1 caused 50.0 % immobility, while adsorption by the materials eliminated the toxicity, demonstrating the effectiveness of the adsorbents in reducing environmental impacts. Additionally, an electrode derived from the adsorption of Cr(VI) on AIB, denoted CPEAIB-Cr-ads-active, presented good performance in the hydrogen evolution reaction (HER), with high current density and low overpotential. The structure of the electrode, with high surface area and the presence of pores and cavities, was favorable for electrochemical catalysis, evidencing its potential for use in applications concerning renewable energy and environmental detoxification.

Impact of agricultural activities on trace element levels in soils of Mato Grosso, Brazil

The expansion of intensive agriculture must be linked to soil quality, especially in strategic regions for global food production, such as the state of Mato Grosso. This study evaluated the distribution and total levels of trace elements (TEs) in the soils of this region, identifying possible influences of agricultural activity and providing support for sustainable management practices. A total of 186 soil samples representing the nine ecoregions of the state, at a depth of 0-20 cm, were analyzed. The levels of TEs were extracted using the aqua regia digestion method and quantified using Inductively Coupled Plasma Mass Spectrometry and Inductively Coupled Plasma Optical Emission Spectrometry. Descriptive, inferential, and multivariate statistical analysis were performed. The results indicated that most of the TEs were below reference values, suggesting that the areas are safe for agricultural production. The state has naturally elevated levels of As, Cr, and Fe, along with low levels of Se. These data contribute to national discussions and prevent misinterpretations about environmental contamination. An increase in Zn and Cd was observed in agricultural areas, within safe levels and related to fertilization. Specific regions, due to high levels TEs, require more monitoring. Spearman correlation and cluster analysis, combined with the study of different soil types, geological provinces, and lithologies, as well as knowledge of natural areas, were essential for understanding the origins of the TEs. This study provides important information for sustainable soil management and food security.

Mechanical properties of raw bamboo fiber-reinforced phosphogypsum-based composite cementitious materials and their strengthening mechanism

Phosphogypsum-based cementitious materials (PGCs) typically exhibit low strength and poor toughness when utilized as construction materials. This study explores the incorporation of raw bamboo fibers of varying lengths into PGCs at different ratios to develop raw bamboo fiber-reinforced phosphogypsum-based composite materials (BFRPGCs). Firstly, the influence of the water-cement ratio on the mechanical properties of BFRPGCs was investigated through a one-factor experimental approach, leading to the identification of an optimal water-cement ratio. Secondly, the effects of the length and content of raw bamboo fibers on the mechanical properties of BFRPGCs at this optimal water-cement ratio were examined, along with an exploration of the mechanisms by which raw bamboo fibers impact the mechanical properties of the composites, considering their damage modes. Finally, the microstructure of BFRPGCs was analyzed using scanning electron microscopy (SEM), which elucidated the mechanisms through which raw bamboo fibers influence the working and mechanical properties of the composites. The results demonstrated that the incorporation of raw bamboo fibers significantly enhanced the mechanical strength of the specimens. Specifically, when the length of the bamboo fibers was 12 mm and the doping amount was 1.0%, the compressive and flexural strengths of BFRPGCs reached their maxima at 28.99 MPa and 8.41 MPa, respectively. These values represent increases of 123.73% and 169.82% compared to the control group. Additionally, hydration-generated calcium silicate hydrate (C-S-H) gels and calcium aluminate (AFt) phases formed a skeletal support around the CaSO₄·2H₂O, reinforcing the matrix structure. Furthermore, numerous hydration products adhered to the surfaces of the raw bamboo fibers, resulting in enhanced adhesion between the fibers and the matrix. This study provides valuable insights for the research and application of fiber-reinforced phosphogypsum-based building materials.

