Geophysical image to root function

Blanchy, Guillaume and Binley, Andrew and Whalley, Richard and Hawkesford, Malcolm (2020) Geophysical image to root function. PhD thesis, Lancaster University.

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Abstract

Our agroecosystems are challenged by climate-induced stresses and the need to increase food production for a growing global population. Improving their resilience and sustainability are key challenges for tomorrow’s agriculture. Alternative agricultural practices (e.g. reduced tillage, cover crops, etc.) and selection of more robust crop varieties have the potential to help meet these challenges. To fully assess the effectiveness of such practices, an improved understanding of the soil-plant-water interactions is needed, however, such improvement is constrained by existing field-based measurement methods. In this work, we used a combination of time-lapse electrical resistivity tomography (ERT) and electromagnetic induction (EMI) to study soil moisture dynamics for a range of agricultural settings. In a study of cover crops, it was found that tap-rooted cover crops had larger impact on the soil drying than shallow-rooted ones. Additionally, the effect on soil drying of long-term cover crops (grown over two seasons) was larger than that over one season. However, in both cases the effect of the cover crops on the soil drying quickly vanished after their destruction. Soil compaction is another issue that might impact crop water availability. In this work, time-lapse ERT measurements enabled the imaging of the restricted drying depth of traffic-induced compacted plots compared to non-compacted ones under potatoes. The impact of tillage and nitrogen levels was also investigated using time-lapse EMI surveys under winter wheat. It was found that nitrogen levels only had an ephemeral effect on the change in electrical conductivity (EC) measured independently of the tillage treatment. Also direct drilled plots showed a smaller drying compared to mouldboard ploughed plots over the season. Aside from different agricultural practices, efforts have also been undergoing to breed varieties of crops more resilient to water stresses. Part of this resilience lies with the root system of the crop and its capacity to extract soil moisture. However, acquiring information on the variety traits (phenotypes) in a field-scale setup is one of the major bottlenecks of crop breeding, especially for below-ground traits. Time-lapse geophysical methods have been successfully used to discriminate soil drying between different winter wheat varieties in large field-scale trials. However, the results here show that the discriminating power of this approach can, under some conditions, be hindered by local variation of the pedophysical relationship used to convert change in EC to change in soil moisture. This study shows that this can have important impacts on the ranking of the varieties; alternative models and experimental procedure are proposed to further account for this heterogeneity. Finally, the emergence of automated field phenotyping platforms offers a unique means for crop breeders to screen large number of varieties in a controlled field environment. These platforms can provide a wealth of above-ground measurements, but they usually lack below-ground data. In this work, time intensive geoelectrical monitoring was performed on four plots of winter wheat with two different nitrogen levels in a field phenotyping platform. The measured seasonal dynamics of the soil EC appear to be related to the evolution of above-ground variables but are also impacted by the different nitrogen levels applied. Large decreases in the soil EC observed after large rainfall events are associated with both crop nitrogen and soil moisture uptake. At the hourly scale, time series analysis enables the identification of diurnal patterns that could be linked to root water uptake. Overall, time-lapse geophysical monitoring has proven to be a useful tool to monitor root zone processes with minimal invasiveness. The work, however, also demonstrates the limitations of the approach; a range of perspectives for future improvement are discussed. While not sufficient on their own, geophysical methods remain a useful tool for the emerging field of agrogeophysics and can provide valuable insights for shaping future agricultural practices.