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.