Impact of traffic intensity and vehicular emissions on heavy metal content in vineyard soils, grapes, and wine: a comparative study of two vineyards in South Moravia (Czech Republic)

The primary objective of this study is to evaluate the impact of vehicular traffic emissions on the contamination of wine production by heavy metals. For this purpose, selected heavy elements (As, Ba, Cd, Cr, Cu, Ni, Pb, and Zn) were determined in the samples of vineyard soils, grapes, final wines, and samples from the various phases of the winemaking process of two South Moravian (Czech Republic) vineyards with different intensity of traffic. After the visualisation of the data, the interpretation of the results and risk assessment calculations were performed. The results obtained indicate that contamination of grapes with aerosol is the most important and soil contamination is of minor relevance. The application of fungicides was the primary source of copper and zinc in soils and grapes. During the winemaking process, there is a significant decrease in the content of heavy elements originating from emissions from vehicular traffic. However, winemaking technology was found to be the most important source of heavy elements in the final wine. The health risk assessment indicates a low risk of consumption of both grapes and wine from both the examined and the reference wineries.

Effect of P rates in long-term conservation agriculture trials on the vertical distribution of soil acidity and nutrient availability

Few studies are published on the long-term impact of phosphorus (P) rates as triple superphosphate (208 g P kg-1) on the vertical distribution of soil acidity and the availability of macro and micronutrients within conservation systems. This study explores the long-term impacts of increasing P rates on the vertical distribution of soil acidity and Mehlich-3 nutrient availability in long-term trials managed under conservation tillage in North Carolina. In 2022, soil samples were collected from two contrasting long-term trials: one on a sandy, poorly drained Portsmouth soil at Tidewater Research Station (67 years) and another on a well-drained, clay-rich Lloyd soil at Piedmont Research Station (37 years), at depths of 0-5, 5-10, 10-20, and 20-30 cm. In general, the greater crop yield and nutrient removal over the years occurred with higher rates of P, resulting in a decrease in the soil available potassium (K). Historical use of P increased the content of other nutrients that are part of phosphate compounds (i.e., calcium [Ca] and magnesium [Mg]) or are contaminants in these fertilizers (i.e., zinc [Zn] and manganese [Mn]). The distinct trend of sulfur (S) in both locations, where soil available S content increased with P rates at Tidewater and decreased at Piedmont, reflects the complexity of S dynamics in soils with contrasting characteristics. These findings reveal differences between soils and the importance of integrated nutrient management in long-term experiments to avoid bias in the crop response over the years, especially in fields managed under conservation tillage.

Evaluation of heavy metal content in agricultural soil samples in the Mekong Delta region, VietNam and human health risks

Salinity intrusion due to climate change, together with industrialization and farming activities, has increased the heavy metal content in soil, causing serious risks to ecosystems and human health. In this study, neutron analysis was applied to determine the metal content in soil samples, exposure dose, pollution index, and multivariate statistical analysis was used to evaluate the characteristics of each area. The results showed that the Zn content (235 mg/kg) in Soc Trang and Cr (213 mg/kg) in An Giang exceeded the permissible limit of Vietnam. Notably, the enrichment factor of U (Kien Giang) elements increased by 5.66 times compared to the background level. Although most elements tended to be enriched, metals such as As went against this trend. Principal component analysis revealed distinct regional distributions of observed variables in soils, while hierarchical clustering (AHC) and correlations between metals revealed strong associations between them in the environment. Children are at higher risk of both cancer and non-cancer events than adults when exposed to heavy metals and environmental pollutants through ingestion, dermal absorption, and inhalation.

Cadmium accumulation, sub-cellular distribution and interactions with trace metals (Cu, Zn, Fe, Mn) in different rice varieties under Cd stress

Rice (Oryza sativa L.) is a staple food in most Asian countries, although it serves as a significant carrier of cadmium (Cd) accumulation. Developing low-Cd accumulating rice varieties is crucial for minimizing Cd contamination in soil and rice grains while also mitigating harmful health consequences. In the present study examined the Cd accumulation and sub-cellular distribution of both high Cd (IR-50) and low Cd (White Ponni) rice varieties under Cd-treated hydroponic nutrient solutions. The results showed that under all Cd treatments, overall plant height, plant fresh and dry biomass reduced substantially in both rice varieties compared to the control. Both rice varieties accumulated more Cd in their roots than shoots, with IR-50 accumulating higher Cd levels. Iron (Fe) concentrations were higher in both roots and shoots of both rice varieties compared to other trace elements. Translocation factor (TF) values were < 1, indicating limited Cd translocation from roots to shoots. Cd was mainly distributed in the epidermis, cortex, and bulliform cells of both rice varieties roots, and shoots. The peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) enzymes activity significantly increased in both IR-50 and WP rice varieties when exposed to Cd treatment. The current study concluded that the IR-50 rice variety accumulated and distributed more Cd than the WP rice variety under different Cd treatments. As a result, WP exhibited higher Cd tolerance, while IR-50 became more susceptible to Cd stress.

