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 (Al-0, Fe-0). 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 = 0.10) relative to the bulk sop. This trend was paralleled by a systematic increase in Al-0 and Fe-0 (alpha = 0.10) at the root-soil interface. Lower amounts of plagioclase were also seen in the rhizosphere samples from five of the six horizons, but the difference from the bulk soil was not significant when all horizons were integrated in the comparison. No rhizosphere effect was detected for K-feldspars. This study provides converging results that emphasize the pedogenic significance of the rhizosphere zone and the role of roots as dynamic weathering agents.