Preferential\nZn and Cd Hotspots in Soil Microarchitecture

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