Physicochemical Properties and Concentration of Metal(oid)s in Soils Used for Different Periods of Time for Sugarcane Cultivation

This study evaluated the physicochemical properties and the concentrations of metal(oid)s in soils with approximately 5, 20 and 40 years of sugarcane cultivation from a locality in Quintana Roo, Mexico. The results showed the metal(oid) concentration order Fe > Cr > V > Pb > Co > Cu > As, with a gradual increase in their concentrations up to 33,834, 76, 67, 59, 32, 22 and 14 mg kg-1, respectively, after 40 years of sugarcane cultivation. The contamination factor indicated considerable contamination for Co, Cr, Pb and V and very high contamination for Fe in soils subjected to 40 years of sugarcane cultivation. The pollution load index revealed medium pollution in soils with 5 and 20 years of sugarcane cultivation, and moderate pollution in soils with 40 years. The organic matter, organic carbon and total nitrogen contents decreased significantly as the length of sugarcane cultivation increased. These results highlight the need to implement measures for proper agricultural management.

New insights into uranium stress responses of Arabidopsis roots through membrane- and cell wall-associated proteome analysis

Uranium (U) is a non-essential and toxic metal for plants. In Arabidopsis thaliana plants challenged with uranyl nitrate, we showed that U was mostly (64-71% of the total) associated with the root insoluble fraction containing membrane and cell wall proteins. Therefore, to uncover new molecular mechanisms related to U stress, we used label-free quantitative proteomics to analyze the responses of the root membrane- and cell wall-enriched proteome. Of the 2,802 proteins identified, 458 showed differential accumulation (≥1.5-fold change) in response to U. Biological processes affected by U include response to stress, amino acid metabolism, and previously unexplored functions associated with membranes and the cell wall. Indeed, our analysis supports a dynamic and complex reorganization of the cell wall under U stress, including lignin and suberin synthesis, pectin modification, polysaccharide hydrolysis, and Casparian strips formation. Also, the abundance of proteins involved in vesicular trafficking and water flux was significantly altered by U stress. Measurements of root hydraulic conductivity and leaf transpiration indicated that U significantly decreased the plant's water flux. This disruption in water balance is likely due to a decrease in PIP aquaporin levels, which may serve as a protective mechanism to reduce U toxicity. Finally, the abundance of transporters and metal-binding proteins was altered, suggesting that they may be involved in regulating the fate and toxicity of U in Arabidopsis. Overall, this study highlights how U stress impacts the insoluble root proteome, shedding light on the mechanisms used by plants to mitigate U toxicity.

Arsenic, cadmium, and chromium concentrations in contrasting phosphate fertilizers and their bioaccumulation by crops: Towards a green label?

