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

© 2019, Springer Nature Switzerland AG. 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