Wetland identification in seasonally flooded forest soils: soil morphology and redox dynamics

Soil morphology provides a long-term record of hydroperiod and soil aeration and is widely used to identify wetlands. This study was conducted to: (i) investigate the influence of soil texture on redox processes, (ii) determine the quantitative relationships between morphology-based hydric soil indicators, redox potential, O2 content, and water table depth, and (iii) evaluate the use of hydromorphic soils to identify wetlands. Transects were established across a floodplain of the Savannah River, South Carolina, and a third-order tributary. Ten plots were monitored for water table depth, redox potential, and soil O2 content at depths of 15, 30, 60, and 90 cm during a 2-yr period. Six plots met the criteria for hydric soils. Redox potential, O2 content, and chroma were strongly correlated when averaged across annual time scales (2 yr). In all cases, anaerobic and reducing conditions near the soil surface were caused by soil saturation. A clay soil in this study, however, was poorly aerated at depths greater than or equal to 60 cm even in a drained condition. Our data provide empirical support for using soil morphology to infer periods of soil saturation and reduction in southeastern wetland forests. Field indicators (mottling, gleying, and histic epipedons) were accurate surrogates for more detailed measurements of redox potential, content, and water table depth on all sites. Contrary to assumptions in the criteria for hydric soils, temperatures were always greater than or equal to 5 degrees C at 50 cm during the non-growing season. Our results suggest anaerobic conditions can develop during the winter months. Because southeastern swamp forests are saturated mainly during the winter or spring, growing-season criteria for hydric soils may require further evaluation.