The role of plants in ironstone evolution: iron and aluminium cycling in the rhizosphere.

The Carajás plateaus in Brazil host endemic epilithic vegetation ("campo rupestre") on top of ironstone duricrusts, known as canga. This capping rock is primarily composed of iron(III) oxide minerals and forms a physically resistant horizon. Field observations reveal an intimate interaction between canga's surface and two native sedges (Rhynchospora barbata and Bulbostylis cangae). These observations suggest that certain plants contribute to the biogeochemical cycling of iron. Iron dissolution features at the root-rock interface were characterised using synchrotron-based techniques, Raman spectroscopy and scanning electron microscopy. These microscale characterisations indicate that iron is preferentially leached in the rhizosphere, enriching the comparatively insoluble aluminium around root channels. Oxalic acid and other exudates were detected in active root channels, signifying ligand-controlled iron oxide dissolution, likely driven by the plants' requirements for goethite-associated nutrients such as phosphorus. The excess iron not uptaken by the plant can reprecipitate in and around roots, line root channels and cement detrital fragments in the soil crust at the base of the plants. The reprecipitation of iron is significant as it provides a continuously forming cement, which makes canga horizons a 'self-healing' cover and contributes to them being the world's most stable continuously exposed land surfaces. Aluminium hydroxide precipitates ("gibbsite cutans") were also detected, coating some of the root cavities, often in alternating layers with goethite. This alternating pattern may correspond with oscillating oxygen concentrations in the rhizosphere. Microbial lineages known to contain iron-reducing bacteria were identified in the sedge rhizospheric microbiome and likely contribute to the reductive dissolution of iron(III) oxides within canga. Drying or percolation of oxygenated water to these anaerobic niches have led to iron mineralisation of biofilms, detected in many root channels. This study sheds light on plants' direct and indirect involvement in canga evolution, with possible implications for revegetation and surface restoration of iron mine sites.

Accelerating microbial iron cycling promotes re-cementation of surface crusts in iron ore regions.

Accelerating microbial iron cycling is an innovative environmentally responsible strategy for mine remediation. In the present study, we extend the application of microbial iron cycling in environmental remediation, to include biocementation for the aggregation and stabilization of mine wastes. Microbial iron reduction was promoted monthly for 10 months in crushed canga (a by-product from iron ore mining, dominated by crystalline iron oxides) in 1 m3 containers. Ferrous iron concentrations reached 445 ppm in treatments and diverse lineages of the candidate phyla radiation dominated pore waters, implicating them in fermentation and/or metal cycling in this system. After a 6-month evaporation period, iron-rich cements had formed between grains and 20-cm aggregates were recoverable from treatments throughout the 1-m depth profile, while material from untreated and water-only controls remained unconsolidated. Canga-adapted plants seeded into one of the treatments germinated and grew well. Therefore, application of this geobiotechnology offers promise for stabilization of mine wastes, as well as re-formation of surface crusts that underpin unique and threatened plant ecosystems in iron ore regions.

Biogeochemical cycling of iron oxides in the rhizosphere of plants grown on ferruginous duricrust (canga).

Goethite-cemented duricrusts, also known as canga, commonly occur as a capping rock protecting underlying iron ore deposits. The processes that govern canga formation are still unclear but include recurrent partial dissolution and recrystallisation of goethite through biogeochemical cycling of iron, hypothesised to be catalysed by plants and bacteria. In the present study, the effect of plant exudates on mobilisation of iron in canga was examined using model plants grown on crushed canga in RHIZOtest devices, which separate roots from substrate by a semi-permeable membrane. Moderate plant-induced acidification of the canga was detected, however the primary driver of mineral dissolution was the synergistic effect of reductive and ligand-promoted dissolution, identified by an increase in organic acids concentration and the presence of low concentrations of free ferrous iron. Whilst organic acids exudation lasted, iron cations were stabilised in solution; once the organic acids were degraded by microorganisms, the free cations precipitated as iron oxy-hydroxides. Mineralogical analysis and high-resolution microscopy confirmed our hypothesis that plants that grow in this iron-rich substrate contribute to iron dissolution indirectly (e.g., during phosphate solubilisation), and that the resulting surplus iron not taken up by the plants is redeposited, promoting the cementation of the residual minerals. Understanding the contribution of plants to the iron cycling in canga is crucial when formulating post-mining rehabilitation strategies for iron ore sites.

Biogeochemical cycling of iron: Implications for biocementation and slope stabilisation.

Microbial biofilms growing in iron-rich seeps surrounding Lake Violão, Carajás, Brazil serve as a superb natural system to study the role of iron cycling in producing secondary iron cements. These seeps flow across iron duricrusts (referred to as canga in Brazil) into hydraulically restricted lakes in northern Brazil. Canga caps all of the iron ore deposits in Brazil, protecting them from being destroyed by erosion in this active weathering environment. Biofilm samples collected from these seeps demonstrated heightened biogeochemical iron cycling, contributing to the relatively rapid, seasonal formation of iron-rich cements. The seeps support iron-oxidising lineages including Sideroxydans, Gallionella, and an Azoarcus species revealed by 16S rRNA gene sequencing. In contrast, a low relative abundance of putative iron reducers; for example, Geobacter species (<5% of total sequences in any sample), corresponds to carbon limitation in this canga-associated ecosystem. This carbon limitation is likely to restrict anoxic niches to within biofilms. Examination of a canga rock sample collected from the edge of Lake Violão revealed an array of well- to poorly-preserved microbial fossils in secondary iron cements. These heterogeneous cements preserved bacterial cell envelopes and possibly extracellular polymeric substances within the microfossil iron-rich cements (termed biocements). Bacterial iron reduction initiates the sequence, and intuitively is the rate-limiting step in this broadly aerobic environment. The organic framework of the active- and paleo-biofilms appears to provide a scaffold for the formation of some cements within canga and likely expedites cement formation. The accelerated development of these resilient iron-rich biocements in the lake edge environment compared with surroundings duricrust-associated environments may provide insights into new approaches to remediate mined land, aiding to stabilise slopes, reduce erosion, restore functional hydrogeology and provide a substrate akin to natural canga for revegetation using endemic canga plant species, which have adapted to grow on iron-rich substrates.

Fate of carbamazepine, its metabolites, and lamotrigine in soils irrigated with reclaimed wastewater: Sorption, leaching and plant uptake.

Irrigation with reclaimed wastewater may result in the ubiquitous presence of pharmaceutical compounds (PCs) and their metabolites in the agroecosystem. In this study, we focused on two highly persistent anticonvulsant drugs, lamotrigine and carbamazepine and two of its metabolites (EP-CBZ and DiOH-CBZ), aiming to elucidate their behavior in agricultural ecosystem using batch and lysimeter experiments. Sorption of the studied compounds by soils was found to be governed mainly by the soil organic matter level. Sorption affinity of compounds to soils followed the order lamotrigine > carbamazepine > EP-CBZ > DiOH-CBZ. Sorption was reversible, and no competition between sorbates in bi-solute systems was observed. The results of the lysimeter studies were in accordance with batch experiment findings, demonstrating accumulation of lamotrigine and carbamazepine in top soil layers enriched with organic matter. Detection of carbamazepine and one of its metabolites in rain-fed wheat previously irrigated with reclaimed wastewater, indicates reversibility of their sorption, resulting in their potential leaching and their availability for plant uptake. This study demonstrates the long-term implication of introduction of PCs to the agroecosystem.