Frédéric Rees

Les biochars sont au cœur de la lutte contre les changements globaux. Ils constituent aussi un enjeu pour la gestion durable des sols contaminés. Ces travaux ont été conduits afin de mieux comprendre les effets du biochar sur la mobilité des métaux dans les sols et leur transfert vers les plantes. Une série d’expériences de complexité croissante ont été conduites, impliquant un biochar obtenu par pyrolyse de bois à 450 °C, et deux sols, l’un acide et l’autre alcalin, contaminés par Cd, Pb et Zn suite à l'activité d'une fonderie. Une gamme d’espèces végétales aux réponses contrastées vis-à-vis des métaux a été testée. Des essais de sorption en batch et de lixiviation en colonnes couplés à une caractérisation microscopique et spectroscopique du biochar ont été réalisés, ainsi que des expériences de croissance végétale en vases de végétation, en rhizotrons et en lysimètres. Les résultats démontrent que les phases minérales carbonatées du biochar jouent un rôle prépondérant dans l'immobilisation des métaux. Elles interviennent dans la sorption directe des métaux à la surface du biochar par co-précipitation. Leur dissolution contribue également à l'augmentation du pH du sol, conduisant à une rétention accrue des métaux à la surface des particules du sol. Le biochar modifie le transfert des métaux vers la plante en diminuant la disponibilité des métaux, mais également en diminuant la mobilité de cations majeurs et en modifiant la surface racinaire développée par la plante. Une diminution du transfert du métal vers les parties aériennes de la plante a généralement été observée avec Lolium perenne ou Zea mays, alors qu'une augmentation du prélèvement de Cd et Zn par l'hyperaccumulateur Noccaea caerulescens a été mise en évidence. En conclusion, le biochar contrôle la mobilité des métaux dans les systèmes sol-plante grâce à une série de mécanismes différents. Au plan pratique, le biochar favorise les stratégies de phytostabilisation et de phytoextraction des métaux dans les sols contaminés

Net nitrogen (N) uptake results from active and water-mediated N inflows, partly offset by N exudation. These processes exhibit significant variations along root axes, suggesting the existence of specific zones of higher N exchanges. However, the precise location, drivers, and significance of such active root zones in root N budget remain unclear. Here, we identified and characterized active root zones using Root-CyNAPS, a new functional-structural plant model that simulates the space-time variations in net N uptake, based on the interactions between nitrogen, carbon, and water flows, and root anatomy in each segment of a 3D root system architecture. Our simulations on wheat for various plant ages and external nitrate concentrations revealed two zones of preferential net N uptake: one near the root apices and the other one coinciding with the lateral root initiation zone, both characterized by a net N uptake activity about 10 times higher than adjacent segments. Higher water uptake and N exudation rates were also located next to apices. The contribution of most active root zones (defined here by the top 10% of root length) varied between 20% and 80\% of plant net N uptake, depending on root environment and age. Our simulations also showed that water-mediated N uptake could represent up to two thirds of gross N uptake, while N losses could offset more than half of it. Root-CyNAPS offers coupling opportunities with other functional-structural models to simulate nitrogen, carbon, and water multiscale interactions across the soil-plant-atmosphere continuum.

Background and Aims: Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in plant-soil interactions. However, its spatial and temporal dynamics remain poorly understood. Methods: We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Results: Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. Conclusion: RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition.

Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in soil-plant interactions. However, its spatial and temporal dynamics remain poorly understood. We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition. Highlight: Using the new model RhizoDep , we simulated distinct spatial and temporal patterns of rhizodeposition along the roots of spring wheat over its complete growth cycle, and identified their main drivers. Graphical

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.

Two types of organic-matter-rich coversoils are used during reclamation in the oil sands region of Alberta: forest floor material (FFM) salvaged from upland forests, and peat material (PM) salvaged from boreal wetlands. In this study, we tested the hypothesis that carbon (C) and nutrient availability may limit microbial activity in these reclamation materials by measuring their response to either ¹³C-labeled glucose or NPKS addition. Coversoil materials were compared with two natural forest soils corresponding to target sites for reclamation. A shift in microbial community structure (determined using phospholipid fatty acid analysis) was detected after both additions, but it was stronger with glucose than NPKS, especially for the two reclamation materials. For all soils, the increase in microbial respiration was stronger after glucose than after NPKS addition. The majority of CO₂ originated from soil organic matter (SOM) for the natural forest soils but from glucose for the reclamation materials. In PM, glucose addition triggered SOM mineralization, as shown by a positive priming effect. Despite the absence of a priming effect for FFM, microbial communities incorporated higher rates of glucose into their biomass and respired double the amount of glucose compared with the other materials. Furthermore, the overall microbial community structure in the FFM became more similar to that of the natural forest soil materials following glucose addition. These findings indicate that C and NPKS limitations were stronger for the two reclamation materials than for the two natural forest soils. Furthermore, microbial communities in the two reclamation materials responded more readily to labile C than to NPKS addition.