Role of freeze-thaw cycles and chlorpyrifos insecticide use on diffuse Cd loss and sediment accumulation

Effects of biochar and compost on microbial community assembly and metabolic processes in glyphosate, imidacloprid and pyraclostrobin polluted soil under freezethaw cycles

Biochar and organic compost are widely used in agricultural soil remediation as soil immobilization agents. However, the effects of biochar and compost on microbial community assembly processes in polluted soil under freezingthawing need to be further clarified. Therefore, a freezethaw cycle experiment was conducted with glyphosate (herbicide), imidacloprid (insecticide) and pyraclostrobin (fungicide) polluted to understand the effect of biochar and compost on microbial community assembly and metabolic behavior. We found that biochar and compost could significantly promote the degradation of glyphosate, imidacloprid and pyraclostrobin in freezethaw soil decrease the half-life of the three pesticides. The addition of immobilization agents improved soil bacterial and fungal communities and promoted the transformation from homogeneous dispersal to homogeneous selection. For soil metabolism, the combined addition of biochar and compost alleviated the pollution of glyphosate, imidacloprid and imidacloprid to soil through up-regulation of metabolites (DEMs) in amino acid metabolism pathway and down-regulation of DEMs in fatty acid metabolism pathway. The structural equation modeling (SEM) results showed that soil pH and DOC were the main driving factors affecting microbial community assembly and metabolites. In summary, the combined addition of biochar and compost reduced the adverse effects of pesticides residues.

Colloid-facilitated mobilization of cadmium: Comparison of spring freeze-thaw event and autumn freeze-thaw event

Freeze-thaw action has the potential to facilitate the mobilization of colloid-associated contaminants in soil. However, the differences in colloid-associated contaminants following autumn freeze-thaw (AFT) events and spring freeze-thaw (SFT) events remain unclear. In this study, the potential influence mechanisms of AFT and SFT on both the generation and migration of colloids and colloid-associated cadmium (Cd) in soil were explored. Higher aggregate stabilities were found in soils after AFT compared with after SFT. After SFT, lower Cd concentrations were found in soil aggregates of 0.25-0.50 mm and <0.106 mm and higher concentrations were found in 0.106-0.25 mm aggregates. Moreover, SFT generated higher amounts of colloidal Cd than AFT, while AFT increased the total Cd concentration in leachates. Additionally, compared with SFT, AFT led to higher Cd concentrations in dissolved and colloid-associated forms in leachates. These findings demonstrate that higher amounts of colloid and fewer loadings of Cd in colloids in Cd contaminated soil can be found after SFT events. Thus, to better understand the environmental risk of contaminants in areas subject to seasonal freeze-thaw cycles, the differences between freeze-thaw processes in spring and autumn should be considered.

Spatiotemporal variation and ecological risk assessment of sediment heavy metals in two hydrologically connected lakes

Excessive accumulation of heavy metals in global lake sediments poses a serious threat to lake water quality and ecosystem security. However, there is still a knowledge gap in comparison of heavy metal variation and pollution in hydrologically connected lakes. In this study, concentrations of As, Cd, Cr, Cu, Hg, Pb, and Zn in sediments of two hydrologically connected lakes, Xingkai Lake and Xiaoxingkai Lake, were determined during the hydrologically connected periods (May and September) and disconnected period (January and July) in 2021. We found the range of As was 2.58∼14.35 mg/kg, Cd was 0.050∼0.21 mg/kg, Cr was 28.58∼262.3 mg/kg, Cu was 3.12∼28.05 mg/kg, Hg was 0.0030∼0.14 mg/kg, Pb was 10.87∼58.86 mg/kg, and Zn was 18.21∼90.73 mg/kg. Heavy metal concentrations were lower than grade I level in Chinese soil quality standards with significant spatial and temporal differences in the basin. Overall, most of the sampling sites in Xingkai Lake and Xiaoxingkai Lake were at the uncontaminated level and moderate ecological risk during the sampling period. Two lakes showed different heavy metal compositions, accompanied by higher contamination level and higher potential ecological risk in the small lake than those in the large lake based on analysis of the geo-accumulation index and potential ecological risk index. Besides, the contamination level and potential ecological risk in May and September were higher than those in January and July, mainly due to human activities and hydrological connectivity. The ecological risks were moderate for Cd and Hg, and low for As, Cr, Cu, Pb, and Zn. Correlation and PCA analyses showed that Cd mainly originated from anthropogenic sources, while other metals mainly came from natural sources. These findings elucidate the effects of agriculture and hydrological connectivity on heavy metals in sediments, and provide scientific basis for the reasonable management of lake ecosystem.

