Investigating interactions between plant roots and phosphorus in soil

Project Information

RootOutP

Grant agreement ID: 101027472

DOI

10.3030/101027472

Project closed

EC signature date 25 March 2021

Start date 1 May 2022

End date 31 March 2025

Funded under

• SOCIETAL CHALLENGES - Europe In A Changing World - Inclusive, Innovative And Reflective Societies

Total cost

€ 219 312,00

EU contribution

€ 219 312,00

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - RootOutP (Investigating interactions between plant roots and phosphorus in soil)

Reporting period: 2022-05-01 to 2024-04-30

Summary of the context and overall objectives of the project

Phosphorus (P) is a scarce resource that is critical for crop production, but it is not being used sustainably. Excessive past fertiliser applications mean large amounts of P have accumulated in soil, losses of which are of major environmental concern. Nonetheless, the majority of soil-P exists in pools of very low bioavailability to plants, due to the high reactivity of P in soil. Increasing the ability of plants to take up P from applied sources (fertilisers) and from accumulated soil reserves would allow for reductions of fertiliser use and decreased potential P losses to the environment.

Through evolution, plant roots have adopted several strategies to improve P capture, including: 1) architectural traits that affect the spatial exploration of the soil profile; 2) adaptive (plastic) responses to zones of high P supply (e.g. around fertiliser granules); and 3) physiochemical alteration of the environment in their rhizosphere. A challenge for the research community is to evaluate these properties and their potential benefits to cropping systems. As soil is inherently opaque, these traits are hard to study. Our current knowledge is fragmented because studies have generally used destructive sampling techniques, artificial media, and experimental setups making observations in 1D or 2D. Further advances of our understanding require in-situ visualization and quantification in real soil.

This project delivered such in-situ information in the pursuit of ultimately selecting crops with benefitial root traits and developing crop systems that make better use of applied and accumulated soil P. Specifically, the overarching objective of the project was to understand how root traits and distribution are affected by the spatial distribution of P availability, and vice-versa (Fig. 1), by achieving the following objectives:

1. Demonstrate how root system architecture of contrasting genotypes affects the response to heterogeneously distributed P fertiliser in time
2. Determine how contrasting genotypes respond plastically to a fertiliser P band and relate this to soil chemical properties (labile P, speciation of elements) across the soil-fertiliser interface
3. Demonstrate how citrate exudation along a root axis affects soil pH and availability of P for contrasting genotypes (high citrate efflux /no citrate efflux)

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Experiments were conducted across three work packages (WP), addressing the three project objectives:

WP1: Demonstrate how RSA of contrasting NILs (narrow/wide angle) affects the response to heterogeneously distributed P fertiliser in time
WP2: Determine how contrasting NILs (narrow/wide angle) respond plastically to a fertiliser P band and relate this to soil chemical properties (labile P, speciation of elements) across the soil-fertiliser interface
WP3: Demonstrate how citrate exudation along a root axis affects soil pH and availability of P for contrasting NILs (high citrate efflux /no citrate efflux)

Within each WP, main activities have involved plant growth experiments followed by in-situ imaging of root growth and/or soil chemical changes around roots. Major techniques include the use of (synchrotron and lab-based) 3D X-ray computed tomography (CT), diffusive gradients in thin films (DGT) coupled with laser ablation-ICP-MS (LA-ICP-MS), Planar optodes, and X-ray absorption near-edge structures (XANES).

Project results demonstrate:
1. synchrotron-based X-ray CT is a potential novel tool for studying plastic responses of root system architecture, in soil, at high spatial resolution.
2. the reactions of P in soil determine the movement and availability of P in and around a fertiliser band, depending on the interaction between fertiliser type and soil type. The resulting P availability dynamics, in turn, determine the occurrence of plastic root responses in a largely genotype-independent manner.
3. the P depletion gradient along the root axis was greater with a high citrate efflux from roots, but this was associated with an increase rather than the expected decrease of rhizosphere pH. We hypothesise that control of the rhizosphere pH was driven by the cation-anion balance, in which excess uptake of nitrate by the plant root would have led to alkalinization rather than acidification - this would require additional experimentation.

Although the project did not directly target market applications or environmental challenges, the tools and knowledge generated may support future research addressing societal needs, including those related to plant breeding, agronomy and sustainability. For example, methodology may ultimately develop towards new ways of selecting root traits for breeding programs. In addition, the X-ray CT method serves as a ‘proof-of-concept’ for in-situ investigation of root-soil interactions, opening a relevant new field for future research and applications.

Data have been disseminated through oral presentations at scientific conferences, peer-reviewed manuscripts, and on-campus stakeholder visits. The planned manuscripts have either been published (van der Bom et al., 2025), or are in preparation for submission in line with the original dissemination plan.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Project results demonstrate:
1. synchrotron-based X-ray CT is a potential novel tool for studying plastic responses of root system architecture, in soil, at high spatial resolution.
2. the reactions of P in soil determine the movement and availability of P in and around a fertiliser band, depending on the interaction between fertiliser type and soil type. The resulting P availability dynamics, in turn, determine the occurrence of plastic root responses in a largely genotype-independent manner.
3. the P depletion gradient along the root axis was greater with a high citrate efflux from roots, but this was associated with an increase rather than the expected decrease of rhizosphere pH. We hypothesise that control of the rhizosphere pH was driven by the cation-anion balance, in which excess uptake of nitrate by the plant root would have led to alkalinization rather than acidification - this would require additional experimentation.

Although the project did not directly target market applications or environmental challenges, the tools and knowledge generated may support future research addressing societal needs, including those related to plant breeding, agronomy and sustainability. For example, methodology may ultimately develop towards new ways of selecting root traits for breeding programs. In addition, the X-ray CT method serves as a ‘proof-of-concept’ for in-situ investigation of root-soil interactions, opening a relevant new field for future research and applications.

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