Notes: Red Spruce Rhizosphere Dynamics: Spatial Distribution of Aluminum and Zinc in the Near-Root Soil Zone

Publisher： Society of American Foresters

ISSN： 0015-749X

Source： Forest Science, Vol.35, Iss.4, 1989-12, pp. : 1114-1124

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Abstract

Red spruce roots, associated with mature, healthy trees occupying canopy positions, were located in three 8.5 m² forest floor plots at 250 m elevation in the Hubbard Brook Experimental Forest, White Mountain National Forest, New Hampshire. Roots, 0.25-1.0 cm diam, were severed, and cut ends were placed in soil in plastic bags. In 14 months, new roots developed behind cut ends and were cultured in wooden trays containing screened forest floor material. Forest floor concentrations of aluminum, calcium, chlorine, iron, lead, and zinc were established. Trays containing roots were supplied with a distilled water treatment or with distilled water containing chloride salts of aluminum, lead or zinc sufficient to increase substrate cation concentration by 500 ppm. Eight weeks following treatment, roots (largely ectomycorrhizal, diam 0.4-1.0 mm) with associated rhizosphere soil were harvested by impregnating specimens in agar and freezing in liquid nitrogen. Cross sections of root-rhizosphere soil samples were freeze dried and examined with a scanning electron microscope and energy dispersive x-ray spectrometer. Gradients of element concentrations were established along transects from root interiors (cortex) through the inner and outer rhizosphere zones (2 mm from root) and in bulk soil (devoid of roots). The pH of rhizosphere soil was approximately 0.5 unit less than bulk soil. Aluminum concentrations exhibited a strongly descending gradient from bulk soil through the rhizosphere to the root. Estimated concentrations ranged from 1000 ppm within 200μ of the root to 10 X this amount in soil beyond 2000μ. Calcium distribution, without aluminum amendment, was relatively constant through the rhizosphere, but with aluminum amendment, calcium exhibited a sharply decreasing gradient near the root. In the unamended treatment, the Al:Ca ratio dropped from 7 to 0.2 across the rhizosphere. Chlorine was constant in the rhizosphere but exhibited pronounced accumulation inside the root. Iron counts were variable in the rhizosphere, but on average suggested constant concentrations across the rhizosphere. Zinc concentrations were marginally detectable and exhibited slight accumulation in cortex tissue. Aluminum uptake appeared to be strongly mediated by rhizosphere processes. Solubility, and more importantly exchangeability, of aluminum may be enhanced in the rhizosphere by lowered pH; greater availability of low molecular weight organic ligands, or some other unique chemical characteristic of the rhizosphere. Judgments made concerning potential uptake of aluminum from bulk soil characteristics of forest soils, without consideration of rhizosphere dynamics, could lead to improper conclusions. For. Sci. 35(4):1114-1124.