Presence, sources, and risk assessment of heavy metals in the upland soils of northern China using Monte Carlo simulation

The spatial dynamics of heavy metal contamination in the upland soils of northern China are relatively unknown, despite the region's high contribution to the national grain output. In this study, the concentrations of As, Cd, Co, Cr, Cu, Mn, Pb, Sb, Sc, Ti, and Zn and subsequent ecological and human health risks were investigated in three major grain producing areas (Hexi Corridor, L1; Hetao irrigation area, L2; and eastern Inner Mongolia, L3) of northern China. Among the heavy metals, Ti had the highest average concentration of 3.02 g/kg, followed by Mn (470 mg/kg), Cr (56.6 mg/kg), Zn (34.3 mg/kg), Pb (19.4 mg/kg), Cu (17.8 mg/kg), Co (9.66 mg/kg), Sc (7.26 mg/kg), As (5.35 mg/kg), Sb (0.73 mg/kg), and Cd (0.17 mg/kg). Generally, the heavy metal concentrations decreased from west to east (L1 > L2 > L3) across northern China. Moreover, three potential sources of the heavy metal were distinguished, including natural process, anthropogenic activities (industrial development and agricultural cultivation), and atmospheric deposition. Although the contamination of the single metal (including Cd, Cr, Cu, and Pb) was moderate in L1 and L2, the combined contamination was low in the upland soils. It was noted that Cd posed a moderate to considerable ecological risk on the upland soils in northern China. This metal was the most sensitive factor in assessing the combined ecological risk, with a contribution rate of 91.56-94.84%. Considering the ingestion exposure, the current concentrations of the metals posed minimal risks to human health. Furthermore, children experienced higher health risks than adults. Present study analyzed the probabilistic distribution of contamination, ecological, and health risk of heavy metals in upland soils of northern China, providing fundamental information for better agricultural heavy metal pollution assessment in China.

Do freeze-thaw cycles affect the cadmium accumulation, subcellular distribution, and chemical forms in spinach (Spinacia oleracea L.)?

To date, although there are many studies investigating the toxicity of heavy metal to plant, little research exists in the seasonal freeze-thaw (FT) regions where FT cycles often happen during the plant growing process. To reveal the adaptive mechanisms of plants to the combination stresses of cadmium (Cd) and FT, the Cd accumulation, subcellular distribution, chemical forms, and antioxidant enzyme activity (peroxidase (POD)) were investigated in spinach (Spinacia oleracea L.) growing under different soil Cd levels (i.e., 0.10 mg Cd kg-1 soil (low), 1.21 mg Cd kg-1 soil (medium), and 2.57 mg Cd kg-1 soil (high)). Compared to the non-freeze-thaw (NFT) treatments, higher Cd concentrations in the root and lower translocation factors from root to leaf were found for the plants experiencing FT cycles. FT significantly decreased the Cd concentrations in the leaves under the low- and medium-Cd treatments, while similar values were found for the high-Cd treatments. Generally, FT could decrease the concentrations and proportions of Cd stored in the cell wall and soluble fractions and increase them in the organelle fractions for the medium- and high-Cd treatments, while opposite tendency was found for the low-Cd treatments. Moreover, larger Cd amounts in the inorganic and water-soluble forms were found for the low- and medium-Cd treated plants under FT, while lower values were found for the high-Cd treatments. Additionally, POD, which presented higher activities at the low- and medium-Cd treatments and lower activities at the high-Cd treatments under FT, were also significantly influenced by the Cd × FT interaction. This study indicated that FT could significantly change the accumulations of Cd in plant, and it provided a new insight into the Cd accumulation by plants in the seasonal FT region.

Lead and Chromium Immobilization Process Subjected to Different Freeze-Thaw Treatments in Soils of the Northeastern Qinghai-Tibet Plateau

The freeze-thaw cycle is one of the important processes that affected heavy metal behaviors in soil. However, information regarding the adsorption and desorption behavior of heavy metals in soils under different freeze-thaw conditions is relatively less. Therefore, different freeze-thaw conditions including unfrozen, 15 freeze-thaw cycles at 60% water content, and 15 freeze-thaw cycles at 100% water content were investigated. Then the adsorption and desorption behaviors of Pb and Cr in freeze-thaw soils were studied. Results showed the Pb and Cr adsorption amount mostly decreased with increasing water-soil ratio, and the soil performance of Pb and Cr adsorption at same water-soil ratios showed variation under different freeze-thaw conditions. The Pb isothermal adsorption was higher for most freeze-thaw treatments compared to the control. The soil performance of Cr isothermal adsorption showed variation under different freeze-thaw conditions. Most electrostatic binding of Pb and Cr were stronger under unfrozen and freeze-thaw conditions than unfrozen conditions. Most Pb and Cr adsorption kinetics patterns of freeze-thaw treated soils were rapid than unfrozen conditions. These results implied that freeze-thaw cycles could change the soil adsorption and desorption patterns of Pb and Cr. Therefore, further studies are urgently needed to investigate Pb and Cr immobilization mechanisms in soils during freeze-thaw cycles. Hence, these findings provided useful information on Pb and Cr immobilization process in soils that underwent freeze-thaw cycles to offer an additional insight into predicting Pb and Cr behaviors in cold and freezing environments.

