Phytoextraction is one of the most promising technologies for the remediation of metal contaminated soils. Changes in soil metal availability during phytoremediation have direct effects on removal efficiency and can also illustrate the interactive mechanisms between hyperaccumulators and metal contaminated soils. In the present study the changes in metal availability, desorption kinetics and speciation in four metal-contaminated soils during repeated phytoextraction by the zinc/cadmium hyperaccumulator Sedum plumbizincicola (S. plumbizincicola) over three years were investigated by chemical extraction and the DGT-induced fluxes in soils (DIFS) model. The available metal fractions (i.e. metal in the soil solution extracted by CaCl2 and by EDTA) decreased greatly by >84% after phytoextraction in acid soils and the deceases were dramatic at the initial stages of phytoextraction. However, the decreases in metal extractable by CaCl2 and EDTA in calcareous soils were not significant or quite low. Large decreases in metal desorption rate constants evaluated by DIFS were found in calcareous soils. Sequential extraction indicated that the acid-soluble metal fraction was easily removed by S. plumbizincicola from acid soils but not from calcareous soils. Reducible and oxidisable metal fractions showed discernible decreases in acid and calcareous soils, indicating that S. plumbizincicola can mobilize non-labile metal for uptake but the residual metal cannot be removed. The results indicate that phytoextraction significantly decreases metal availability by reducing metal pool sizes and/or desorption rates and that S. plumbizincicola plays an important role in the mobilization of less active metal fractions during repeated phytoextraction.

Although biochar supports were widely adopted to fabricate the biochar (BC) supported layered double hydroxides (LDHs) composites (LDH-BC) for efficient environmental remediation, few studies focus on the important role of biochar support in alleviating the stacking of LDHs and enhancing LDH-BC's performance. Through the analysis of the material structure-performance relationship, the “support effect” of fine biochar prepared by ball milling was carefully explored. Compared with the original LDHs on LDH-BC, the LDHs on ball milled biochar (LDH-BMBC) had smaller particle size (from 1123 nm to 586 nm), crystallite size (from 20.5 nm to 6.56 nm), more abundant O-containing functional groups, and larger surface area (370 m² g⁻¹) and porous structure. The Langmuir model revealed that the maximum theoretical phosphate adsorption capacity of LDH-BMBC (56.2 mg P g⁻¹) was significantly higher than that of LDH-BC (27.6 mg P g⁻¹). The leaching experiment proved that the addition of LDH-BMBC in calcareous soil could significantly reduce the release of soil total phosphate (46.1%) and molybdate reactive phosphate (40.4%), even though pristine BC and BMBC significantly enhanced the soil phosphate leaching. This work fabricated high-performance and eco-friendly LDH-BMBC for phosphate adsorption in solution and phosphate retention in soil and also provide valuable insights into fine biochar support effect on LDHs exfoliation, extending the practical use of the engineered ball milled biochars in environment remediation.

Synthetic inhibitors and organic amendment have been proposed for mitigating greenhouse gas N₂O emissions. However, their combined effect on the N₂O emissions and ammonia-oxidizer (ammonia-oxidizing bacteria and archaea, AOB and AOA) communities remains unclear in calcareous soils under climate warming. We conducted two incubation experiments (25 and 35 °C) to examine how N₂O emissions and AOA and AOB communities responded to organic amendment (urea plus cattle manure, UCM), and in combination with urease (N-(n-butyl) thiophosphoric triamide, NBPT) and nitrification inhibitor (nitrapyrin). The treatments of UCM + nitrapyrin and UCM + nitrapyrin + NBPT significantly lowered total N₂O emissions by average 64.5 and 71.05% at 25 and 35 °C, respectively, compared with UCM treatment. AOB gene abundance and α-diversity (Chao1 and Shannon indices) were significantly increased by the application of urea and manure (P < 0.05). However, relative to UCM treatment, nitrapyrin addition treatments decreased AOB gene abundance and Chao 1 index by average 115.4 and 30.4% at 25 and 35 °C, respectively. PCA analysis showed that UCM or UCM plus nitrapyrin notably shifted AOB structure at both temperatures. However, fertilization had little effects on AOA community (P > 0.05). Potential nitrification rate (PNR) was greatly decreased by nitrapyrin addition, and PNR significantly positively correlated with AOB gene abundance (P = 0.0179 at 25 °C and P = 0.0029 at 35 °C) rather than AOA (P > 0.05). Structural equation model analysis showed that temperature directly increased AOA abundance but decrease AOB abundance, while fertilization indirectly influenced AOB community by altering soil NH₄⁺, pH and SOC. In conclusion, the combined application of organic amendment, NBPT and nitrapyrin significantly lowered N₂O emissions via reducing AOB community in calcareous soil even at high temperature. Our findings provide a solid theoretical basis in mitigating N₂O emissions from calcareous soil under climate warming.

