Changes in water‐soluble and exchangeable ions, cation exchange capacity, and phosphorusmax in soils under alternating waterlogged and drying conditions

Abstract

The effects of waterlogging and subsequent drying on pH, Eh, dissolved oxygen (DO), water soluble ions, exchangeable cations, cation exchange capacity (CEC), and phosphorus sorption capacity (Pmax) in three soils were studied (krasnozem, sandy loam, and highly organic material from a Melaleuca wetland). Fifty grams of each soil were mixed with 75 mL of distilled water, and left in a waterlogged state for 21 days. After removing a sample of the ponded water and sediment for analysis, the remaining water was discarded and the soils allowed to dry at room temperature for 21 days. All soils demonstrated a decrease in Eh, PO4, NO3 and DO, and an increase in pH, soluble organic carbon (SOC), iron (Fe), manganese (Mn), NH4, calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na), in the solution phase due to waterlogging, but the magnitudes of change were greatest in soils with the higher levels of organic matter. Increases in Fe and Mn were attributed to the dissolution of Fe and Mn oxides under chemically reducing conditions. Increases in Ca, Mg, K, and Na were attributed to increased solubility of organic carbon, and from increased competition for the CEC sites due to elevated Fe and Mn concentrations. Increases in CEC due to the presence of colloids with variable charge were largely balanced by Fe and Mn. Mineralization of organic nitrogen (N) by anaerobic bacteria, and the absence of DO, was considered responsible for the accumulation of water soluble NH4. Decreases in water soluble NO3 and PO4 due to waterlogging were attributed to denitrification and sorption by amorphous Fe(OH)2, respectively. Reoxidation by drying caused increases in Eh and DO, decreases in exchangeable Fe and Mn, but had no significant effect on pH, exchangeable cations or CEC. Waterlogging nearly doubled Pmax in the krasnozem A horizon and wetland soils, and produced only small changes in the other soils. Sorption by freshly precipitated Fe(OH)2 is considered the major mechanism increasing PO4 sorption.