Grassland soil depressions: Relict bison wallows or inherent landscape heterogeneity?

Wallows are circular soil depressions created by repeated bison (Bison biso n L.) dust-bathing. Despite more than a century of bison absence from the G reat Plains and lack of evidence on wallow persistence, many studies have c lassified grassland soil depressions as 'relict' wallows. We studied bison wallowing on a tallgrass prairie site in Oklahoma where bison were reestabl ished in late 1993. Bison use of existing soil depressions fitting descript ions for relict wallows located before reintroduction and bison formation o f new active wallows were documented from 1993-1995. Bison avoided existing depressions, instead forming active wallows in different locations. Bulk d ensity and soluble salt, sodium and clay content of soils in existing depre ssions were significantly higher than soils in active wallows or nearby ref erence sites. These depressions occurred primarily on shale-derived soils r ich in silt and clay whereas active wallows were formed mostly on sandy loa m soils overlying sandstone. The spatial distribution and soil conditions o f these depressions suggested pedogenic, rather than animal disturbance, or igins. Soil sampling beneath the depressions revealed a dense clay lens loc ated above a natric (high exchangeable sodium content) horizon, both of whi ch were absent from soils in nearby reference sites. Natric soil horizons, known as claypans, are formed by pedologic processes common to soils derive d from marine shales. Thus, these soil depressions, and possibly other 'rel ict' bison wallows, are not persistent soil disturbances resulting from bis on wallowing, but small patches of landscape and soil heterogeneity resulti ng from variation in underlying geological materials.