Management-Dependent Soil Variability and Surface Hydraulic Properties of Southeastern U.S. Coastal Plain Plinthic Kandiudults

Abstract

Soil surveys are the foundation for natural resource planning and management and have been developed to illustrate and characterize the spatial variability of our soil resource; however, decade-scale temporal variability in near-surface properties largely induced by management may be more critical from an interpretation standpoint. With the initial survey of the soil resource nearly complete, future goals of the National Cooperative Soil Survey will concentrate on improving soil interpretations within the context of anthropogenic soil change. An improved understanding of variability of near-surface soil properties can greatly improve soil survey applications, land use management, and policy development. Soil hydraulic properties are of particular importance due to their role in ecosystem function and wide application within soil interpretations. The National Cooperative Soil Survey defines a management-dependent property as a type of dynamic soil property which changes on a human time-scale due to anthropogenic disturbances. Interest in soil change and C sequestration has led to increased emphasis on characterization and inventory of these properties. Decades of research has shown that management can have vast effects on near-surface soil properties, but to what extent these effects have occurred on southeastern Coastal Plain soils is unclear. The Wiregrass region has a broad range of natural and agroecosystems ranging from native longleaf pine (Pinus palustris) forests to conventional monoculture row crop land. The upper 50 cm in most upland soils of the region are coarse textured with relatively high permeability and low water holding capacity. Because short-term droughts during the growing season often limit ecosystem productivity, near-surface hydraulic properties are of utmost importance. For a prime farmland, benchmark soil map unit in the Wiregrass region of the Alabama Coastal Plain, objectives of this study were to: 1) assess the degree of management-dependent versus use-invariant soil property variability, 2) evaluate the impacts of long-term land use systems on near-surface soil properties, 3) evaluate soil C pools and sequestration potentials, and 4) develop improved relationships between management-dependent soil properties and near-surface soil hydraulic properties, including development of pedotransfer functions. Near-surface (0-50 cm) soil physical, chemical, and hydraulic properties within Dothan (Fine-loamy, kaolinitic, thermic Plinthic Kandiudults) consociations were measured under long-term (>10 years) conventional and conservation row cropping, pasture, pine plantation, and old-growth Longleaf pine forest. Pedon description and characterization were conducted at each site to quantify use-invariant variability. Variability in the investigated management-dependent properties was greater than observed variability of use-invariant soil properties. Significant anthropogenic influences were observed on near-surface soil properties as a function of long-term land use systems (best expressed in chemical and C properties). Based on C contents, stratification of C and N pools, surface exchange capacities, and aggregate stability, longleaf and pasture systems had superior soil quality. Differences in C sequestration were observed across natural and agroecosystems, with pastures having the highest (57 Mg C ha-1) and conventional row crop (38 Mg ha-1) the lowest C contents (0-50 cm). Relationships among near-surface hydraulic properties and management-dependent soil properties allowed development of regression based algorithms that adequately described near-surface hydraulic property variability using both use-invariant and management-dependent soil properties. Because many of these properties are correlated, it is likely that a subset of properties to be measured can be developed to characterize management-dependent variability. Considering the importance of management-dependent properties to ecosystem function and map unit interpretations, creating management schemes and soil interpretations based on land use may be better suited than heavy reliance on use-invariant properties.