Aging shapes the distribution of copper in soil aggregate size fractions.

Soil aggregates are often considered the basic structural elements of soils. Aggregates of different size vary in their ability to retain or transfer heavy metals in the environment. Here, after incubation of a sieved (<2 mm) topsoil with copper, bulk soil was separated into four aggregate-size fractions and their adsorption characteristics for Cu were determined. By combining nano-scale secondary ion mass spectrometry and C-1s Near Edge X-ray Absorption Fine Structure Spectroscopy, we found that copper tends to bind onto organic matter in the <2 μm and 20-63 μm aggregates. Surprisingly, Cu correlated with carboxyl-C in the <2 μm aggregates but with alkyl-C in the 20-63 μm aggregates. This is the first attempt to visualize the spatial distribution of copper in aggregate size fractions. These direct observations can help improve the understanding of interactions between heavy metals and various soil components.