Labelling plants in the Chernobyl way: A new Cs-137 and C-14 foliar application approach to investigate rhizodeposition and biopore reuse

Biopores as microbial hotspots provide additional nutrients to crops - but only if their roots grow within the biopores. Such reuse has never been quantified as pre-crop-specific biopores are hardly differentiated from the multitude of pre-existing biopores. Quantification requires e.g. radionuclide labelling of pre-crops (Cs-137, to label their biopores) and main crops (C-14, to detect new roots). Preliminary testing was performed on simulated biopore reuse: both nuclides given to the same plant were excreted into the same rhizosphere. Cichorium intybus (cv. Puna) and Medicago sativa (cv. Planet) were each sequentially labelled via the leaves with Cs-137 and (CO2)-C-14. beta-signals were visualised by imaging of horizontal soil cuts - with and without shielding off the weaker C-14. Both species allocated 7.1-9.4% of the Cs-137 and 21-63% of the C-14 below ground. The first image gave both activities; while the second gave only Cs-137. Subtracting the second from the first image gave the C-14 distribution, resulting in successful separation of the signals. Thus, separate spatial representations of the roots were obtained. Main root locations by Cs-137 and C-14 showed a very high spatial overlap coefficient (> 0.95). Biopore reuse quantification likely becomes feasible with this sequential labelling and shielding approach.