Continuum and particle scale analysis of nutrient gradients in the rhizosphere

Plant roots create chemical gradients within the soil rhizosphere but little information exists on the effect of root types and ages on the distribution of chemical gradients. Research aim was to develop an imaging workflow and to analyze and model the effects of radial root geometry, root hairs, and ages on nutrient gradients around roots.

The presented correlative imaging workflow is suitable for targeted sampling of roots in their 3D context and assessing the imprint of roots on chemical properties of the root-soil contact zone at µm to mm scale. Maize (Zea mays) was grown in ¹⁵N-labelled soil columns and pulse-labelled with ¹³CO₂ to visualize the spatial distribution of carbon inputs and nitrogen uptake together with the redistribution of other elements. Soil columns were scanned by X-ray computed tomography (X-ray CT) at low resolution (45 µm) to enable image-guided subsampling of specific root segments. Resin embedded subsamples were then analysed by X-ray CT at high resolution (10 µm) for their 3D structure and chemical gradients around roots using micro X-ray fluorescence spectroscopy (µXRF), nanoscale secondary ion mass spectrometry (NanoSIMS), and laser-ablation isotope ratio mass spectrometry (LA-IRMS). NanoSIMS and LA-IRMS detected the release of ¹³C into soil up to a distance of 100 µm from the root surface, whereas ¹⁵N accumulated preferentially in the root cells.

Concentration gradients with different spatial extents could be identified by µXRF. The observed concentration gradients were compared to simulated gradients generated by a process-based, radially symmetric 1D rhizosphere model. An accumulation of calcium and sulfur was observed, particularly around old root segments. Our model simulations indicated that this phenomenon originates from the radial structure of the root, leading to enhanced nutrient transport towards the root surface. Gradients of calcium and sulfur could be accurately predicted by the model around a single growing root, when they were mainly caused by sorption.

However, at the pore-scale, phenomena like local precipitation, which could be visualized using our methodology, were inadequately accounted for by the classic model approach. Nonetheless, the observed extension of the gradients was well described by the model. The presented approach combining targeted sampling of the soil-root system and correlative microscopy opens new avenues for unravelling rhizosphere processes in situ.