Imaging the interaction of roots and phosphate fertiliser granules using 4D X-ray tomography

Aims: Plant root system architecture adapts to the prevailing soil environment and the distribution of nutrients. Many species respond to localised regions of high nutrient supply, found in the vicinity of fertiliser granules, by elevating branching density in these areas. However, observation of these adaptations is frequently limited to plants cultured in idealised materials (e.g., hydrogels) which have a structure-less, homogenous matrix, which are spatially limited and in the case of rhizotron observation provide only 2D data that are not fully quantitative. Methods: In this study, in vivo, time resolved, micro-focus X-ray CT imaging (μCT) in 3D was used to visualise, quantify and assess root/fertiliser interactions of wheat plants in an agricultural soil during the entire plant life cycle. Two contrasting fertilisers [Triple superphosphate (TSP) and struvite (Crystal Green®)] were applied according to 3 different treatments, each providing an equivalent of 80 kg P₂O₅ ha⁻¹ (struvite only, TSP only and a 50:50 mixture) to each plant. μCT scans (60 μm spatial resolution) of the plant roots were obtained over 14 weeks. Results: This is the first time that in situ root/soil/fertiliser interactions have been visualised in 3D from plant germination through to maturity. Results show that lateral roots tend to pass within a few millimetres of the phosphorus (P) source. At this length scale, roots are able to access the P diffusing from the granule. Conclusions: Quantitative analysis of root/fertiliser interactions has shown that rooting density correlates with granule volume-loss for a slow release, struvite fertiliser.