Short-term Soil Formation in an Abandoned Sand Borrow Pit, Georgia, USA.
Eric C. Brevik, Valdosta State University, Dept. of Physics, Astronomy, and Geosciences, Valdosta, GA 31698-0055
This study investigates soil properties in an abandoned borrow pit and compares those developing soils to natural soils surrounding the pit. The study site is located about 16 km south of Valdosta, GA at 30.7220 N latitude, 83.2560 W longitude. The pit was excavated in 1961 and then abandoned with no efforts at reclamation, giving an excellent opportunity to investigate short-term soil formation processes over the last ~40 years. The pit contains two tiers, a deep lower part and an upper terrace. There are distinct vegetative zones within the pit; three are included in this study. The lowest part of the pit (referred to as the pit bottom) has mixed slash pine (Pinus elliottii Englelm.) and deciduous trees including sweetgum (Liquidambar styraciflua L.) and swamp tupelo [Nyssa sylvatica var. biflora (Walter) Sarg.] with little underbrush and abundant leaf litter. Water ponds at the surface during wet periods and trees are characterized by buttressed trunks. The edge of the upper terrace (referred to as the sparsely vegetated area) has sparse vegetation limited to a few stunted slash and longleaf (Pinus palustris Mill.) pines and a sparse lichen cover, including “deer moss” [Cladina evansii (Abbayes) Hale & W.L. Culbertson] and no leaf litter cover. The edge of the pit along the upper terrace (referred to as the grassy area) has a healthy stand of mixed pine and deciduous trees similar to the lowest part of the pit, but includes a thick cover of grass (Andropogon sp.). Water ponds at the surface during wet periods here as well. Native deciduous trees including live oak (Quercus virginiana Mill.), water oaks (Quercus nigra L.), and magnolia (Magnolia spp.) characterize the area outside the pit. Soils in the pit were sampled using shovels and bucket augers in 2002 and 2003. Soil descriptions were made using standard U.S. Soil Survey methods and samples for further analysis were taken by horizon. Soil samples were stored in sealed plastic bags and refrigerated until laboratory analyses could be conducted. Four piezometers were installed in each of the four areas (pit bottom, sparsely vegetated area, grassy area, and native soils) studied and monitored about once every 2 weeks from February 2002-May 2004. Bulk density was determined using rings of known volume for the upper 0.2 m in the developing soils and 1.0 m of the natural soils. Total C was determined using a Perkin-Elmer 2400 CHNS and a detailed topographic survey of the pit was made using a Sokkia laser-based total station. Historical aerial photographs were scanned, georeferenced, and imported into ArcView 3.2 to establish the re-vegetation history of the study site. Although soil formation over only 40 years is slight, there are distinct differences between the vegetative zones being studied. Soil formation is consistently deeper in the grassy area (~0.25 m on average) and pit bottom (~0.13 m on average) than in the sparsely vegetated area (~0.06 m on average). Carbon accumulation rates follow the same trend, being highest in the grassy area (0.09 kg C m-2 yr-1), intermediate in the pit bottom (0.05 kg C m-2 yr-1), and lowest in the sparsely vegetated area (0.01 kg C m-2 yr-1). When investigating potential controls on the rates of soil formation in these three areas, it was noted that water was available at relatively shallow depths in both the grassy area and pit bottom for significant portions of the year, but was often found at depths greater than 2 m in the sparsely vegetated area. Bulk density was also significantly higher in the sparsely vegetated area (1.74 g cm-3) than in the grassy area (1.20 g cm-3) and pit bottom (1.45 g cm-3). Comparisons with natural soils outside the pit showed that all soils developing within the pit have much more pedogensis to undergo before reaching equilibrium with the natural environment. Soil differences in different parts of the studied pit are attributed primarily to differences in vegetation at different locations in the pit. Vegetative differences are in turn attributed to differences in soil bulk density and to topographically controlled access to water. Study of surrounding developed soils coupled with rates of pedogensis within the pit to date reveals that soils within the pit will probably continue to undergo pedogensis for several hundred years.