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Abstract Artificial soils constructed from wastes and by-products have been considered as a sustainable option for land reclamation. In particular, they could contribute in a complementary manner to natural soils to global climate regulation by storing large quantities of carbon (C). However, the evolution of C stocks in such newly formed soils remains largely unknown. This work aimed at evaluating the dynamics of C in constructed Technosols, focusing on two experimental sites in Lorraine, France, where Technosols were constructed from thermally-treated industrial soil, papermill sludge and green waste compost, and planted with grasses. Soil samples were collected over 12 years, and stocks of C were calculated, taking into account the increase in soil bulk density and the associated decrease in soil thickness over the years. The evolution of the stocks of organic C was compared to the evolution reported for natural grassland soils from the same region and for other Technosols. Initial organic C stocks in the two constructed Technosols were 50% higher than in natural analog soils when calculated over 30 cm, and up to 5 times higher when calculated over 100 cm. Organic C stocks in the two Technosols decreased over the first three years, but increased during the following years, most likely due to the accumulation of plant-derived C. A similar evolution was observed in other planted Technosols. We conclude that Technosols constructed from an adequate set of parent materials can keep large amounts of organic C over time and sequester additional C under permanent plant cover. Graphical abstract Unlabelled Image Highlights • Organic C stocks of two Technosols were 30% to 50% higher than in natural soils. • C stocks decreased by 30% for the first three years, and then slowly increased. • A similar increase was observed with other Technosols under permanent plant cover. [ABSTRACT FROM AUTHOR]

Abstract Agromining is a technology based on the phytoextraction of metals by hyperaccumulator plants, combining agronomic and hydrometallurgical processes, to produce metal-based compounds. So far, it has been primarily developed to recover Ni from ultramafic soils, but secondary materials, namely industrial wastes containing Ni, may offer new opportunities for agromining. However, because of the toxicity of such materials, plants cannot be grown without formulating suitable substrates. The aim here was to assess the feasibility of growing Ni-hyperaccumulating plants on a Technosol containing a toxic industrial sludge and to test the influence of a biochar amendment on plant growth and Ni uptake. A constructed soil was prepared by mixing a decontaminated soil with an industrial sludge containing high concentrations of Fe, Ni, P and Zn, and amending it with biochar at different rates (0 to 5 wt%, dry matter). An ultramafic, Ni-rich soil was used as a reference material. Pot experiments were conducted with the hyperaccumulator Alyssum murale and the non-accumulating plant Lolium multiflorum used as a reference plant. After twelve weeks of growth, plant shoots and roots and soil samples were collected and analysed. Soil pore water was also collected over the experiment and analysed. Results showed that the growth of both plants was higher on the constructed soil than on the ultramafic soil, and increased with biochar amendments. The highest amounts of phytoextracted Ni were reached by A. murale on the ultramafic soil in the presence of biochar, whereas they remained low on the constructed soil. Contrary to the ultramafic soil, the constructed soil contained high amounts of Zn which was shown to impair Ni uptake as a result of the strong competition between Ni and Zn. Further investigations should therefore focus on practical solutions for decreasing this competition in order to maximize Ni uptake. In conclusion, agromining was proven feasible on soils constructed from industrial waste containing metals, providing that such soils are carefully designed to meet hyperaccumulator requirements. Highlights • Plants can mine strategic metals from secondary resources if appropriate Technosols are designed. • Growth of Ni-hyperaccumulator and a reference plant was proven possible on a constructed Technosol. • Biochar amendment increased plant growth and Ni extraction on both soils. • Ni:Zn balance in the Technosol controlled Ni hyperaccumulation. [ABSTRACT FROM AUTHOR]

Biochar may be used as an amendment in contaminated soils in phytoremediation processes. The mechanisms controlling plant metal uptake in biochar-amended soils remain however unclear. This work aimed at evaluating the influence of biochar on root development and its consequence on plant metal uptake, for two non-hyperaccumulating plants ( Zea mays and Lolium perenne ) and one hyperaccumulator of Cd and Zn ( Noccaea caerulescens ). We conducted rhizobox experiments using one acidic and one alkaline soil contaminated with Cd, Pb and Zn. Biochar was present either homogeneously in the whole soil profile or localized in specific zones. A phenomenon of root proliferation specific to biochar-amended zones was seen on the heterogeneous profiles of the acidic soil and interpreted by a decrease of soil phytotoxicity in these zones. Biochar amendments also favored root growth in the alkaline soil as a result of the lower availability of certain nutrients in the amended soil. This increase of root surface led to a higher accumulation of metals in roots of Z. mays in the acidic soil and in shoots of N. caerulescens in the alkaline soil. In conclusion, biochar can have antagonist effects on plant metal uptake by decreasing metal availability, on one hand, and by increasing root surface and inducing root proliferation, on the other hand. [ABSTRACT FROM AUTHOR]

