Long Term Soil Management Options Affect Iron Oxide Properties and Reactivity.

Long-term soil management can impact soil properties such as organic-matter (OM) content, nutrient dynamics and mineral weathering; however, little information is available on the effect of soil management and tillage on mineral transformations. The objective of this study was to evaluate the effect of long-term management options on the concentration and reactions of the reactive Fe oxide, ferrihydrite. The soil (Thermic Fluventic Ustochrept) utilized in this study is part of a long term (26 year) cropping system experiment. Four management systems were considered: conventional continuous wheat, no-till (NT) continuous wheat, a conventional rotation (sorghum-wheat-soybean), and a no till rotation (NTR). Soil samples were collected from 0-5 cm and 5-20 cm depth. Ferrihydrite concentration and reactivity was evaluated using pH 3 ammonium oxalate extraction (in the dark) and time dependent DTPA, respectively, both before and after removal of soil organic matter using 6% NaOCl (pH 8). The NT systems allowed for greater accumulation of soil OM compared to the conventional systems. The poorly crystalline Fe oxide contents as assessed by ammonium oxalate extraction were approximately the same for NT and conventional systems. Without OM removal, DTPA-extractable Fe was considerably greater with the convention compared to the NT systems, probably due to the role of OM in blocking reactive Fe-oxide sites. When OM was removed prior to DTPA extract, exactable Fe levels were approximately 7 fold higher and the original trend of NT versus convention treatments were reversed, i.e., more Fe was extracted with the NT treatments. These results indicate that the long term NT treatment results in a more reactive Fe oxide. These reactions will be discussed in terms of carbon sequestration, Fe-oxide reactivity and implications to agricultural sustainability.