Potentially toxic elements such as arsenic (As), cadmium (Cd), and chromium (Cr) are severely regulated in fertilizers and deserve continuous investigation. Phosphate-derived Cd has been a stepping-stone toward achieving sustainable and safe worldwide food production, especially after a new regulation aiming for reduced limits of Cd in P fertilizers (EU, 2019/1009). Three pot experiments were conducted to assess the variability of As, Cd, and Cr concentrations - with a particular focus on Cd - from monoammonium phosphates (MAP 1, MAP 2, and MAP 3 from different geographic origins) and their accumulation in limed and unlimed soils, and contrasting crops, representing staple food and significant sources of these elements for humans (i.e., potato, tobacco, and rice). A diverse array of sensitive techniques for trace elements determination were used to reveal the highest level of Cd of MAP 3 (20.71 mg kg-1 MAP), which loaded the highest amounts of this element to the soil matrix and solution, plant shoots, and xylem sap, contrasting with results for MAP 1 (0.87 mg kg-1 MAP), which has almost ten times less Cd than that required for low-Cd labeling of P fertilizers (≤20 mg Cd kg-1 P2O5). MAP 3 also had the highest Cr concentration (139.3 mg kg-1 MAP). Among crops, rice accumulated 16-fold less Cd than potato plants. Liming decreased Cd in tobacco and potato shoots up to 35%. Moreover, reductions of about 20% were also observed for Cd accumulation in tubers and sap. Conversely, Cd from MAP 3 was always much more accumulated in soil solution, achieving up to 20 μg L-1, while values < 5 μg L-1 (i.e., a groundwater limit) were obtained from MAP 1. Our findings may be used as a reference in developing green labels for fertilizers in scenarios where Cd accumulation represents a potential risk for soil and human health.

Phenotypic and genomic characterization of phosphate-solubilizing rhizobia isolated from native Mimosa and Desmodium in Brazil

The phosphate (P)-solubilizing potential of rhizobia isolated from active root nodules of Brazilian native Mimosa and Desmodium was assessed. Out of the 15 strains selected, five Paraburkholderia isolated from Mimosa spp. grown in rocky outcrops stood out. The Ca3(PO4)2-solubilizing efficiency of these strains ranged from 110.67 to 356.3 mgL-1, with less expressive results for FePO4 and Al(H2PO4)3, that might be attributed to the low solubility of these two P compounds. Paraburkholderia strains CNPSo 3281 and CNPSo 3076 were the most efficient siderophore producers (44.17 and 41.87 µMol EDTA) and two of the top FePO4 solubilizers. Acidification of the culture media was observed for all the strains and P sources. Regarding Ca3(PO4)2 solubilization, the main organic acids detected were glucuronic (an important component of rhizobia exopolysaccharides) and gluconic acids. Genomic analysis of P. nodosa CNPSo 3281 and CNPSo 3076 along with other phosphate-solubilizing Paraburkholderia species of the genus pointed out a conserved gene organization of phoUBR, pstSCAB, ppk and ppx. Greenhouse experiment revealed that P. nodosa CNPSo 3281 and CNPSo 3076 promoted maize growth under low P. Our results indicate the relevance of native rhizobia as multifunctional plant-associated bacteria and the rocky outcrops ecosystems as hotspots for bioprospection.

Deciphering the pollution risks, sources and their links of heavy metals in soils

Heavy metals in soils pose serious ecological and health risks. To make efficient strategies for mitigating the underlying hazards, it is critical to reveal the pollution sources and their links with the risks. Researchers have investigated source identification and risk evaluation of heavy metals in soils, yet few have systematically deciphered the source-sink relationship of soil metals and the links between source apportionment and risk assessment. In the study, an integrated technological framework has been proposed to address the gaps, and applied to characterize the pollution risks, sources and their links of soil metals in a typical coal resource city in China. The assessment using geochemical tool and ecological risk index shows the soils in study area are polluted by Cd, Hg, Cr, As and Pb in varied degrees, and particularly, Cd and Hg present significant ecological risk. Two advanced receptor models (multivariate curve resolution-weighted alternating least-squares and multilinear engine 2) are comparatively applied for apportioning the potential sources of soil metals, and the results suggest the two models have identified similar sources (r2 > 0.90), including agricultural activities, atmospheric depositions and industrial discharges with contributions of 35.5 %-38.3 %, 30.3 %-35.1 %, and 26.6 %-34.1 %, respectively. Then, apportionment results of the two models are jointly employed for evaluating the source-specific health risks of metals in the environment using a probabilistic risk assessment model. The risk levels within the area are overall acceptable or tolerable, and relatively, the industrial discharges present higher contribution on the non-carcinogenic and carcinogenic risks of soil metals to public. Findings will help the managers to design targeted policies for reducing the risks of soil metals, and the framework proposed provides a useful guideline to better understand the source-risk relationship of soil metals in other environments worldwide.