Aims: Biochar could be used as a soil amendment in metal contaminated soils, for safe crop production or soil remediation purposes. This work was conducted to study the effects of biochar amendments on metal uptake by two contrasted plants grown on metal-contaminated soils. Methods: A non-hyperaccumulating plant ( Lolium perenne) and a Cd- and Zn-hyperaccumulator ( Noccea caerulescens) were grown in pots on acidic (A) and alkaline (B) soil contaminated by Cd, Pb and Zn, both amended by a wood-derived biochar. Results: Biochar amendments decreased the availability of metals by increasing soil pH, but also decreased Ca, P and N availability. Growth of L. perenne was increased and shoot metal uptake decreased by biochar addition in both soils, although increasing biochar dose above 0.5 % resulted in a progressive decrease of shoot production on soil B. Growth of N. caerulescens was not significantly affected by biochar. But an increase of Cd uptake with 5 % biochar was recorded on both soils, and of Zn uptake on soil B. Conclusions: Beside immobilizing metals, biochar may decrease the availability of nutrients, leading either to plant deficiency or to a decreased competition with cations for metal uptake, thus enhancing extraction of metals by hyperaccumulators. [ABSTRACT FROM AUTHOR]

Amendments of biochar, the residual solid of biomass pyrolysis, have been shown to enhance metal phytoextraction from contaminated soils with hyperaccumulating plants in specific situations. In order to investigate this phenomenon over successive harvests in field conditions, two identical undisturbed soil cylinders (1-m2 section × 1.85-m height) were excavated from a contaminated agricultural plot and monitored with instrumented lysimeters. Wood-derived biochar was added at a rate of 5% (w/w) in the first 30 cm of one of the two lysimeters. The Cd/Zn-hyperaccumulator Noccaea caerulescens was then grown for the next four years on both lysimeters. Our results showed that the hyperaccumulating plant was able to remove about 2 g m−2 of Cd and 12–16 g m−2 of Zn within four years, representing about 40% and 4% of the initial Cd and Zn soil contamination, respectively. Biochar amendment improved plant germination and survival and increased root surface density. However, no significant effect of biochar on shoot metal content of N. caerulescens was observed. Mass balances suggested that up to 10% the metal contamination moved from the disturbed Ap horizon to the deeper horizons, particularly in the biochar-amended soil profile. Furthermore, shoot Cd and Zn concentration generally decreased over the successive harvests, together with soil metal availability. Depending on the way to account for this progressive decrease in efficiency, our estimations of the time necessary to remove the excess of metals in the topsoil in these conditions ranged from 11 to 111 years for Cd and from 97 years to an infinite time for Zn. In conclusion, the simultaneous use of N. caerulescens and biochar amendment can lead to a significant removal of specific metallic elements from the topsoil, but the risk of metal movement down the soil profile and the observed decrease in phytoextraction efficiency over time deserve further investigations. Unlabelled Image • 40% of soil Cd had been phytoextracted by N. caerulescens after four years. • Biochar amendment favored seed germination and plant survival. • Biochar amendment did not affect shoot Cd and Zn concentrations. • Phytoextraction efficiency decreased over successive harvests. [ABSTRACT FROM AUTHOR]

