Iron (Hydr)Oxide Crystallinity Effects on Soil Aggregation

Differences in crystallinity may explain why total Fe (hydr)oxide content has a variable effect on aggregate stability. Therefore, surface soil samples with a range of poorly crystalline Fe (hydr)oxide contents were characterized for water‐stable aggregates >0.25 mm (WSA), mean‐weighted diameter (MWD), soil organic C (OC), particle‐size distribution, pH, exchangeable cations, citrate/bicarbonate/dithionite (subscript d), and acid ammonium oxalate (subscript o) extractable Fe, Al, and Si. The WSA and MWD range from 23 to 95%, and 0.3 to 5.1 mm, respectively. The effects of Fe o (1.1–6.8 g kg −1 ), Fe d (3.2–19.6 g kg −1 ), OC (2.4–24.0 g kg −1 ) and clay (141–467 g kg −1 ) contents on WSA and MWD of both A and B horizons of these soils was studied using linear regression. The poorly crystalline Fe (hydr)oxide (Fe o ) and OC contents are significantly correlated with WSA in the A horizons ( r 2 = 0.95, n = 6, p = 0.001, and r 2 = 0.93, n = 6, p = 0.002, respectively) and in the B horizons ( r 2 = 0.73, n = 6, p = 0.029, and r 2 = 0.76, n = 6, p = 0.024, respectively). When regressed against MWD, Fe o has an r 2 of 0.89 ( n = 6, p = 0.004) in the A, and 0.97 ( n = 6, p = 0.000) in the B horizons. The coefficient of determination of MWD vs. OC contents is 0.98 ( n = 6, p = 0.000) in the A and 0.79 ( n = 6, p = 0.018) in the B horizons. Clay and Fe d contents are not significantly correlated to WSA or MWD. Apparently, the Fe o component (poorly crystalline) is more effective than Fe d at stabilizing soil aggregates, even though it is present in lower concentrations. The Fe o component appears more important than OC in terms of WSA and MWD for soils with relatively low soil organic matter contents.