Potential errors caused by roots in analyses of rhizosphere soil

Abstract

Roots contain high concentrations of many elements, and have the potential to interfere with measurements of chemical change in rhizosphere soil. To assess potential interferences, maize (Zea mays L.) roots (flee of soil) and soil (free of roots) were extracted separately with several extractants commonly used to assess the status of soil nutrients. The maize roots were grown within filter envelopes which prevented direct contact with soil, but permitted passage of mineral nutrients and water from the adjacent soil, Water, ammonium acetate (pH7), DTPA (pH7.3), Morgan's solution (pH4.8), and dilute HCI were used as extractants. Most elements were released readily into soluble forms from roots killed by freezing to lyse the cells. Significantly lower amounts were extracted from flesh roots, with the greatest differences between fresh and killed roots for the extractants H20 and DTPA, which were the mildest in terms of acidity and salt concentration. Extraction of P from the fresh roots by H20 and HCI was particularly low. Contamination of rhizosphere samples with root materials would almost certainly prevent the accurate measurement of water-soluble P, K, Mn, Zn, Cu, and Na in the slightly alkaline soil used in this experiment. Large errors would be likely also for P, Mn, and Cu extracted by ammonium acetate. The DTPA extractant is normally used only for micronutrient metals or heavy metals, and the small amounts of these elements released by roots should not contribute to significant error. With Morgan's solution, errors would likely be large only for P. Dilute HCI is a reasonably strong extractant for many elements in soil, and major errors from roots contained in rhizosphere samples are unlikely. The relatively high probability of errors in extractions of soluble elements from rhizosphere soil is unfortunate, because these elements are among the most readily available to plants and the most likely to be altered by the normal activities of roots.

The Effects of AlCl3Additions on Rhizosphere Soil and Fine Root Chemistry of Sugar Maple (Acer Saccharum)

“…Rhizosphere and bulk soil samples were air-dried and sieved through a 1 mm screen to minimize root contamination in rhizosphere samples (Norvell and Cary, 1992). Exchangeable Ca, Mg and Al were extracted with 1 N KCl (10:1 solution:soil), by shaking samples for 1 h, centrifuging at 4000 rpm for 20 min and filtering through a 0.2 µm polycarbonate membrane syringe filter (Bickelhaupt and White, 1982).…”

“…In addition, element ratios could be quite sensitive to the amount of soil collected because concentration gradients vary among elements (Marschner, 1995). Finally, root contamination of the rhizosphere sample is an inevitable cost of mechanically separating soil from roots and may influence the exchange chemistry of rhizosphere soil (Norvell and Cary, 1992).…”

Correlative Imaging of the Rhizosphere─A Multimethod Workflow for Targeted Mapping of Chemical Gradients

“…5−8 Furthermore, destructive rhizosphere samples can be contaminated with root cells, i.e., root hairs being also brushed off. 9 Current knowledge with respect to chemical gradients in rhizosphere soil has primarily been based on systems not considering the radial geometry of transport to and from roots such as rhizobox or split-compartment experiments. Not accounting for this geometry in planar experimental setups leads to an amplification of the extent and magnitude of gradients.…”

“…Most of our knowledge on rhizosphere properties is based on operationally defined ways of sampling the rhizosphere, such as brushing, shaking, or washing off soil adhering to the roots after extracting them from bulk soil. These approaches do not refer to a certain distance from the root surface, although nutrient gradients are reported to extend over less than 1 mm up to several centimeters. − Furthermore, destructive rhizosphere samples can be contaminated with root cells, i.e., root hairs being also brushed off . Current knowledge with respect to chemical gradients in rhizosphere soil has primarily been based on systems not considering the radial geometry of transport to and from roots such as rhizobox or split-compartment experiments.…”

Characteristics of rhizosphere soil from natural and agricultural environments

“…This difficulty in collecting the ectorhizosphere has resulted in the devising of a number of procedures to solve the problems they have encountered. All the procedures are able to concentrate the ectorhizosphere over the bulk soil, but they can also introduce experimental artefacts that may lead to erroneous considerations (Norvel and Cary, 1992;Wenzel et al, 2001). These problems evidently make it difficult to compare the results obtained by different researchers.…”