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Rickard, William (2025) Deciphering soil structure: linking soil physics, water dynamics, carbon storage, and agricultural resilience in long-term experiments. PhD thesis, University of Nottingham.

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Abstract

Background and Objectives: Soil structure is a key component of soil health, governing processes like water retention, gas exchange, root growth, and carbon storage. However, quantifying its impact on long-term agricultural productivity and ecosystem functions remains challenging. This thesis addresses that gap by leveraging long-term field experiments to explore how soil structure influences (i) crop yield resilience to drought, (ii) soil organic carbon (SOC) dynamics, and (iii) the development of improved measurement techniques for soil structural properties. The overarching aim was to advance our understanding of soil structure and functional relationships in agricultural soils. Methods: Four complementary studies were undertaken. (1) Yield resilience under drought was analysed using decades of data from the Broadbalk Wheat Experiment (Rothamsted, UK), where treatments with varying fertiliser and organic inputs have created differences in soil properties. We related soil physical indices, notably the saturated hydraulic conductivity (Kₛₐₜ) and air-entry value (α) derived from soil water retention curves, to wheat yield performance in identified drought years. (2) Soil structure and SOC were investigated at the North Wyke farm platform (UK) by comparing permanent grassland, improved grass, and arable plots maintained over long periods. We measured changes in SOC stocks alongside soil water release characteristics and performed high-resolution X-ray Computed Tomography (CT) scans of soil aggregates, using image analysis and modelling to quantify pore network connectivity and substrate transport capacity. (3) To improve structural assessment, we developed a novel multi-tension infiltration method based on the Green–Ampt infiltration model. This technique was deployed in the Highfield Ley-Arable Experiment (Rothamsted) to partition soil macroporosity from capillary matrix porosity, allowing us to track how long-term grass vs arable management affected macropore volume and its seasonal dynamics. (4) We developed a new method to measure daily root water uptake, root water potential, and radial root water permeability at different soil depths under field conditions. Using this approach, we monitored wheat and grass plants throughout drying and rewetting cycles and found that both species adjusted their root permeability depending on soil moisture conditions. This allowed us to uncover how plants actively coordinate water uptake from shallow and deep soil layers to optimise water use during periods of water stress.

Key Findings: (1) In the Broadbalk study, nutrient management outweighed soil structural differences in determining yield stability. Yields under severe drought were sustained best in plots with optimal nitrogen fertilisation (around 144 kg N/ha), while unfertilised or excessively fertilised plots showed larger yield reductions. Notably, soil structural parameters (Kₛₐₜ, α) varied across treatments, but these did not correspond to improved yield resilience during drought years. This suggests that in a long-term, high-input system, adequate fertility and baseline soil water availability are the primary buffers against drought, whereas incremental differences in physical structure have a limited effect on yield in extreme conditions. (2) The SOC and soil structure investigation revealed a strong linkage between soil physical architecture and carbon storage. Soils with more connected pore networks accumulated significantly higher SOC than those with a fragmented structure, despite decades of similar climate and soil type. SOC content increased with the calculated substrate transport capacity of soil pores, approaching an asymptotic maximum. This implies a physical limit to carbon sequestration set by soil structure: once pore connectivity and volume reach a certain point, additional carbon inputs result in diminishing increases in stable SOC. These findings provide novel evidence that pore-scale connectivity facilitates microbial access to substrates, enhancing carbon stabilisation, and that without a well-developed structure, added organic matter may not fully convert to long-term SOC. (3) The new infiltration-based method successfully quantified macroporosity differences induced by long-term land use. Grass ley management produced substantially greater macropore volume than annual cultivated or bare fallow soils, confirming qualitative expectations with quantitative measurement. Macroporosity remained relatively constant between years, whereas capillary matrix porosity exhibited seasonal fluctuation. Additionally, the macropore hydraulic conductivity did not differ much between treatments, indicating that while the number of macropores changed with land use, the flow efficiency per macropore was similar. This methodology innovation is a key outcome. Furthermore, (4) plants reduce water uptake from dry topsoil by lowering the permeability of shallow roots and increase uptake from deeper soil by boosting permeability there. After rainfall, they quickly reverse this pattern, highlighting a dynamic strategy to optimise water use based on soil moisture.

Implications: Collectively, the results emphasise that soil structure matters for both agricultural performance and environmental outcomes, but its impact is context-specific. In intensive cropping systems with high inputs, structural improvements alone may not translate to immediate yield benefits under extreme weather unless coupled with sufficient fertility, highlighting a potential risk of “false resilience” if soils are degraded but compensated by inputs. On the other hand, for long-term soil sustainability and ecosystem services, good structure is indispensable: it underpins greater carbon sequestration potential and regulates hydrological behaviour, which in turn benefits climate adaptation and resource use efficiency. These findings also show that plants use more than just root depth; they actively adjust root function to manage water uptake, which helps them cope with drought. This insight opens new possibilities for breeding crops with dynamic root traits that improve resilience to changing climate conditions. This thesis's successful use of long-term experiments demonstrates their importance in revealing slow-building effects and complex interactions that short experiments miss. From a management perspective, the findings support strategies that integrate soil physical management with prudent nutrient management. Such integrated approaches can achieve more resilient crop production, combining healthy, well-structured soils that hold water with adequate nutrition to support crops during drought. Additionally, the new measurement technique for macroporosity could be adopted in soil monitoring programs or further research, improving our ability to detect and quantify structural improvements or degradation in situ.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Whalley, Richard Mooney, Sacha
Keywords:	Soil structure; Long-term field experiments; Yield stability; Soil organic carbon; Soil macroporosity
Subjects:	S Agriculture > S Agriculture (General)
Faculties/Schools:	UK Campuses > Faculty of Science > School of Biosciences
Item ID:	82372
Depositing User:	Rickard, William
Date Deposited:	12 Dec 2025 04:40
Last Modified:	12 Dec 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/82372

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