In China, excessive phosphorus (P) application in protected vegetable fields has led to high legacy P stores. Soil amendment with alum or dolomite is one of many best management practices (BMPs) used to reduce P losses in calcareous soils. However, both the kinetics and mechanisms of P sorption and soil available P in amended soils are understudied. Herein, both aspects were looked at under controlled conditions. Firstly, a sorption study which coupled P concentrations with poorly-crystalline Al hydroxides and dolomite was conducted. Results from this batch experiment showed that P sorption on poorly-crystalline Al hydroxides was homogenous and occurred mainly via displacement of inner-sphere hydroxyl (Al–OH) instead of the formation of AlPO₄. However, the amount of sorbed P reached maximum sorption of 73.1 mg g⁻¹ and did not change with further increase in P concentration. It was observed that P adsorbed onto the dolomite surface at low P concentrations, whereas hydroxyl replacement and uneven cluster precipitation of Ca₃(PO₄)₂ occurred at high P concentrations. A second 90 day incubation experiment investigated changes to soil available P and sorption-desorption across variable rates of amendments (0–50 g kg⁻¹). Results showed that alum amendment at a rate of 50 g kg⁻¹ decreased soil CaCl₂–P and Olsen-P concentrations by 91.9% and 57.8%, respectively. However, Olsen-P increased when the dolomite rates were <20 g kg⁻¹. Phosphorus sorption-desorption of the amended soil showed alum had higher P sorption efficiency than dolomite at low addition rates (<10 g kg⁻¹). However, soil amended with high dolomite rates (>10 g kg⁻¹) could sorb more P in comparison with alum when P concentrations were increased. The P status of the amended soil was closely connected to the P sorption mechanisms on mineral amendments, soil P concentrations and soil properties.

Efficiency and the preservation of soil functions are key requirements for sustainable remediation of contaminated soil. Microbial decomposition and conversion of substrates is a fundamental soil function. Pilot-scale EDTA-based soil washing recycled chelant generated no wastewater and removed 78% of Pb from acidic farmland soil with 860 mg kg⁻¹ Pb and 60% of Pb from calcareous garden soil with 1030 mg kg⁻¹ Pb. Remediation had an insignificant effect on microbial respiration in acidic soil induced by sequential additions of glucose, micro-cellulose, starch and alfa-alfa sprout powder (mimicking litter components, C-cycle). In contrast, remediation of calcareous soil reduced cumulative CO₂ production after glucose (simple) and alfalfa (complex substrate) addition, by up to 40%. Remediation reduced the nitrification rate (denoting the N-cycle) in acidic soil by 30% and halved nitrification in calcareous soil. Remediation in both soils slightly or positively affected dehydrogenase and β-glucosidase activity (associated with C-cycle), and decreased urease activity (N-cycle). Generally, EDTA remediation modestly interfered with substrate utilisation in acidic soil. A more prominent effect of remediation on the functioning of calcareous soil could largely be attributed to the use of a higher EDTA dose (30 vs. 100 mmol kg⁻¹, respectively).

The quantification of heavy metal contents in soils and their sources are essential for contamination monitoring and the assessment of the potential risks to the ecosystems. This study aims to investigate the source of heavy metals and other elements in soils from a uranium-phosphate deposit using integrated multivariate and geostatistics techniques. For this, 50 soil samples in Itataia deposit, Northeastern, Brazil, were collected at 0–0.2-m depth for the determination of U, Fe, Al, Mn, Ti, Zn, Cu, Ni, Mo, Co, Cr, Cd, Pb, As, Se, V, B, and Zr. The Pb, Se, Ni, Cr, As, and Mo mean contents were closer or exceeded The Brazilian Environmental Council (CONAMA) prevention values for soils. Uranium content was about 500 times higher than the mean levels reported for Brazilian soils. The cluster analysis indicates three geochemical groups based on different contamination levels. The first principal component was associated with lithological origin, the second principal component may be related to anthropogenic sources, and the third and fourth principal components indicated a joined source (natural and anthropogenic), indicating different sources of contamination. Mo was not related to other heavy metals, being found independent in the area. The accumulation of heavy metals in soils is associated not only with the parent material but also with the minerals of the soil. In the area of study, calcareous soils favored alkaline conditions that influenced the dynamics of heavy metals. The multivariate and geostatistical analyses were able to provide preliminary information regarding the metal contents in soil for environmental management.

