Truly Predicting Root Uptake of Water: Case Study with Wheat

我们严重依赖土壤来养活我们赖以生存的农作物。不那么明显的是，我们还依赖土壤来提供大量我们从中受益的“免费服务”。例如，土壤缓冲了水文系统，大大降低了暴雨后洪水的风险；土壤含有非常大量的碳，否则就会释放到大气中，在那里它会导致气候变化。鉴于它的重要性，土壤，特别是它与植物根的相互作用，得到了广泛的研究，这并不令人惊讶。然而，土壤的复杂和不透明的性质一直使其成为难以研究的媒介。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，它们聚集在一起形成复杂的多孔材料。虽然土壤的功能是由微观尺度(通常称为孔隙尺度)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(通常称为田间尺度)上测量和观察土壤功能。我们可以在宏观尺度上操纵土壤系统，并从经验上观察发生了什么，这种经验描述是有用的，但它没有提供真正预测系统将如何应对修改的范围。这一点很重要，因为我们有潜力，而且很可能未来需要在微观尺度上操纵潜在的过程(在植物和土壤中)。例如，我们需要知道：我们的作物是否应该扎根更深？更改根架构是否有用？根能在多大程度上适应土壤物理环境中的压力？对于未来的环境来说，哪些管理措施会导致土壤结构的变化是可取的？目前在实地范围内评估这种可能性需要逐个案例的实证调查，几乎没有任何基本理论提供指导；这是当前知识的一个巨大缺口。即使已经存在了在微观尺度上解释土壤-根系相互作用的良好理论，但如何将其应用于田间尺度尚不清楚。在&lt；1 mm的尺度上理解和操纵系统是很好的，但我们想要在&gt；10公里的尺度上有所不同！我们需要能够将我们的微观知识“放大”到一个有用的规模。在解决对土壤-根系相互作用的微观理解方面可以取得进展，然而，只有当我们也找到扩大到田间情况的方法时，这一进展才具有真正的重要性。这也是一个巨大的知识鸿沟。由于最近的两项方法学发展，这些知识差距现在可以得到弥补。首先，基于X射线计算机层析成像(CT)的新的实验技术使以非侵入性方式可视化和量化土壤和根的微结构变得更容易。其次，数学均匀化理论提供了正确地将微观过程放大到宏观模型的新方法，从而解决了尺度问题。首次将这两种新方法结合起来，我们将考虑土壤中水分运动的具体问题，以及小麦对水分的吸收，小麦是英国农业的重要作物。我们将进行实验，测量土壤的微观结构，并研究水分如何通过这些土壤到达植物的根部。我们的目标将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，并使用这些模型来评估不同小麦根构型的性能。我们将以一种专门设计的方式来实现这一点，使我们能够“放大”结果，这样我们就可以根据土壤的可观察到的微观特征，在田野尺度上进行预测。因此，由于该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.