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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]

Metals can be immobilized on biochars by precipitation with carbonate. The distribution of metal-carbonate phases at the surface of biochars and the conditions of their formation, however, are unknown. Electron microscopy and X-photon spectroscopy were used to characterize carbonate phases in various morphological groups of particles of a wood-derived biochar, both before and after a metal-sorption experiment. Our results showed that the distribution of metals at the surface of biochar particles depended on the corresponding wood tissues and the presence of carbonate phases. Metals were particularly concentrated (i) within calcium carbonate crystals in bark-derived particles, which originated from calcium oxalate crystals formed prior to pyrolysis, and (ii) as new phases formed by the reprecipitation of carbonate on specific tissues of biochar. The formation of biochar carbonate phases and their redistribution by dissolution-precipitation mechanisms may primarily control the localization of metals on biochar particles and the durability of metals immobilization. [ABSTRACT FROM AUTHOR]

Key priorities in biochar research for future guidance of sustainable policy development have been identified by expert assessment within the COST Action TD1107. The current level of scientific understanding (LOSU) regarding the consequences of biochar application to soil were explored. Five broad thematic areas of biochar research were addressed: soil biodiversity and ecotoxicology, soil organic matter and greenhouse gas (GHG) emissions, soil physical properties, nutrient cycles and crop production, and soil remediation. The highest future research priorities regarding biochar’s effects in soils were: functional redundancy within soil microbial communities, bioavailability of biochar’s contaminants to soil biota, soil organic matter stability, GHG emissions, soil formation, soil hydrology, nutrient cycling due to microbial priming as well as altered rhizosphere ecology, and soil pH buffering capacity. Methodological and other constraints to achieve the required LOSU are discussed and options for efficient progress of biochar research and sustainable application to soil are presented. [ABSTRACT FROM PUBLISHER]

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]

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]