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DIVISION S-5—PEDOLOGY

Iron (Hydr)Oxide Crystallinity Effects on Soil Aggregation

^a Dep. of Crop and Soil Sciences, The Pennsylvania State University, 116 ASI Building, University Park, PA 16802-3504
^b USDA-ARS-NSL, P.O. Box 1157, Oxford, MS 38655
^c Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
^d School of Natural Resources, The Ohio State University, 2021 Coffey Rd., Columbus, OH 43210

^* Corresponding author (swd10{at}psu.edu)

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 (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, Fe_o 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 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.

Abbreviations: subscript d, citrate/bicarbonate/dithionite extractable • MWD, mean-weighted diameter • subscript o, acid ammonium oxalate extractable • RR, redness ratio • WSA, water-stable aggregates

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