Post-mining land reclamation of Athabasca Oil Sands (AB, Canada) involves the reconstruction of soil profiles able to support a mosaic of boreal forest communities. However, the use of coarse-textured reclamation materials to recreate forest ecosystems represents a challenge in terms of soil water and nutrient availability. This work aimed to quantify nutrient leaching in reclaimed coarse-textured soils constructed with two coversoils (peat mineral mix and forest floor mineral mix) underlain by mineral materials, including a blended B/C subsoil reclamation material, lean oil sand overburden substrate, and tailing sand. Water retention and conductivity curves were estimated for each material, and their retention capacity for inorganic N and P was measured in sorption isotherm experiments. The redistribution of water, inorganic N and P five days after an intense rain event was evaluated in six different reclaimed soil profiles using a laboratory-controlled leaching experiment in 1.2-m deep columns. The redistribution of fertilizer nutrients was also measured following the addition of 15N-labelled ammonium and phosphate over the top 10 cm of the columns. In addition, a 25-day incubation experiment with the two coversoils enabled us to estimate the timing of N immobilization and nitrification processes. Our results show that, depending on the combination of materials used for land reclamation, the soil profiles may provide equal or higher amounts of inorganic N and P in the rooting zone compared to natural, coarse-textured soils of the region. Following the simulated intense rainfall, the peat-mineral mix was able to retain 44% of its initial inorganic N within the top 20 cm of the reclaimed soil profiles, while 84% of the inorganic N present in the forest floor mineral mix was leached down. Compared to the movement of water, the leaching of N down the soil profiles was slower and partly restricted by the presence of lean oil sand, and to a lesser degree tailing sand. Most of the introduced fertilizer-N remained in the first 20 cm of the soil profiles under the form of nitrate, although the incubation experiment suggested that nitrification only occurred after the simulated rainfall event. Based on our experimental data and on additional simulations of water and nutrient transport, we conclude that nutrient leaching in reclaimed soils can be significant if specific materials such as forest floor mineral material and coarse-textured subsoil are combined and when an intense rainfall occurs at a period coinciding with a high concentration of nitrate-N in the topsoil. • The stock of nutrients was comparable between reclaimed and natural soils. • N leaching depended on the design of soil cover and on the underlying substrate. • The downward movement of N was slower than the movement of water. • Fertilizer-N remained in the top 20 cm after an intense rainfall. [ABSTRACT FROM AUTHOR]

Soils contaminated with heavy metals represent a serious threat for humans and all ecosystems. In some cases, they can be remediated by metal phytoextraction, using accumulator or hyperaccumulator plants, however the fate of harvested metal-enriched biomass has to be addressed. Cadmium (Cd) and zinc (Zn) are very common contaminants of urban, industrial and agricultural soils. To date, no process has been developed to recover these metals from plant biomass. This contribution presents a novel hydrometallurgical process designed to recover Cd and Zn from the biomass of a Zn/Cd hyperaccumulator plant Noccaea caerulescens . Dried plants are first ashed at 620 °C, a process which has been carefully investigated to avoid metal loss, and then the ash is acid leached. Following this, the process comprises two main steps: one to extract Cd and Zn and the other for Cd cementation using Zn powder and selective precipitation of Zn. Optimal cementation conditions have been determined with synthetic solutions using a Box-Behnken experimental design. In this context, a mole ratio Zn:Cd of 2:1, a temperature of 25 °C and a duration of 50 min proved optimal. The full process was tested on a sample of plant biomass. It demonstrated that Cd and Zn recovery was possible by cementation and precipitation. This new process still has to be optimized and up-scaled but it paves the way to Cd and Zn recovery from diverse types of soils, and even to ‘metal cultivation’, an emergent phytotechnology now referred to as agromining. [ABSTRACT FROM AUTHOR]

There is currently an intense debate about the potential for additional organic carbon storage in soil, the strategies by which it may be accomplished and what the actual benefits might be for agriculture and the climate. Controversy forms an essential part of the scientific process, but on the topic of soil carbon storage, it may confuse the agricultural community and the general public and may delay actions to fight climate change. In an attempt to shed light on this topic, the originality of this article lies in its intention to provide a balanced description of contradictory scientific opinions on soil carbon storage and to examine how the scientific community can support decision-making despite the controversy. In the first part, we review and attempt to reconcile conflicting views on the mechanisms controlling organic carbon dynamics in soil. We discuss the divergent opinions about chemical recalcitrance, the microbial or plant origin of persistent soil organic matter, the contribution of particulate organic matter to additional organic carbon storage in soil, and the spatial and energetic inaccessibility of soil organic matter to decomposers. In the second part, we examine the advantages and limitations of big data management and modeling, which are essential tools to link the latest scientific theories with the actions taken by stakeholders. Finally, we show how the analysis and discussion of controversies can guide scientists in supporting stakeholders for the design of (i) appropriate trade-offs for biomass use in agriculture and forestry and (ii) climate-smart management practices, keeping in mind their still unresolved effects on soil carbon storage. [ABSTRACT FROM AUTHOR]

Here, we present our collective musings on soil research challenges and opportunities and, in particular, the points raised by Philippe Baveye (Baveye, 2020a, 2020b) and Johan Bouma (Bouma, 2020) on I bypass i and I hyperbole i in soil science. Furthermore, developing a healthy and constructive post-publication peer-review system, where bypasses and hyperbolic approaches can be identified and discussed, would ultimately boost publication quality and contribute to a more open discourse in soil science. [Extracted from the article]