Preferential Zn and Cd Hotspots in Soil Microarchitecture

Soil system properties control the fate of essential Zn and toxic Cd, which can have pervasive influences on ecosystem health. However, direct evidence of the spatial distribution of trace metals within the organo-mineral soil architecture is lacking, though this knowledge is important to better predict the fate of trace metals in ecosystems. Here, we present a spectromicroscopic approach to map the distribution of Zn and Cd between mineral phases and organic matter at a resolution of approximately 150 nm. This was achieved using nanoscale secondary ion mass spectrometry (NanoSIMS) applied to a spiked gradient of Zn and Cd concentrations in an arable soil. We observed patchy-distributed Zn and Cd hotspots in all soils, including at a native concentration in the nonspiked soil. With increasing Zn and Cd spikes, these metals preferentially colocated with Fe-dominated mineral phases, whereas with the highest spike, Zn and Cd increasingly colocated with clay minerals and soil organic matter. Soil extracts suggested that large proportions of spiked Zn and Cd were tightly bound to the colocated phases. Our direct measurements of patchy-distributed Zn and Cd hotspots indicate that distinct soil surfaces preferentially allocate and govern the fate of trace metals in soils.