The Identification of the Reactive Pore Space in Soils

Project Information

EXPOSOIL

Grant agreement ID: 101054917

DOI

10.3030/101054917

EC signature date 11 May 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 498 535,00

EU contribution

€ 2 498 535,00

Coordinated by

KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium

Periodic Reporting for period 1 - EXPOSOIL (The Identification of the Reactive Pore Space in Soils)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Ensuring the quality of our soils is essential for a sustainable world. Typically, soil quality testing occurs by soil sampling, sieving and extraction, thereby disturbing the soil’s hierarchical pore size structure. Soil sieving and extraction disrupt the macro-aggregates and overestimate the accessible reactive surfaces of soil. Plant roots are exposed to only a fraction of these soil-reactive surfaces. Hence, traditional soil tests for bioavailability underscore the physical non-equilibrium of nutrients and contaminants in soil. EXPOSOIL aims to identify the reactive pore space to which roots are exposed in undisturbed soil. The research objectives are to quantify the effects of soil structure and mobile colloids on the bioavailability of nutrients and contaminants and to develop methods to understand and diagnose these effects. We speculate that these effects create local heterogeneities that are most important in soils with stronger aggregate structure, for contaminants or nutrients that are relatively immobile and less aged in soil and for elements strongly associated with mobile colloids. Experimental studies will be set up to test these hypotheses in soils with surface-amended trace metal contaminants, fertilisers or lime, using isotopes to trace local provenances and using novel visualisation tools. Novel reactive membranes acting as zero sinks for solutes and colloids will be developed to mimic plant roots and to make 2D images of the locally available elements. That development is of high risk but high gain because no other method has yet assayed diffusive fluxes of solutes, let alone of colloids, in unsaturated and undisturbed soils. The new method will disclose the enigmatic roles of soil physical factors and colloids on bioavailability. This knowledge will advance the practical use of soil chemistry in environmental applications, e.g. to improve existing soil testing assays and to facilitate the development of novel, more efficient fertilisers.

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

The EXPOSOIL project investigates reactive pore space around plant roots in undisturbed soil. Objectives include quantifying how soil structure and mobile colloids influence nutrient and contaminant bioavailability and developing diagnostic methods for these effects. Traditional soil tests may miss physical non-equilibrium near roots due to macro-aggregate destruction during testing, potentially leading to significant local heterogeneities. This is especially true in soils with strong aggregate structures, where relatively immobile contaminants or nutrients interact with mobile colloids.

In WP1, we are developing a novel DGT method to map the mobility of metals, nutrients, and colloids in situ in undisturbed, unsaturated soil. This DGT, with higher binding capacity for organometal colloids, enables 2D visualisation of available elements via LA-ICP-MS. WP2 investigates soil structure's impact on nutrient and toxic element bioavailability, including trace metal uptake, liming efficiency, and phosphorus fertiliser efficiency, requiring extensive soil sampling and testing.

Initial trials examined soil structure effects on metal mobility using pot experiments with five soils of varying structures, modified through sieving to create intact, 8 mm, and 2 mm sieved soils. Stable metal isotopes (62Ni, 65Cu, 70Zn, 108Cd, 204Pb) were surface-applied before maize growth. Results showed isotopes penetrated deeper into undisturbed soil, with 2D imaging DGT revealing enrichment in intact soils, indicating preferential flow paths and highlighting soil structure's influence on solute mobility.

Plant uptake confirmed higher bioavailability in intact soils, with 65Cu uptake 1.5–5 times greater due to increased isotope presence in macropores. Native metal availability was lower in intact soils due to mass transfer limitations, underscoring soil structure's role in heavy metal availability.

Unexpectedly, liming experiments showed lime infiltration limited to 1 cm in all soil types, likely due to minimal structural differences or strong sorption. 2D spatial pH measurements revealed pH heterogeneities. A pot trial is underway to assess soil structure's effect on phosphorus fertiliser efficiency in tropical Madagascar soil, comparing local TSP granule applications to powdered phosphorus broadcasting. Analyses are pending, alongside studies of nanoparticle phosphorus bioavailability using novel DGT methods.

Challenges remain in expanding fertiliser and colloid studies and diagnosing soil-plant impacts. WP4 will apply findings from WP2 and WP3 in the field upon completion of current research.

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)

a) We confirmed that soil structure significantly affects the availability of nutrients and metals. Plants took up more trace metals from intact soils than from sieved soils, with differences ranging from 1.5 to 5 times. This is crucial given the narrow margins between acceptable limits and actual trace metal concentrations in the food chain. While we predicted that metals like Pb and Cu, which have a high distribution coefficient (Kd), would be more affected by structural changes, the data did not support this. Changes in soil structure primarily influenced Cu uptake, while Pb uptake was minimally affected, likely due to airborne contamination. Over time, the differences in plant uptake between sieved and intact soils diminished, as added metals became less available in larger soil aggregates. This study uniquely highlights the impact of soil structure on metal availability.

b) The pot trial indicated that native metals in the soil were significantly less available in intact soils than in sieved ones, with a maximum difference of 2.5 times. This suggests that native metals are within aggregates more common in intact soils, leading to greater metal depletion in the rhizosphere in these structured soils. This finding, while unexpected, makes sense upon reflection.

c) We developed colloid DGTs to visualize spatial heterogeneity in mobile soil colloids during a long-term manure application field trial, focusing on anaerobic microsites to identify colloidal P hotspots. Sampling occurred in winter 2023-2024 during record high drainage due to heavy rainfall. Unexpectedly, we detected mobile clay mineral colloids using advanced LA-TOF-ICP-MS. One ERC post-doc received a travel grant to measure DGTs with synchrotron and LA-TOF-ICP-MS in Australia in 2024. This method allows for rapid, simultaneous elemental analysis, identifying clay colloids through the co-localisation of Al, Si, Rb, and Cs. The low Ca concentration in the soil solution, influenced by prolonged winter rainfall rather than anaerobic conditions, likely accounts for the nature of these mobile colloids

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