Truly Predicting Root Uptake of Water: Case Study with Wheat

我们严重依赖土壤来支持我们赖以生存的作物。不太明显的是，我们也依赖土壤提供大量“免费服务”，从中受益。例如，土壤缓冲了水文系统，大大降低了暴雨后发生洪水的风险；土壤中含有大量的碳，否则这些碳会被释放到大气中，从而导致气候变化。考虑到它的重要性，土壤，特别是它与植物根系的相互作用，被广泛研究就不足为奇了。然而，土壤的复杂和不透明的性质一直使其成为一种难以研究的介质。土壤是复杂的，因为它是由各种形状和大小（从厘米到微米）的不同材料（矿物颗粒、有机物、水、微生物）组成的，这些材料聚集在一起形成复杂的多孔材料。虽然土壤的功能是由在微观尺度（通常称为孔隙尺度）上发生的过程决定的，但在这种复杂的材料中，我们传统上只能在更大的宏观尺度（通常称为场尺度）上测量和观察土壤的功能。我们可以在宏观尺度上操纵土壤系统，并通过经验观察发生了什么，这种经验描述是有用的，但它没有提供真正预测系统如何响应修改的范围。这很重要，因为我们有潜力，而且未来很可能需要在微观尺度上（在植物和土壤中）操纵潜在的过程。例如，我们需要知道：我们的作物是否应该扎根更深？根架构的改变有用吗？根系在多大程度上能适应土壤物理环境的胁迫？什么样的管理引起的土壤结构变化对未来的环境是可取的？目前，在实地规模上评估这种可能性需要逐案的实证调查，任何基础理论都没有提供多少指导；这是当前知识的一个巨大差距。即使存在很好的理论来解释微观尺度上土壤-根的相互作用，也不清楚如何将其应用于田间尺度。在小于1毫米的尺度上理解和操作系统是很好的，但我们想在bbb10公里的尺度上有所作为！我们需要能够将我们的微观知识“放大”到有用的规模。可以在解决对土壤-根相互作用的微观理解方面取得进展，但是，只有当我们也找到扩大到实地情况的方法时，这一进展才会真正重要。这也是一个巨大的知识缺口。由于最近两种方法的发展，这些知识差距现在可以得到解决。首先，基于x射线计算机断层扫描（CT）的新实验技术使得以非侵入方式可视化和量化土壤和根的微观结构变得更加容易。其次，数学均匀化理论为正确地将微观尺度过程扩展到宏观尺度模型提供了新的方法，从而解决了尺度问题。首次整合这两种新方法，我们将考虑土壤中的水分运动及其对小麦的吸收的具体问题，小麦是英国农业的重要作物。我们将进行实验来测量土壤的微观结构，并研究水如何通过这些土壤到达植物的根部。我们的目标是利用这些信息来开发和测试水分运动和吸收的理论模型，并用这些模型来评估不同小麦根系结构的表现。我们将以一种特别设计的方式来做这件事，使我们能够“扩大”结果，这样我们就可以根据可观察到的土壤微观特征在现场尺度上做出预测。因此，由于本项目产生的通用方法，结果不仅适用于小麦，而且适用于广泛的农作物。

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.