The concentration and size distribution of iron‐rich colloids in pore waters are related to soil organic matter content and pore water calcium concentration.

Iron (Fe)‐rich colloids in the environment have a high surface reactivity and facilitate transport of oxyanions and trace metal cations in soil. This study was set up to help identify the soil properties that might affect the concentrations of colloidal Fe and its size distribution in pore waters of soils. The pore water was isolated by centrifugation from a collection of 97 topsoils with contrasting properties and land use. The total Fe concentration in the pore waters ranged between 0.2 and 26 μM (10th–90th percentile) and was largest at low soil solution calcium (Ca) and high dissolved organic carbon (DOC), indicating that enhanced colloidal stability determines total Fe in solution. The colloidal size distribution (<100 nm) of Fe in the pore waters of 11 soils was determined with flow field flow fractionation (FlFFF‐UV‐ICP‐MS), which yielded three different fractions that varied strikingly among soils. The smallest fraction (<5 nm) of Fe co‐eluted with high DOC and copper (Cu), suggesting mononuclear Fe‐organic carbon (OC) complexes. This fraction increased with an increasing DOC/Fe ratio in pore waters. The 5–50‐nm fraction of Fe prevailed in soils with >3.5% organic carbon (%OC), whereas the 50–100‐nm fraction was dominant in soils with low %OC content. This suggests that natural organic matter inhibits crystallization and growth of mineral Fe particles. The elemental ratios of mineral colloids (>5 nm) indicated the presence of clay minerals and Fe oxyhydroxides, which was confirmed by scanning electron microscopy (SEM) analysis. This study thus identified soil %OC and Ca in soil solution as the dominant factors predicting both the concentration and size of pore water Fe colloids, with the smallest and, potentially, most mobile Fe colloids occurring in OC‐rich soils. Highlights: Pore water iron concentrations in topsoils are governed by colloid stabilityFlow field flow fractionation analysis allows characterization of iron colloids in the low nanometre rangeIron colloid size and composition vary strikingly among soilsSmall iron colloids prevail in organic carbon‐rich soils due to colloid growth inhibition [ABSTRACT FROM AUTHOR]