Phytoremediation of Pb-polluted soil using bermudagrass: Effect of mowing frequencies

Plant lead (Pb) tolerance and accumulation are key characteristics affecting phytoremediation efficiency. Bermudagrass is an excellent candidate for the remediation of Pb-polluted soil, and it needs to be mowed regularly. Here, we explored the effect of different mowing frequencies on the remediation of Pb-contaminated soil using bermudagrass. Mowing was found to decrease the biomass and photosynthetic efficiency of bermudagrass under Pb stress, thereby inhibiting its growth. Although mowing exacerbated membrane peroxidation, successive mowing treatments alleviated peroxidation damage by regulating enzymatic and nonenzymatic systems. A comprehensive evaluation of Pb tolerance revealed that all the mowing treatments reduced the Pb tolerance of bermudagrass, and a once-per-month mowing frequency had a less negative effect on Pb tolerance than did more frequent mowing. In terms of Pb enrichment, mowing significantly increased the Pb concentration, total Pb accumulation, translocation factor (TF), and bioenrichment factor (BCF) of bermudagrass. The total Pb accumulation was greatest under the once-a-month treatment, while the TF and BCF values were greatest under the three-times-a-month mowing treatment. Additionally, the decrease in soil pH and DOC were significantly correlated with the soil available Pb content and plant Pb accumulation parameters. The results showed that changes in the rhizosphere are crucial factors regulating Pb uptake in bermudagrass during mowing. Overall, once-a-month mowing minimally affects Pb tolerance and maximizes Pb accumulation, making it the optimal mowing frequency for soil Pb remediation by bermudagrass. This study provides a novel approach for the remediation of Pb-contaminated soil with bermudagrass based on mowing.

Development of an integrated framework for dissecting source-oriented ecological and health risks of heavy metals in soils

Heavy metals (HMs) in soils pose significant risks on ecosystem and human health. To design targeted regulatory measures for mitigating and controlling the risk, it is necessary to accurately identify the pollution sources and environmental risks of soil HMs, as well as to reveal the linkages between them. To date, yet systematic investigation aimed at deciphering the links between source apportionment of soil HMs and their associated environmental risks is still lacking. To fill the gap, an integrated framework has been developed in this study and applied for dissecting the source-sink relationship and source-oriented ecological and health risks of soil HMs in Shanxi, a province with rich coal resource, in which long-term coal mining activities in history has resulted in soil HMs pollution and unavoidably posed environmental risks. Two advanced receptor models, multivariate curve resolution alternating least squares based on maximum likelihood principal component analysis (MCR-ALS/MLPCA) and multilinear engine 2 (ME2), have been employed for apportioning the potential sources, and their apportionment results are jointly incorporated into a modified ecological risk index and a probabilistic health risk assessment model for identifying the source-oriented ecological and health risks posed by soil metals. The results show that the soils in study area have been polluted by HMs (i.e., Cd, Cr, Hg and As) to varying degrees. Industrial activities (35%-35.8%), agricultural activities (11.1%-20.5%), atmospheric deposition (10.5%-13%) and mix source (31.5%-42.6%) are apportioned as the main contributors of soil HMs in the area. The source-oriented ecological risk assessment suggests Hg has presented significant ecological risk and largely contributed by the sources from atmospheric deposition and industrial activities. The source-oriented health risk assessment shows the non-carcinogenic hazard level and carcinogenic risk posed by soil HMs in the study area are acceptable. Relatively, industrial activities and mix source have contributed more on the health risks.

