Southeastern U.S. Upper Coastal Plain Ecological Sites for Dynamic Soil Property Characterization

Abstract

The Ecological Site (ES) concept groups soils that respond similarly to management to meet conservation goals, develop site interpretations and models for ecological transitions, and construct restoration pathways. The USDA-NRCS and the National Cooperative Soil Survey (NCSS) have a nationwide plan to classify all lands into ESs. Dynamic soil properties (DSPs) are near-surface properties that respond to management and are critical to soil function and ecosystem services. Measurement of DSPs is also used to evaluate anthropogenic impacts on soil (termed soil change). The NCSS is developing methodologies to characterize and inventory DSPs, and we hypothesize the ES framework is a valid approach for doing this. The ability to characterize DSPs using ES would facilitate hydrological modeling, soil carbon sequestration, and provide soil health benchmarks and goals. In this study, we developed an ES for Longleaf Pine (Pinus palustris) -Bluestem (Schizachrium scoparium and Andropogon spp.) systems in the Alabama Fall Line Hills region, and subsequently characterized and inventoried DSPs for four states. The reference state consisted of relatively undisturbed Longleaf Pine and Bluestem sites; cultivated states included conventional row crop, pine plantation, and pasture sites that had been in place >20 yrs. The soil components in the ES were described, sampled, characterized, and verified to be Kanhap- and Kandiudults with sandy surfaces, loamy subsoils, and low activity mineralogy. Measured DSPs (0-50 cm) included carbon pools, chemical (e.g. extractable nutrients, exchangeable bases, CEC), and physical (bulk density, aggregate stability, clay dispersion ratio) properties critical to soil function. Soil organic carbon pools (0-30 cm, p = 0.0037) were higher (> 4 kg m-2) in reference and pine plantation than pasture and row crop states (< 4 kg m-2). However, the highest active carbon was found in the pasture state (p = 0.0011). As expected, intensive management (i.e. amendment additions) resulted in higher base saturation (p <0.0001) and Mehlich 1 extractable P (p = 0.0004), and lower Al saturation (p < 0.0001) in row crop and pasture states. Water stable aggregates (WSA) decreased with disturbance and tillage, with the lowest stable aggregates (77 %) in the row crop state. Surface bulk density (ρb) was highest in row crop (1.56 g cm-3) and lowest in the reference state (1.07 g cm-3). Bulk density decreased (r = -0.83) and CEC increased (r = 0.79) with increasing SOC, illustrating the critical role soil organic matter plays in soil quality for these systems. Multivariate analyses showed that measured sites generally grouped by state. While differences in DSPs were observed among states, differences were not as extensive as similar studies since the reference sites had legacy effects associated with a history of disturbance. However, the reference state represents a realistic management goal. Considering the range of a reference state concept, the aggregate of data suggests ESs are effective for characterizing DSPs and soil change in this region. The second objective of this study was to measure and compare soil hydraulic properties and functions among states of the ecological site. Measured hydraulic properties include soil water retention (van Genuchten) parameters, infiltration rate, and saturated hydraulic conductivity (Ksat). van Genuchten moisture retention parameters were measured at 15 cm in each site. The moisture retention parameter alpha (α) was higher in the reference than other states, indicating that these states lose gravitational water more quickly (p = 0.0058). Infiltration rate, Ksat (five depths to 100 cm), and water retention parameters (three depths to 50 cm) were measured in all states within Marvyn units. Paired t-tests (by depth) indicated significantly higher (p < 0.062) Ksat to 50 cm in the reference as compared to other states. These differences among the reference and cultivated states suggested some change in hydrologic function with ecological state. HYDRUS 1-D was used to simulate water flow for the reference and row crop states in Marvyn units under wet and dry conditions. In a 100-yr storm simulation (24 cm over 24 hour), the reference Marvyn site displayed no runoff while the row crop site showed 5.5 cm runoff. However, in a dry down event, the row crop Marvyn unit retained more water than the reference Marvyn unit over 60 days. These differences are likely due to inherent properties (e.g. soil texture) at sites rather than ecological state effects. Additional hydraulic data and simulations are necessary for further understanding of hydrologic function in states of an Alabama Fall Line Hills Ecological site. Comparisons of site and soil response to heavy rainfall and drought conditions under differing ecological states demonstrate the use of hydrologic modeling in Ecological Sites.