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Visualization and quantification of root exudation using 14C imaging: challenges and uncertainties

Holz, Maire, Zarebanadkouki, Mohsen, Carminati, Andrea, Kuzyakov, Yakov
Plant and soil 2019 v.437 no.1-2 pp. 473-485
air, autoradiography, barley, carbon, energy, exudation, image analysis, porosity, radionuclides, rhizosphere, root exudates, roots, soil, soil microorganisms, uncertainty
BACKGROUND AND AIMS: Root exudation is an important carbon (C) and energy source for soil microorganisms but quantifying its spatial distribution is challenging. We tested whether ¹⁴C imaging (analogue of previous autoradiography) can be used to quantitatively estimate the spatial distribution of root exudates in the rhizosphere. METHODS: First, the attenuation coefficient of ¹⁴C β⁻ rays in soil and in water was measured and expected gradients of ¹⁴C in the rhizosphere were modelled. Secondly, barley plants were pulse labelled in ¹⁴CO₂ atmosphere and the origin (roots or root exudation) and locations of ¹⁴C signal in soil were detected with imaging. RESULTS: The attenuation coefficient of ¹⁴C was 148 cm⁻¹ for soil and 67 cm⁻¹ for water, corresponding to a maximum distance that ¹⁴C β⁻ rays pass through a dry soil of 0.37 mm. Based on the measured coefficients we calculated the effect of exudation intensity, root radius and root position on the imaged ¹⁴C signal. The distribution of the imaged signal was strongly affected by: a) ¹⁴C activity in the root, b) root radius, c) distance from the root surface to the imaging screen, d) amount of root exudates in the soil, and e) presence of an air gap (or a region with high porosity) between the soil and the imaging screen. CONCLUSIONS: Neglecting the effects of these factors (a-e) may cause biases in the estimation of root exudates using ¹⁴C imaging of the rhizosphere. The ¹⁴C imaging approach should therefore be accompanied by accurate measurement of these factors and calculation of the β⁻ ray transmission through the soil.