Monday, November 13, 2006

Effects of Redox on Aggregate Stability of Upland Soils.

Alfredo B. De-Campos1, Chi-Hua Huang2, and Amrakh I. Mamedov2. (1) Purdue Univ, 275 S Russell St, West Lafayette, IN 47906, (2) USDA-ARS National Soil Erosion Research Lab, 275 S Russell St, West Lafayette, IN 47907-2077

Upland soils in the US Midwest often undergo reducing conditions when soils are temporally flooded during the spring. The redox effect on the aggregate/structural stability of upland soils is not well understood. We hypothesized that aggregate stability would decrease under reducing conditions. Three cultivated and three uncultivated soils with different organic matter contents and similar mineralogy were selected to test this hypothesis. Changes in the redox status (Eh), pH, dissolved organic matter (DOC), metal concentrations, electrical conductivity (EC) in the soil solution, and aggregate stability were determined for all soils in anaerobic batch experiments for 0, 1, 3, 7 and 14 day incubation treatments. A biogeochemical reactor was designed for the anaerobic incubation and the aggregate stability was measured by the high-energy moisture characteristic (HEMC) procedure. In general, redox potential (up to -450mV) and aggregate stability decreased with increasing incubation time for all six soils. An increase in pH (up to 2 units), EC (up to 1.9 mS cm-1), metal concentrations (Fe, Mn, Na, K, Ca, Mg), and DOC (up to 350 mg L-1) was also observed. Cultivated soils were less stable than uncultivated soils for the same incubation period. Comparisons within soils showed that soils under stronger reducing conditions (14 days incubation) are less stable. Renewing soil pore water significantly decreased aggregate stability. This trend depended on the initial stability of the soil after the incubation treatments. This study showed that under reducing conditions, changes in concentrations of redox sensitive metals (e.g. Fe, Mn), organic matter decomposition, and salinity of the soil solution can affect aggregate stability.