Biogeochemical alteration of iron oxyhydroxide in the pine (mycor)rhizosphere: new evidence from in-situ X-ray imaging

Abstract:

Iron (Fe) is vital to the metabolic functions of living organisms and is an “essential nutrient” for plants but exists in soil as sparingly-soluble minerals (e.g., oxyhydroxides). Plants can increase soil bioavailable Fe by interacting with soil microorganisms. We used the well-established ectomycorrhizal Pinus-Suillus model symbiont system to investigate Fe dynamics across the plant-mycorrhiza-soil interface. Two species of Suillus (S. brevipes SB 120 and S. spraguei EM 44) were paired with P. contorta and then planted for two months in a nutrient-poor sand culture supplemented with ferrihydrite-coated sand. Morphological changes to roots were visibly evident for the +EMF treatments. Stereomicroscope analysis showed that Fe addition had a significant and negative effect on the fungal colonization rate in the early stage. In-situ X-ray fluorescence (XRF) imaging revealed spatial and chemical differences between the +EMF and -EMF treatments. Microcosms without host-specific EMF contained predominantly unaltered grains of iron-coated sand following 2 months growth while those with EMF contained severely altered grains that had ‘disintegrated coatings’ near roots, and their roots had a fungal sheath enriched with iron. Time-series XRF imaging of mycor(rhizosphere) showed an expanding interaction zone around roots. Spatially-resolved Ca K-edge X-ray absorption spectroscopy (μXAS) revealed the presence of tiny oxalate crystals in rhizosphere soil and Fe K-edge μXAS identified an Fe(III) organometallic complex associated with the fungal sheath surrounding +EMF roots. In this talk we will discuss the preliminary findings of the study and explain why we think these two spectroscopic observations are connected mechanistically. Findings support the hypothesis that ectomycorrhizal fungi can dissolve Fe coatings and transform them into plant-available forms, which in turn enhances plant biomass.