Mineralogy of the rhizosphere in forest soils of the eastern United States

Abstract

Chemical and mineralogical studies of forest soils from six sites in the northeastern and southeastern United States indicate that soil in the immediate vicinity of roots and fine root masses may show marked differences in physical characteristics, mineralogy and weathering compared to the bulk of the forest soil. Examination of rhizosphere and rhizoplane soils revealed that mineral grains within these zones are affected mechanically, chemically and mineralogically by the invading root bodies. In SEM/EDS analyses, phyllosilicate grains adjacent to roots commonly aligned with their long axis tangential to the root surface. Numerous mineral grains were also observed for which the edge abutting a root surface was significantly more fractured than the rest of the grain. Both the alignment and fracturing of mineral grains by growing roots may influence pedogenic processes within the rhizosphere by exposing more mineral surface to weathering in the root-zone microenvironment. Chemical interactions between roots and rhizosphere minerals included precipitation of amorphous aluminium oxides, opaline and amorphous silica, and calcium oxalate within the cells of mature roots and possible preferential dissolution of mineral grains adjacent to root bodies. Mineralogical analyses using X-ray diffraction (XRD) techniques indicated that kaolin minerals in some rhizosphere samples had a higher thermal stability than kaolin in the surrounding bulk forest soil. In addition, XRD analyses of clay minerals from one of the southeastern sites showed abundant muscovite in rhizoplane soil adhering to root surfaces whereas both muscovite and degraded mica were present in the immediately surrounding rhizosphere soil. This difference in mineral assemblages may be due to either K-enrichment in rhizoplane soil solutions or the preferential dissolution of biotite at the root-soil interface.

Properties of soils influenced by ectomycorrhizal fungi in hybrid spruce [Picea glauca × engelmannii (Moench.) Voss]

“…2), may indicate higher rates of mineral weathering in ECS compared to N-ECM soils. These findings are consistent with those of Arocena et al (1999) where amounts of mica and chlorite in ECS samples were significantly lower compared to N-ECM soils for subalpine fir, and they corroborate results from other studies (April and Keller 1990;Hinsinger et al 1991;Kodama et al 1994;Courchesne and Gobran 1997). In these four studies cited, bulk and rhizosphere materials were compared and soil minerals other than mica and chlorite (e.g., amphibole) were affected by increased weathering.…”

“…In the rhizosphere zones, both physical and chemical processes increase rates of mineral weathering. The pressures exerted by growing roots and associated fungal hyphae could mechanically alter minerals by causing realignment, bending, and fracturing (April and Keller 1990). In addition, Robert and Berthelin (1986) showed micrographs of hyphae probing between mica flakes to extract essential K + .…”

Biological impact on mineral dissolution: Application of the lichen model to understanding mineral weathering in the rhizosphere

“…The extensive transformation of biotite to vermiculite has been attributed to root-induced pH decreases in the rhizosphere and acid dissolution of the mica structure (104). In a similar experimental study (93), plant roots and ectomycorrhizal fungi actively produced oxalate and increased biotite weathering when they were stressed for K and Mg. Plant roots and associated microbial populations also physically disrupt sheet silicates, exposing new surface area for chemical alteration (107).…”

Legacy of Rice Roots as Encoded in Distinctive Microsites of Oxides, Silicates, and Organic Matter

“…Concerning the Al compounds, April and Keller [29] found that chemical interactions between roots and rhizosphere minerals included the precipitation of amorphous Al oxides and hydroxides within the cells of mature roots. Cloutier-Hurteau et al [30] revealed that Al-organic complexes were generally the dominant species in the rhizosphere of forest soils.…”