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The mineralogy of bulk and rhizosphere soils was compared to assess the effect of roots on mineral weathering in a Typic Dystrochrept supporting Norway spruce (Picea abies [L.] Karst) and oak (Quercus sessiliflora Smith). In an experimental forest site (Breuil-Chenue, France), systematic soil sampling was performed in forty pits. The soil adhering to the roots was considered as rhizosphere soil. The remaining material was regarded as bulk soil. The mineralogy of the clay-sized particles of both fractions was determined by x-ray diffraction (XRD). Amorphous solid phases were estimated by extracting Fe and Al with hot Na-citrate and dithionite-citrate-bicarbonate. Total chemical analyzes were performed on the clay-sized particles via Ba[Cl.sub.2]-saturation and an iso-zirconium weathering balance was calculated. The XRD demonstrated an increase of illite-like minerals in the rhizosphere of both species and the selective extractions revealed a decrease in the amorphous phases. The total chemical analyzes showed that the rhizosphere clay-sized fraction contained significantly more Si and K (for oak only) and less Fe and Al than the bulk soil. By way of the iso-zirconium weathering balance, these losses of Fe and AI in the rhizosphere were estimated at several tens of kilograms by hectare for the surface horizon (0-3 cm). This study demonstrates that, despite the short duration of the contact between the active part of a root and the solid mineral phase, the intensity of the processes occurring in the rhizosphere significantly increases mineral weathering. Abbreviations: B, bulk soil; BS, base saturation; subscript c, hot Na-citate treatment; CEC, cation exchange capacity; subscript d, dithionite-citrate-bicarbonate treatment; EA, exchangeable acidity; EG, ethylene glycol treatment; HIS, hydroxy-interlayered smectite; HIV, hydroxy-interlayered vermiculite; ICP-AES, inductively coupled plasma spectrometry-atomic emission spectrometry; ICP-MS, inductively coupled plasma spectrometry- mass spectrometry; R, rhizosphere; subscript [tot], total; XRD, x-ray diffraction.

Summary: Mineral weathering by microorganisms is considered to occur through a succession of mechanisms based on acidification and chelation. While the role of acidification is established, the role of siderophores is difficult to disentangle from the effect of the acidification. We took advantage of the ability of strain Collimonas pratensis PMB3(1) to weather minerals but not to acidify depending on the carbon source to address the role of siderophores in mineral weathering. We identified a single non‐ribosomal peptide synthetase (NRPS) responsible for siderophore biosynthesis in the PMB3(1) genome. By combining iron‐chelating assays, targeted mutagenesis and chemical analyses (HPLC and LC‐ESI‐HRMS), we identified the siderophore produced as malleobactin X and how its production depends on the concentration of available iron. Comparison with the genome sequences of other collimonads evidenced that malleobactin production seems to be a relatively conserved functional trait, though some collimonads harboured other siderophore synthesis systems. We also revealed by comparing the wild‐type strain and its mutant impaired in the production of malleobactin that the ability to produce this siderophore is essential to allow the dissolution of hematite under non‐acidifying conditions. This study represents the first characterization of the siderophore produced by collimonads and its role in mineral weathering. [ABSTRACT FROM AUTHOR]

Soil analyses was combined with cultivation-dependent analyses of the culturable bacterial communities associated with the widespread mycorrhizal fungus Scleroderma citrinum to demonstrate a significant enrichment of bacterial isolates with efficient mineral weathering potentials around the oak and beech mycorrhizal roots compared to bulk soil. The study highlights the presence of bacterial strains harboring efficient mineral weathering potential in the soil surrounding the trees regardless of the tree species considered.

Two column experiments with a quartz-biotite substrate were conducted to quantify the respective effects of plant- and root-associated bacteria on mineral weathering and their consequences on tree seedling growth and nutrition. It was observed that there was no improvement in growth when the seedlings were supplied with all the necessary nutrients suggesting that the principal nutrient source for forest trees is derived from the weathering of soil minerals, which results from water circulation and from plant and microbial activity.