Transport of dissolved organic matter through a sandy forest soil

We assessed the transport of dissolved organic matter (DOM) through a sandy, Ultisol forest soil on the southeastern U.S. Coastal Plain, and contrasted the results with similar studies from other forest regions, to test the hypothesis that DOM transport is greater through sandy Ultisols than finer textured Ultisols and Spodosols. Within a small headwater catchment, concentrations of dissolved organic carbon (DOG), a measure of DOM, were measured in soil solution at three depths (10, 30, and 90 cm) in sand A and E horizons of soil profiles, and in sand along the valley bottom at 198- to 264-cm depth in shallow groundwater. Water samples were collected after every rainfall event for 21 mo using zero-tension lysimeters, suction samplers, and piezometers. Mean concentration of DOC in soil water decreased from 25.5 mg C L-1 at 10-cm depth to 13.7 mg C L-1 at 30 cm, and to 1.8 mg C L-1 at 76 to 99 cm, before contacting clay-enriched horizons. All valley bottom stations consistently averaged between 0.3 and 2.1 mg C L-1. We did not find significant seasonal patterns, nor a correlation between DOC concentration and magnitude of rainfall events. We estimate that the nux of DOC decreased more sharply with soil depth than concentration due to attenuation of water transport through the soil profile. Compared with literature data from other forest regions, our results do not support the hypothesis that there is greater DOM transport through sandy upland soils on the Coastal Plain. Our results suggest that this is due to strong DOM retention within deep sand E horizons of these soils. Thus, strong DOM retention in forest soils appears to occur across a broader range of soil types than those exhibiting podzolization or having high clay content.