The rhizosphere is the volume of soil around living plant roots which is influenced by root activity. Depending on which activity one considers, the outer boundary of the rhizosphere can vary from micrometric to decimetric distance from the root surface.
Terminology and equations
Although rhizosphere is defined at first glance as a volume of soil, there is no consensus on its dimension and on the units to be used. These also fairly much depend on the method used to measure the size of the rhizosphere. As explained below, most of these methods are based on 2-D or 1-D approaches and thus the size of the rhizosphere is most often expressed as its width or distance from the root surface (D, m). Its actual volume (V, m3) can then be calculated assuming cylindrical geometry and knowing the root radius (Rroot, m) and the total root length (Lroot, m): V = ℼ[(D+Rroot)2 – Rroot2] Lroot. Reciprocally, the width of the rhizosphere can be deduced from the total volume of rhizosphere, when accounting for the corresponding (usually measured) mass of soil (M, kg) and its volumetric mass (ρ, kg m-3): D = √[Rroot2 + M/(ℼ ρ Lroot)] – Rroot.
The rhizosphere has an ill-defined outer boundary. It is frequently sampled in field-grown plants, which provides access to a direct measurement of the corresponding mass of soil, and hence volume of the rhizosphere, with quite an arbitrary choice of the outer boundary. Alternatively, the use of probes that provide access to a direct measurement of the gradient of the soil property of interest enables one to directly measure the width of the rhizosphere for a given root activity. The direct measurement of the width of the rhizosphere can be facilitated by using specially designed microcosms, often referred to as rhizoboxes, which simplify the 3-D geometry of the rhizosphere, by e.g. using a 2-D root mat. The choice of method will depend on the target root activity of rhizosphere effect one considers.
In situ methods of sampling
Basically, roots and adhering soil are sampled in field-grown plants, after removal (by shaking) of the bulk soil (loosely adhering to the roots). The size of the largest particles strongly adhering to the roots and thus defined as rhizosphere is arbitrarily set (usually millimetric size), and depends on the water content and particle size of the soil, as well as abundance/length of root hairs and mucilage. These particles are brushed off the roots, but sometimes require the use of water to be fully recovered. Mass of adhering soil, M, is then measured, and volume of rhizosphere, V, deduced. The measurement of total length, Lroot, and average radius, Rroot, of corresponding roots provides access to calculation of the average width of the rhizosphere, D.
In situ probe-based methods
Different types of techniques and probes, both invasive and non-invasive, can be used to measure the gradient of a given soil physical or chemical property as a function of the distance to the root surface, thereby providing direct access to the width of the rhizosphere, D (i.e., distance beyond which the property is not significantlty altered relative to the bulk soil). Non-invasive probes are e.g. microelectrodes or optodes that can be used to measure pH or pO2 gradients, with a millimetric spatial resolution. Invasive techniques are soil solution samplers such as microsuction cups or rhizons. These techniques are often combined with observation plates or rhizotrons to position the probes close to growing, active roots, with potential artefacts due to this interface.
Root mat rhizobox methods
Most published values on the width of the rhizosphere have been obtained from purpose-built microcosms, rhizobox and alikes, in which the geometry of the rhizosphere is simplified. Roots are grown along a mesh (with pore diameter finer than the roots, commonly around 30 μm) in order to form a 2-D mat of roots. The soil is then sliced parallel to the root mat and each slice analysed with conventional soil testing methods. Millimetric to centimetric-wide gradients can easily be measured, while smaller spatial resolution (down to 10-30 μm) can be achieved by using a refrigerated microtome to collect the rhizosphere samples.
Ranges of values
Rhizosphere width (distance from root surface), D: m
Gradients of poorly mobile nutrients (P or micronutrients) or exudates : <0.001-0.005
Gradients of more mobile nutrients (N and K) or exudates : 0.005-0.05
Gradients of water or volatile compounds : 0.01->0.10
Health, safety and hazardous waste disposal considerations
Care in handling cutting or razor blade/microtome for slicing the soil
Blossfeld S. & Gansert D. (2007) A novel non-invasive optical method for quantitative visualization of pH dynamics in the rhizosphere of plants. Plant, Cell and Environment, 30,176-186.
Hinsinger P., Gobran G.R., Gregory P.J. & Wenzel W.W. (2005) Rhizosphere geometry and heterogeneity arising from root-mediated physical and chemical processes. New Phytologist, 168, 293-303.
Luster J., Göttlein A., Nowack B. & Sarret G. (2009) Sampling, defining, characterising and modeling the rhizosphere The soil science tool box. Plant and Soil, 321, 457-482.
Wenzel W.W., Wieshammer G., Fitz W.J. & Puschenreiter M. (2001) Novel rhizobox design to assess rhizosphere characteristics at high spatial resolution. Plant and Soil 237, 37-45.