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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]

Biochar amendments, i.e., the solid product of biomass pyrolysis, reduce soil metal availability, which may lower the toxicity of metal-contaminated soils. A direct link between the decrease in soil metal availability and improved plant development is however often difficult to establish, as biochar may induce undesirable side effects on plant growth, e.g., a modification to plant nutrition. In order to investigate toxicity processes at a cellular level, roots of Vicia faba were exposed for 7 days to three metal-contaminated substrates and one control soil, amended with a 0 or 5% ( w/ w) addition of a wood-derived biochar. Exposure to pure biochar was also tested. Root tip cells were then observed to count the number of micronuclei as an estimation of DNA damage and the number of cells at mitosis stage. Results showed that biochar amendments led to a significant decrease in soil metal availability (Cd, Cu, Ni, Pb, and Zn) and to enhance root development on acidic substrates. The micronucleus frequency in root tip cells was positively correlated and the number of mitotic cells negatively, to the extractability of Zn in soils and to the concentration of Zn in secondary roots. Exposure to pure biochar caused a lower production of roots than most soil substrates, but led to the lowest number of observed micronuclei. In conclusion, biochar amendments can reduce the genotoxicity associated with the presence of metallic contaminants in soils, thereby potentially improving plant growth. [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]