High-resolution elemental mapping of the root-rhizosphere-soil continuum using laser-induced breakdown spectroscopy (LIBS)

Abstract

Abstract Understanding the complex chemical nature of root-soil interactions is essential for building the next generation of sustainable agricultural systems and facilitating long-term environmental remediation strategies. Techniques currently suited to investigate spatial controls on nutrient exchange in plant rhizospheres, however, are hindered by limitations in throughput, cost, analytical scope, and sample preparation needs. We describe here a method for rapid, high-resolution (∼100\u202fμm), multi-element imaging of both organic content and inorganic constituents in root-rhizosphere-soil systems using laser-induced breakdown spectroscopy (LIBS). A switchgrass assemblage (Panicum virgatum) was grown in compact rhizotrons containing a sandy loam Alfisol from the Kellogg Biological Station (KBS), Michigan, USA. Root-soil samples were extracted using custom plastic coring devices for live root sampling and a modified drill press for sectioning frozen stabilized soil. A 266\u202fnm, Nd:YAG laser was rastered over ∼10\u202fmm2 sample surfaces and broadband spectra from single-pulse ablations were collected and mapped to discrete XY spatial coordinates for simultaneous imaging of 17 macronutrients, micronutrients and matrix elements. In order to rapidly process LIBS raster data and investigate chemical trends in the rhizosphere, an open-source Python module was developed, in which we used a novel calibration-free masking algorithm (based on principal components analysis (PCA) of normalized spectral intensities) to discriminate soil mineral grains, root fragments, and associated rhizosphere regions. We observed fine-scale chemical gradients within only a millimeter of switchgrass roots, consistent with rhizodeposition of organic compounds and proximal uptake of inorganic nutrients. Detection of trace element and carbon accumulations with diagnostic spectral signatures in the rhizosphere suggested the presence of residues (detritusphere) that could serve as sites of preferential microbial accumulation. These results highlight potential applications of LIBS within the growing field of spatially-resolved plant-soil analysis and extend its versatile imaging capabilities to the complex root-rhizosphere-soil network.