Heavy metal accumulation, geochemical fractions, and loadings in two agricultural watersheds with distinct climate conditions

The main aim of this study was to explore the effects of climate conditions on the transport and transformation of heavy metals. Sedimentary geochemical analysis and watershed modeling were used to investigate the distinctions between heavy metal pollution under different climate conditions. The results showed that the average concentrations of Cu, Cd, and Pb in sediments of the subtropical watershed (36.64, 0.60, and 133.69 mg/kg, respectively) were higher than those of the temperate watershed (26.58, 0.19, and 23.17 mg/kg, respectively) because of surface runoff-induced heavy metal loadings under higher precipitation. Also, the labile fractions, which mainly originated from anthropogenic sources, showed higher percentages in the subtropical watershed (67.84-91.33%), thereby indicating that the transport of heavy metals was promoted by surface runoff. Moreover, higher percentages of acid-soluble fractions of Cu and Pb (23.55-33.60%) in the subtropical watershed suggested that higher temperatures accelerated the transformation of heavy metal fractions, thus contributing to the transportation of heavy metals. Overall, climate conditions were the dominant factors for the differences between the subtropical and temperate watersheds. The results of this study suggest that the effects of climate conditions on the transport, enrichment, and bioavailability of heavy metals are of great significance. Such effects should therefore be the focus of future studies.

Impacts of different freeze-thaw treatments on the adsorption and desorption behaviors of Cd in black soil

Seasonal freeze-thaw cycle (FTC) is one of the key processes that affect heavy metal behaviors in soil. However, previous studies are mainly focused on extreme FTC treatments which may exaggerate the real FTC effects in the field. This study aimed to compare the effects of different FTC conditions on the adsorption and desorption behaviors of Cd in the surface black soil. Different minimum freezing temperatures (- 2, - 5, and - 15 °C), FTC rates (1 and 20 °C h^-1), freezing lengths (2 and 24 h), and FTC frequencies (1, 3, and 9) were investigated. The thawing temperature was set at 5 °C. The amplitude for the FTC rate, length, and frequency experiments ranged from 5 to - 2 °C. Our results indicated that the adsorption amounts of Cd presented an order of - 2 °C > - 15 °C > - 5 °C and 24 h > 2 h for different FTC amplitude- and freezing length-treated soils, and the adsorption amounts decreased with increasing FTC rate and frequency. Soil maximum adsorption amount of Cd increased with the increases of FTC frequency, freezing length, and FTC rate, while it decreased with the decreases of freezing temperature. Soil Cd desorption ratio decreased with the increases of FTC frequency, freezing length, and TFC rate, and it increased with the increasing freezing temperature. Our results suggested that FTC conditions can significantly influence the adsorption and desorption behaviors of heavy metal in soil.

Pollution assessment of heavy metal accumulation in the farmland soils of Beijing's suburbs

In this study, we used GIS to assess heavy metal concentrations and their spatial variations in Beijing's suburban farmlands. The Beijing soil heavy metal background values (background values), the primary standard of the Chinese Environmental Quality Standard of Soil (primary standard), and the Environmental Quality Standard of Green Food Production Site (green standard) were used as the standards for assessing soil heavy metals. Results showed that the average Hg value was higher than that of the primary standard. The Ni and Pb values were lower than the corresponding background values. From the percentage area, the concentration is 100% area with As, more than 93% area with Cr, Cu, Pb and Zn, and 67% area with Cd and Hg, meeting the primary standard; over 97% area met the green standard. The calculated Nemerow index showed that less than 3% of the samples were slightly polluted, indicating good environmental quality. But the accumulated pollution of Cd and Hg was relatively high in some areas which was mainly affected by the high density of the industry. The farmland soils in Beijing suburbs were found to be suitable for farming, but a small percentage of the soils exceeded the green standard; Cd and Hg levels require special attention.

Contribution of root uptake to cadmium accumulation in two peanut cultivars: evidence from a split-column soil experiment

Cadmium (Cd) accumulation and internal Cd translocation in the peanut (Arachis hypogaea L.) are highly related to root uptake, which may largely depend on the cultivar variation and the depth of the Cd-contaminated soil. A split-column soil experiment was conducted using two common Chinese peanut cultivars (Huayu-20 and Huayu-23) known to relocate Cd to different tissues. The growth medium was separated into four layers and Cd solution was solely applied to one layer to determine the key depth affecting the Cd accumulation in a plant via root uptakes. The results showed that the biomass of Huayu-23 was significantly higher biomass (3.28-94.0%) than that of Huayu-20, especially in the aerial parts (stems and leaves) and kernels, implying the dilution of Cd. Following the addition of Cd to the soil, the Cd concentrations in peanut tissues increased on average by 28.9-172 and 28.3-111% in Huayu-20 and Huayu-23, respectively. The largest presence of Cd in a peanut plant was observed in the aerial parts, followed by the kernels. Huayu-20 accumulated more Cd in plant tissues than did Huayu-23 due to the former's high Cd translocation. These findings imply that peanut cultivars vary widely in biomass, Cd accumulation, and the percentage distribution of Cd among various plant tissues, especially kernels. Different Cd treatments in the full depth of the root zone induced significant alterations in Cd accumulation of peanut tissues, especially kernels, for both cultivars. The percentage distribution of Cd accumulation by kernels was significantly higher in the deeper layer than in the top layer of the root zone for both peanut cultivars. This study suggests that soil modifications performed during agronomic activities should take into account the full depth of root exploration as well as the peanut cultivars to manage plant Cd uptake.