The spatial distribution of Zn and Cd across the soil microscale architecture as mediated by different mineral phases in a supplemented arable soil

Zinc (Zn) is an essential trace element for human nutrition as well as for plant growth and soil organisms. Cadmium has similar biogeochemical properties like Zn, but is non-essential for most biota and highly toxic. Due to the on the heterogeneous arrangement of soil mineral phases and organic compounds within a functional soil architecture, there is a lack of knowledge on how the microscale arrangement is interrelated with ecosystem-relevant soil functions such as the storage and cycling of nutrients and contaminants. Here, we present an analytical approach aiming to resolve the spatial distribution of Zn and Cd in a soil at the microscale. Zn and Cd were supplemented in three increasing concentrations to an arable soil from the Jura region, Switzerland. Our image-based investigation was obtained using a dual primary ion source workflow by Nanoscale secondary ion mass spectrometry (NanoSIMS) combining the Cs⁺ and the RF plasma O⁻ source. The dual workflow enabled correlating the distribution of Zn and Cd with Fe, Al, Si, P, Mg, Ca, S, C and N at a lateral resolution of 120nm. Our observations indicate a high co-localization of Zn and Cd hotspots, whereas these were not related with organic matter patches. Of the three mineral phases identified using a machine-learning image segmentation, most areas were occupied by Al-dominated regions followed by Si-dominated and Fe-dominated parts. Across the increasing supplementation, the Zn and Cd hotspots were preferably co-localized to mineral phases in the following order: Fe-dominated > Al-dominated > Si-dominated. With increasing Zn and Cd supplementation, the Cd/Zn ratio as well as the N/C ratio decreased indicating changes in the biochemical composition of . Our model soil approach illustrates how the spatial arrangement of essential and toxic trace elements at the microscale regulates their fate in the soil. The developed NanoSIMS-based dual primary ion source workflow enables emerging opportunities to characterize how environmental changes affect the spatial distribution of nutrients and contaminants in dynamic soil architectures.