Uptake and translocation of cadmium and trace metals in common rice varieties at different growth stages

Rice (Oryza sativa) is an important nutritional grain for the majority of Asian countries, but it is also a major source of cadmium (Cd) accumulation. A pot experiment was carried out to investigate the Cd uptake and translocation of high Cd (IR-50) and low Cd (White Ponni) rice cultivars in Cd-contaminated soils. The findings revealed that Cd impacts on rice development and growth differed depending on rice cultivars. Soil Cd levels in the seedling stage exceeded the critical levels (3-6 mg kg-1) only 5.0 mg kg-1 Cd treatment for the IR-50 (7.47 mg kg-1). At higher Cd treatments (1.0 and 5.0 mg kg-1), morphometric characteristics and yield of grains showed a declining and increasing trend in both rice varieties, respectively. The accumulation of Cd was higher in soil and roots during seedling and tillering stages, whereas in booting and maturity stages increased in stems and leaves in IR-50 and WP rice varieties. Cd levels in rice grains above the maximum allowable limit (0.4 mg kg-1) only in IR-50 (0.51 mg kg-1) rice cultivar at maturity stage. The EF of Cd were classified as minor enrichment to 'moderate enrichment' in both rice cultivars. TF values exhibited > 1 in booting and maturity stages in both rice cultivars at higher Cd treatments. The study concluded that the IR-50 rice variety exhibited increased Cd intake and transported to various parts of rice plants, particularly grains. The findings indicate that WP rice cultivar is more resistant to Cd toxicity, reducing health hazards for persons who preferred the staple food rice.

Cadmium, zinc, and copper leaching rates determined in large monolith lysimeters

Soil mass balances are used to assess the risk of trace metals that are inadvertently applied with fertilizers into agroecosystems. The accuracy of such balances is limited by leaching rates, as they are difficult to measure. Here, we used monolith lysimeters to precisely determine Cd, Cu, and Zn leaching rates in 2021 and 2022. The large lysimeters (n = 12, 1 m diameter, 1.35 m depth) included one soil type (cambisol, weakly acidic) and distinct cropping systems with three experimental replicates. Stable isotope tracers were applied to determine the direct transfer of these trace metals from the soil surface into the seepage water. The annual leaching rates ranged from 0.04 to 0.30 for Cd, 2.65 to 11.7 for Cu, and 7.27 to 39.0 g (ha a)-1 for Zn. These leaching rates were up to four times higher in the year with several heavy rain periods compared to the dry year. Monthly resolved data revealed that distinct climatic conditions in combination with crop development have a strong impact on trace metal leaching rates. In contrast, fertilization strategy (e.g., conventional vs. organic) had a minor effect on leaching rates. Trace metal leaching rates were up to 10 times smaller than fertilizer inputs and had therefore a minor impact on soil mass balances. This was further confirmed with isotope source tracing that showed that only small fractions of Cd, Cu, and Zn were directly transferred from the soil surface to the leached seepage water within two years (< 0.07 %). A comparison with models that predict Cd leaching rates in the EU suggests that the models overestimate the Cd soil output with seepage water. Hence, monolith lysimeters can help to refine leaching models and thereby also soil mass balances that are used to assess the risk of trace metals inputs with fertilizers.

Zinc speciation in highly weathered tropical soils affected by large scale vegetable production

Agriculture in highly weathered tropical soils often requires considerable application of lime and fertilizers to ensure satisfactory plant nutrient levels. The consequences of these continue long-term applications is not well understood may induce changes in soil chemical properties, the abundance, and speciation of potentially toxic trace element and as well as of micronutrients in agriculture soils. In this study, we evaluated the adsorption (at pH 5) and speciation of Zn in tropical soils (both agricultural and native vegetation) as a function of fertilization and contact time using chemical fractionation analyses and X-ray absorption spectroscopy. The soils overall had high Zn adsorption capacities (∼ 700 mg kg-1), but the agricultural soil was approximately 30 % higher than of the soil under native vegetation, and the proportion of Zn in the mobile fraction was 35 % in native vegetation and 21 % in agricultural soils. Zn speciation via linear combination analysis showed a strong relationship with soil mineralogical composition and reveled that Zn associated with organic matter decreased while Zn associated with P increased after the conversion of soils from native vegetation to highly fertilized soil. Aluminosilicate soil minerals were identified as major sinks of soil Zn, accounting for 34 % of total Zn retention regardless of soil origin and land use. Association of Zn with phosphate (i.e., hopeite) was observed in the agricultural soil samples, which might be an unexpected Zn-bearing mineral in highly weathered tropical soils and could have impacts on Zn plant nutrition.