Truly Predicting Root Uptake of Water: Case Study with Wheat

We heavily rely on soil to support the crops on which we depend. Less obviously we also rely on soil for a host of 'free services' from which we benefit. For example, soil buffers the hydrological system greatly reducing the risk of flooding after heavy rain; soil contains very large quantities of carbon which would otherwise be released into the atmosphere where it would contribute to climate change. Given its importance it is not surprising that soil, especially its interaction with plant roots, has been extensively researched. However the complex and opaque nature of soil has always made it a difficult medium to study. Soil is complex in that it is composed of different materials (mineral particles, organic matter, water, microrganisms) of all shapes and sizes (from centimetres to microns) which aggregate together to form a complex porous material. While the function of soil is determined by the processes taking place at the micro-scale (often called pore scale), within this complex material we have traditionally only been able to measure and observe soil function at the larger, macro-scale (usually referred to as the field scale). We can manipulate soil systems at the macro-scale and empirically observe what occurs, and this empirical description is useful, but it offers no scope to truly predict how the system would respond to modification. This is important because we have the potential and most likely the future need to manipulate the underlying processes at the microscale (in both plants and soil). For example we will need to know: should our crops root deeper? Would a change in root architecture be useful? To what extent can roots adapt to stresses in the soil physical environment? What management induced changes to soil structure are desirable for future environments? Evaluating such possibilities at the field scale currently requires case by case empirical investigation with little direction offered by any underlying theory; this is a huge gap in current knowledge. Even if good theories existed to explain soil-root interactions at the micro-scale, it is not clear how this could be applied to the field scale. Understanding and manipulating the system at the scale of <1mm is all very well, but we want to make a difference at the scale of >10 kms! We need to be able to 'scale up' our micro-knowledge to a scale that is useful. Progress can be made to address the microscale understanding of soil-root interactions, however this progress will only be of real importance if we also find ways to scale up to the field situation. This is also a huge gap in knowledge. These knowledge gaps can now be addressed as a result of two recent methodological developments. Firstly new experimental techniques based on X-ray Computed Tomography (CT) are making it easier to visualise and quantify soil and root micro-structure in a non-invasive manner. Secondly, mathematical homogenisation theory offers new ways to correctly scale up micro-scale processes to macro-scale models thereby addressing the scale problem. Integrating these two new methods for the first time we will consider the specific question of water movement in soils and its uptake by wheat, an important crop for UK agriculture. We will undertake experiments to measure the micro-structure of soils and investigate how water passes through these soils to the roots of plants. Our aim will be to use this information to develop and test theoretical models of water movement and uptake and use these to evaluate the performance of different wheat root architectures. We will do this in a way that is specifically designed to enable us to 'scale up' the results so we can make predictions at the field scale, based on the observable micro-scopic characteristics of soil. Thus, because of the generic methodology produced within this project the results are not only applicable for wheat, but for wide range of agricultural crops.

Recent work by our team and others has found that the multiscale nature of soils, especially soil micropores, can strongly influence how plant roots function in soil. This in turn has led us to develop dual porosity models for nutrient diffusion in the soil. In this project we will investigate how the dual porosity effects influence water movement in soil and how this in turn influences the crop, in particular wheat, water uptake. Our general approach will be to start from the simplest element in our system, a soil particle/aggregate, and work our way up in three steps by considering the single root scale and finally the soil profile/crop scale. On the smallest scale we will use X-ray CT to image the internal 3-D pore structure and water content of soil aggregates. At this scale we will import observed soil surfaces into Comsol to solve the mechanistically based Stokes equations for fluid flow enabling us to determine the Darcian permeability and soil water suction characteristic on the basis of the observed micro-structure over a range of different sizes of 'unit cell'. Through mathematical homogenization we will select the appropriate size of unit cell so that these micro-scale results can be applied in the classical macro-scale model of water flow in soil (i.e. the Richards equation). Using the imaging of a single root scale we will address issues of root-soil contact via the water film. Previously this has only been addressed theoretically with idealised soil structures. Using imaging and modeling at the single root scale we will answer questions about the functionality of different root branches for water uptake. Finally we will model and measure water uptake by the full plant root architecture allowing a comparison of the behaviour of different wheat root architectures. We will make the first simultaneous measurements of root architecture and water uptake by living plants at scales relevant to the field situation.

WHO WILL BENEFIT AND HOW? A direct target group is the international agricultural research and training sector concerned with improving the well-being of farmers and consumers by enhancing the quantity and quality of food crops. A key priority is to breed crops for changing climate conditions, in particular, for conditions of severe drought (Africa, Asia etc), but also for areas of severe flooding (Asia, Europe etc). A further requirement is to substantially increase yields with some suggestions indicating by as much as 40% in the next 20 years. Our research will directly benefit crop breeders in guiding which plant root system traits are important for specific soil moisture conditions. Similarly the work will have impacts on the understanding of how soil structure interacts with crop performance which will ultimately be relevant to those involved in management/tillage of soil (i.e. farmers) and those involved in the formulation of agricultural policy and guidance. A second group of beneficiaries are agricultural and environmental policy makers concerned with the fate of pollutants and fertilizers in the environments. Flooding can cause serious eutrophication and understanding the role of plants in this process can enable us to choose crops/plants with special traits that enable the reduction of eutrophication and increase bioremediation. In the longer term our research will help refine methodologies that address persistent flooding and crop growth in extreme environments (very dry and very wet) through appropriate management of soil structure. A third group of beneficiaries are the international development community who can use the results of our research to guide the policy recommendations to areas likely to undergo significant environmental climate change, again with respect to the appropriate management of soil. COMMUNICATION AND ENGAGEMENT PLAN In addition to standard peer-reviewed publications we will also use the web and newsletters as a main means to communicate with the above stakeholders. We will produce reports designed for more general readers distributed through the partner Universities Media Relations Office, which publicizes the results of the research directly to practitioners in industry (especially the water and insurance sectors), and policymakers (especially Defra and the Environment Agency). PI Tiina Roose is a member of technology transfer committee for Environmental Knowledge Transfer Network (KTN) and thus we will disseminate the results of this research via this network to water and waste industry. Further we propose to arrange a workshop (organised by the British Soil Physics Group of which Sacha Mooney is the Chair) for scientists at the end of the project, in which we will disseminate the applications of multiscale studies of soil imaging and modelling studies and how the results would be translated into practical crop growth situation. Since the mathematical modelling and imaging techniques developed during this project are in some sense generic we will invite representatives of other science domains, such as biomedical modelling and imaging community, to attend the workshop.