Iron (Hydr)Oxide Crystallinity Effects on Soil Aggregation

Abstract

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 Feo (1.1–6.8 g kg−1), Fed (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 (Feo) and OC contents are significantly correlated with WSA in the A horizons (r2 = 0.95, n = 6, p = 0.001, and r2 = 0.93, n = 6, p = 0.002, respectively) and in the B horizons (r2 = 0.73, n = 6, p = 0.029, and r2 = 0.76, n = 6, p = 0.024, respectively). When regressed against MWD, Feo has an r2 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 Fed contents are not significantly correlated to WSA or MWD. Apparently, the Feo component (poorly crystalline) is more effective than Fed at stabilizing soil aggregates, even though it is present in lower concentrations. The Feo component appears more important than OC in terms of WSA and MWD for soils with relatively low soil organic matter contents.