The aim of this study was to investigate the impact of incorporating Walnut leaves (WL) and their biochars produced at three temperatures (200, 400, and 600 °C) on fractionation, availability and maize indices in a naturally calcareous highly contaminated soil of Central Iran. A pot experiment was conducted considering soils treated with 0, 0.5, 1, and 2% (w/w) of WL and their derived biochars. After maize (Zea mays L.) planting, shoot and root dry matter and Pb and Zn concentration in shoots and roots and DTPA-extractable and fractions of Zn and Pb in soils were determined. Results showed showed that biochar amendments substantially modified the partitioning of Zn and Pb from easily available forms to less available forms. The results showed that DTPA-extractable of Zn and Pb and their bioaccumulation were reduced upon the addition of biochars produced at different temperatures and application rates in a calcareous soil. Treating soil with 2% biochar produced at 600 °C increased significantly shoot and root dry matter by 131.4% and 116.7%, respectively and reduced the bioavailability of Zn and Pb (DTPA-TEA extraction) by 49.1%, and 34.9%, respectively (P < 0.05) in comparison to the control. Therefore, biochars were able to reduce metals contamination in treatments and increase maize dry matter. Biochar decreased Zn and Pb concentration in plant tissues and promoted gradual maize growth responses through changing metals fractions. Therefore, biochar as a sorbent for contaminants can assist in maize to mitigate and phytostabilize Zn and Pb in highly contaminated soils.

A study on pH and chemical composition of precipitation was carried out in two Italian sites, one urban (site 1) and one rural (site 2), located approximately 30 km far from Bologna, during a 3-year period. No significative site variation was found. In both locations, bulk deposition pH ranged from slightly acid to slightly alkaline, despite the volume weighted mean concentration of acidic species, NO ₃ ⁻ and SO ₄ ²⁻ (67.4 and 118.4 μeq l⁻¹ in site 1 and 88.7 and 103.8 μeq l⁻¹ in site 2), that were similar to those of typical acidic rainfall region. This might be ascribed to the neutralization reaction of the Ca²⁺, attributed to the calcareous soil and the frequent dusty air mass intrusion from the Sahara. The pair correlation matrix and the analysis of the main components suggested also ammonium and other crustal elements as neutralization agents. The depositional rate of SO ₄ ²⁻ and NO ₃ ⁻ , chemical elements of agricultural interest, amounted to 38 and 28 and 32 and 35 kg ha⁻¹ for site 1 and site 2, respectively. These supplies of nutrient were not negligible and had to be considered on cultivated lands. NH ₄ ⁺ deposition rate on site 2 was 7 kg ha⁻¹, 23% over site 1, probably due to nitrogen fertilization in the fields around the monitoring station. In site 1, SO ₄ ²⁻ presented a seasonal trend, indicating that its principal source was the residential heating. Results emphasized that the entity of the bulk deposition acidification is linked not only to the ions local emission sources (fossil fuel combustions, heating, and fertilizers) but also to the surrounding territory and the prevalent wind that transports through kilometers air masses which may contain acidic and alkaline species.

This study was performed on 21 soils with the aim of establishing whether Pb and Ni adsorption/desorption parameters could be considered as good indicators of the risk of groundwater pollution. Results showed that high pH values in soil caused a totally irreversible Pb adsorption, thus excluding any risk of Pb groundwater pollution. Sorption/desorption studies, quantified by the desorption index (DI), showed that Ni retention was only partly affected by the basic pH values but it was also due to the electrostatic attraction processes occurring on soil surfaces, as demonstrated by the partial reversibility of the Ni sorbed. This justifies possible risks of Ni groundwater pollution. The results of a monitoring research confirmed these findings. Results suggested that the adsorption/desorption parameters, namely DI, are promising indicators to predict the risk of groundwater pollution from metals in calcareous soils.