Selenium distribution in French forests: Influence of environmental conditions

Climate and soil properties regulate the vertical heterogeneity of minor and trace elements in the alpine topsoil of the Hengduan Mountains

Soil minor and trace elements are vital regulators of ecological processes that sustain alpine ecosystem functions. In this study, the vertical pattern and driving factors of element concentrations in alpine soils of the Tibetan Plateau were investigated. Three snow mountains (Meili, Baima, and Haba) part of the Hengduan Mountain range, were selected as the study area to determine the vertical distribution of 12 typical elements (Cr, Ni, Cu, Fe, Mn, Zn, Cd, Pb, Ca, Sr, As, and Se) in topsoil with increasing and decreasing elevation, as well as the dominant driving factors of their spatial heterogeneity. Results showed that all elements, except Se, showed strong vertical heterogeneity, among which Cr, Ni, Cu, and Fe showed peak concentrations at 2700-3000 m; the highest concentrations of Mn and Zn were at 3200 m and 2700 m, with Cd and Pb at 2500 m. Ca and Sr levels gradually decreased with increasing elevation. According to the structural equation model and random forest analysis, the vertical heterogeneity of soil elements is directly regulated by the variability of climate and soil properties due to changes in elevation. A three-way PERMANOVA further quantized the contributions of climate and soil properties on vertical heterogeneity of all soil elements, which were 35.2 % and 50.5 %, respectively. This study used various statistical tools to reveal the dominant factors affecting the vertical heterogeneity of soil elements. These findings provided a scientific overview of element distribution on the Tibetan Plateau and significant references for the vertical distribution of elements in the topsoil of other snow mountains worldwide.

Effect of Soil Acidification on the Production of Se-Rich Tea

Selenium (Se)-enriched tea is a well-regarded natural beverage that is often consumed for its Se supplementation benefits. However, the production of this tea, particularly in Se-abundant tea plantations, is challenging due to soil acidification. Therefore, this study aimed to investigate the effects of changes in Se under acidified soil conditions. Eight tea plantation soil monitoring sites in Southern Jiangsu were first selected. Simulated acid rain experiments and experiments with different acidification methods were designed and soil pH, as well as various Al-ion and Se-ion concentrations were systematically determined. The data were analyzed using R statistical software, and a correlation analysis was carried out. The results indicated that as the pH value dropped, exchangeable selenium (Exc-Se) and residual selenium (Res-Se) were transformed into acid-soluble selenium (Fmo-Se) and manganese oxide selenium (Om-Se). As the pH increased, exchange state aluminum (Alex) and water-soluble aluminum (Alw) decreased, Fmo-Se and Om-Se declined, and Exc-Se and Res-Se increased, a phenomenon attributed to the weakened substitution of Se ions by Al ions. In the simulated acid rain experiment, P1 compared to the control (CK), the pH value of the YJW tea plantation decreased by 0.13, Exc-Se decreased by 4 ug mg-1, Res-Se decreased by 54.65 ug kg-1, Fmo-Se increased by 2.78 ug mg-1, and Om-Se increased by 5.94 ug mg-1 while Alex increased by 28.53 mg kg-1. The decrease in pH led to an increase in the content of Alex and Alw, which further resulted in the conversion of Exc-Se to Fmo-Se and Om-Se. In various acidification experiments, compared with CK, the pH value of T6 decreased by 0.23, Exc-Se content decreased by 8.35 ug kg-1, Res-Se content decreased by 40.62 ug kg-1, and Fmo-Se content increased by 15.52 ug kg-1 while Alex increased by 33.67 mg kg-1, Alw increased by 1.7 mg kg-1, and Alh decreased by 573.89 mg kg-1. Acidification can trigger the conversion of Exc-Se to Fmo-Se and Om-Se, while the content of available Se may decrease due to the complexation interplay between Alex and Exc-Se. This study provides a theoretical basis for solving the problem of Se-enriched in tea caused by soil acidification.

Mechanisms of changing speciation and bioavailability of selenium in agricultural mollisols of northern cold regions

The distribution, speciation, and bioavailability of selenium (Se) - an essential micronutrient for human beings - in agricultural soils influence the resource recovery of agricultural benefits and the sustainable use of Se in agroecosystems. Quantitative understanding in this regard however remains limited in the world's mollisol agroecosystems, despite their critical importance in securing global food supply. Herein, a systematic investigation of Se in the river sediment-irrigation water-mollisols-rhizosphere-rice seeds continuum, at the core zone of the northern mollisol regions, was conducted to elucidate the hydrological-hydrogeochemical processes and mechanisms responsible for the distribution and bioavailability of Se. The content of total Se in the mollisols ranged between 0.12 and 0.54 mg/kg with an average of 0.31 mg/kg. At the riverside flood plains, humic-acid bound Se accounted on average for 39 % of total Se. This pool of Se can be transformed to water-soluble and ion-exchangeable Se(VI), supporting a higher potential of Se bioavailability at riparian agricultural mollisols. For mollisol lands far from the river channels, the topography affects the speciation and partitioning of Se presumably through regulating water retention and organic matter transport. Moreover, altering pH and redox conditions in response to irrigation with the river water may boost Se bioavailability in weakly acidic and high Eh mollisols. It can be in part ascribed to the transformation of organic-bound Se along with infiltrated oxygenated water that leads to the increase of water-soluble and ion-exchangeable Se. These findings reinforce that hydrological-hydrogeochemical perturbations due to irrigation with surface water need to be assessed carefully in the management of Se resources in the mollisol agroecosystems.

