Chemical properties and mineralogy of soils with plinthite and petroplinthite in Iranduba (AM), Brazil

Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. This work was carried out to investigate the chemical behavior, mineralogical composition and weathering stage of four representative soil profiles with plinthite and petroplinthite, in Iranduba, AM (Central Amazon). Three well-drained soil profiles at high elevations were studied (P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol) and a contrasting poorly drained soil (P4 Haplic Plinthosol), located at low elevation. After profile descriptions, soil samples were collected from each horizon, air-dried, sieved (2 mm), and analyzed for particle-size distribution, pH, exchangeable cations (Al3+, Ca2+, Mg2+, K+, and Na+), as well as available P and total organic carbon (TOC) content. The minerals present in the clay and sand fractions, as well as in the ferruginous materials were identified by X-ray Diffraction (XRD). The weathering stage of these soils was assessed by means of Ki and Kr indexes, and the amounts of free and amorphous Fe and Al oxides by using dithionite citrate bicarbonate (DBC) and ammonium oxalate dissolution procedures, respectively. The results showed that all soils were extremely unfertile, with pH levels ranging between strong and moderate acidity, very low sum of bases and organic matter content, and of available P. The mineralogy of the soil profiles was very similar, mainly of the well-drained soils, with predominance of kaolinite and quartz in the clay and sand fractions, respectively. In the poorly-drained P4, 2:1 clay particles were also observed. These profiles can be considered highly developed according to the Ki index, however, the Ki value of P4 was higher, indicating that this soil was less developed than the others. In summary, these profiles with plinthite and petroplinthite can be characterized as highly developed and infertile soils and are, with exception of P4, well-drained.

Monophyly of beta-tubulin and H+-ATPase gene variants in Glomus intraradices: consequences for molecular evolutionary studies of AM fungal genes.

Arbuscular mycorrhizal fungi (AMF) are an ecologically important group of fungi. Previous studies showed the presence of divergent copies of beta-tubulin and V-type vacuolar H+-ATPase genes in AMF genomes and suggested horizontal gene transfer from host plants or mycoparasites to AMF. We sequenced these genes from DNA isolated from an in vitro cultured isolate of Glomus intraradices that was free of any obvious contaminants. We found two highly variable beta-tubulin sequences and variable H+-ATPase sequences. Despite this high variation, comparison of the sequences with those in gene banks supported a glomeromycotan origin of G. intraradices beta-tubulin and H+-ATPase sequences. Thus, our results are in sharp contrast with the previously reported polyphyletic origin of those genes. We present evidence that some highly divergent sequences of beta-tubulin and H+-ATPase deposited in the databases are likely to be contaminants. We therefore reject the prediction of horizontal transfer to AMF genomes. High differences in GC content between glomeromycotan sequences and sequences grouping in other lineages are shown and we suggest they can be used as an indicator to detect such contaminants. H+-ATPase phylogeny gave unexpected results and failed to resolve fungi as a natural group. beta-Tubulin phylogeny supported Glomeromeromycota as sister group of the Chytridiomycota. Contrasts between our results and trees previously generated using rDNA sequences are discussed.