Mineralogical variations of bulk and rhizosphere soils from a Norway spruce stand

The mineralogy of bulk and rhizosphere soil materials was compared to assess the effect of roots on mineral weathering in a Podzol supporting a homogeneous Norway spruce [Picea abies (L.) Karst] stand. Six horizons were sampled in two profiles at the Skogaby site in southwestern Sweden. The soil adhering to the roots after shaking was considered as rhizosphere soil, a layer 3 mm thick intimately associated with root surfaces. 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) and normalized with respect to quartz (I/I(QZ)). Amorphous solid phases were estimated by extracting Fe and Al with acid-ammonium oxalate (Al0, Fe0). Mineral abundance (I/I(QZ)) near roots differed consistently from that in the bulk soil. The rhizosphere contained significantly lower amounts of amphiboles and expandable phyllosilicates (alpha