A radioecological model with moisture-dependent Kd: Application to 129I and 79Se natural release to a grassland

A radioecological model previously developed to simulate chlorine cycling in a Scots pine forest was modified to examine the effect of soil hydrochemical conditions on the fate of 129I and 79Se released to a grassland through natural discharge of contaminated groundwater. To this end, the constant solid-liquid distribution coefficient (Kd) in the original model was replaced by a parametric equation to estimate 129I and 79Se Kd values from soil saturation - as a proxy for soil redox potential - and a set of Kd values determined experimentally under oxic and anoxic conditions. Additionally, the multi-compartment Scots pine tree module was replaced by a two-compartment module to represent 129I and 79Se cycling in grass. Simulations undertaken with the model indicated a considerable effect of soil redox conditions on 129I and 79Se accumulation in the soil column, especially in the saturated subsoil above the water table. The constant Kd overestimated 129I accumulation in the soil in relation to the parametric Kd. In contrast, the constant Kd underestimated 79Se accumulation in the soil. These results have implications for radiological impact assessments, specifically regarding the degree of conservatism in the Kd used in the assessment. In respect of bioavailability to grass, the simulated soil-to-plant transfer factors of 129I and 79Se compared favourably with values reported in the literature for similar soils and plant species, giving confidence in the model performance. The model presented here is a step forward in radioecological modelling as it includes the key processes that drive radionuclide transfers in soil-plant systems and the effect of soil redox conditions on sorption. The model can be readily extended to other cultivated lands and release scenarios to predict radionuclide transfer up the food chain.

Recycling and persistence of iodine 127 and 129 in forested environments: A modelling approach

Differences in the source and behaviour of ¹²⁹I compared to ¹²⁷I isotopes have been described for a variety of surface environments, but little is known about the cycling rates of each isotope in terrestrial ecosystems. We developed a compartment model of the iodine cycle in a forest ecosystem, with a labile and non-labile pool to simplify the complex fate of iodine in the forest floor and soil. Simulations were performed using atmospheric ¹²⁷I and ¹²⁹I inputs for sites differing in climate, vegetation, and soil. In general, considering dry deposition in addition to wet deposition improved model simulations. Model results support the view that soil is the sink for atmospheric iodine deposited in forest ecosystems, while tree vegetation has little influence on long-term iodine budgets. Modelling also showed that iodine cycling reaches equilibrium after a period of about 5000 years, mainly due to a gradual incorporation of iodine into the bulk stabilised soil organic matter. At steady state, this pool of non-labile iodine in soil can retain about 20% of total deposition with a mean residence time of 900 years, while the labile iodine pool is renewed after 90 years. The proportions of modern anthropogenic ¹²⁹I in each modelled pool reflect those of stable ¹²⁷I at least several decades after input to the forest; this result explains why isotopic disequilibrium is common in field data analysis. Volatilisation plays a central role in regulating iodine storage in soil and, therefore, its residence time, while drainage is a minor export pathway, except at some calcareous sites. Dynamic modelling has been particularly helpful for gaining insight into the long-term response of iodine partitioning to continuous, single or even varying deposition. Our modelling study suggested that better estimates of dry deposition of atmospheric iodine, weathering of parent rock, and volatilisation of the deposited iodine from soil and vegetation will be required for reliable predictions of iodine cycling in specific forests, because these processes remain insufficiently explored.

Interaction between Se(IV) and fulvic acid and its impact on Se(IV) immobility in ferrihydrite-Se(IV) coprecipitates during aging

Selenium (Se) is regarded as a trace element for humans, but it is toxic in excess. In natural environments, the mobility of Se is dominantly controlled by the Se oxyanions with high solubility such as selenite (Se(IV)). Se(IV) is often associated with the omnipresent ferrihydrite and coexisting organic matter. However, there is little information on the dynamic interactions among Se(IV), fulvic acid, and ferrihydrite. This study investigated the influence of fulvic acid on ferrihydrite-Se(IV) coprecipitates (Fh-Se) transformation for 8 days and the subsequent behavior of Se(IV) at varied pH (5.0, 7.5, and 10.0). Results showed that fulvic acid had different effects on Fh-Se transformation at varied pH values. Fh-Se transformation was promoted by fulvic acid at initial pH 5.0 whereas it was inhibited at initial pH 10.0. Interestingly, at initial pH 7.5, Fh-Se transformation was promoted at a low C/Fe ratio while it was suppressed at a high C/Fe ratio. Besides, fulvic acid induced the generation of more extractable Se(IV) at initial pH 5.0 and more coprecipitated Se(IV) at initial pH 7.5 and blocked the release of Se(IV) at initial pH 10.0. Fulvic acid possibly interacted with Se(IV) via carboxyl complexation and weakened the inhibition of Se(IV) on Fh-Se transformation. Thus, fulvic acid increased the transformation rate of Fh-Se. These findings help to uncover the environmental behavior of Se(IV) and organic matter during ferrihydrite transformation.