Chemical imaging of phosphorus dynamics in the rhizosphere

Phosphorus uptake by plants is a key factor controlling crop productivity. The bioavailability of P in soils is generally low, as is the P use efficiency of mineral fertilisers by plants. Detailed understanding of plant P uptake is therefore crucial to increase P use efficiency by crops eg. through plant breeding programmes. Many processes that contribute to P cycling in the rhizosphere and thus plant P uptake have been identified. Roots actively increase the bioavailable fraction of soil P by proton and carboxylate anion (eg. citrate) exudation. Furthermore, organic P compounds can get mineralised in the rhizosphere and the phosphate liberated can be taken up by plants. Although much information on rhizosphere processes contributing to plant P nutrition has been gathered, the exact localisation of these processes along the root axis and their spatial extent into rhizosphere soil are less well understood. Difficulties in sampling include the necessity of taking samples at small spatial scales (preferably c.100 m) and very small sample volumes. This is further complicated if the dissolved fraction of P is measured as its concentration in the rhizosphere is generally in the low mol L-1 range. In a previous study doing 2D chemical imaging of the dissolved P concentration in the Brassica napus rhizosphere we could for the first time demonstrate the potential of in situ chemical imaging for gaining novel insights in rhizosphere P cycling at high spatial resolution. Based on this preliminary work, we plan to develop a set of two-dimensional chemical imaging techniques capable of measuring major parameters controlling the P cycling in the rhizosphere. Diffusive gradients in thin films (DGT) techniques for chemical imaging of P, Fe, Al and Ca in the rhizosphere will be developed. P, Fe, Al and Ca will be analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Furthermore a planar optode method will be used for simultaneously mapping the rhizosphere pH. These novel methods will be extensively tested. The new set of methods will be applied in this project to (1) investigate the differences in the localisation of rhizosphere P dynamics between B. napus cultivars of differential P uptake efficiencies and (2) to study the localised changes in the P solubility in the rhizosphere of B. napus in calcareous and noncalcareous soils after rhizosphere acidification and alkalinisation. The methods developed in this project will provide a powerful and versatile set of techniques for chemical imaging in the rhizosphere. As the range of analytes for both, DGT and planar optodes can be increased, this project will also set the base for chemical imaging of other important nutrients and contaminants in the rhizosphere and in soils in general. Wer also anticipate the results of this project to enhance the current knowledge of P cycling in the rhizosphere. More detailed understanding of P aquisition by plant roots will help in the formulation of mathematical models for plant P uptake. In addition, the results of the project may serve to enhance crop breeding and selection by including parameters of root/rhizosphere traits of P use efficiency and thus contribute to reduce the need for P fertilization in view of the world`s limited P fertiliser stocks.

The main outcomes of this research project were the development and detailed characterization of a versatile methodology to measure and visualize the distribution of plant-available nutrients, contaminants and major controlling soil-chemical parameters (pH, O2) at a very fine (sub-mm) spatial scale. Our work enables us and other researchers to use this set of methods for investigating the micro-environment around plant roots in soil and marine environments, where small-scale biogeochemical variations are of high importance, at unprecedented spatial resolution. During the project we applied these chemical imaging techniques for enhancing our understanding of the plant-availability of the major, and often limiting, plant nutrient phosphorus, primarily in crop production systems. We could show that active solubilisation of phosphorus in the root zone of important crops (wheat, buckwheat, lupine) is often located near root tips. Moreover we were able the clearly differentiate phosphorus fertilizer materials based on the mobility of the P released. In an ongoing study on the phosphorus acquisition by marine seagrass roots we could demonstrate the role of active P solubilisation also in this environment. Other applications of chemical imaging within this project included the localization of contaminant (Zn, As, Cd, Pb) solubilisation and uptake processes by plants which are utilized for soil remediation. The methodology developed in this project is a set of powerful tools for understanding the interaction of plant roots, and potentially also other highly active (biological) hotspots, with their surrounding soil or sediment environment. Therefore we expect increasing interest in the application of chemical imaging in environmental research in future.