Characterizing deep soils from an impacted subtropical isolated wetland: implications for phosphorus storage

Purpose Soils within the Lake Okeechobee drainage basin, FL, USA, have been impacted by beef cattle and dairy operations and the landscape ditched and drained to facilitate stocking and grazing pastures. Restoring wetlands located on pastures has been proposed to reduce overland loss of phosphorus (P) by retaining it within the soils. However, soil properties of deeper horizons within impacted wetlands are rarely investigated due to the assumption that most dominant biogeochemical interactions occur at the soil-water interface. In this paper, we investigate soil properties up to 160 cm below the surface from an impacted isolated wetland and its surrounding upland pasture. Materials and methods Four intact soil cores were collected using the vibracoring technique, sectioned at 2-cm intervals, and analyzed for organic matter content by loss on ignition, total P by acid persulfate digestion, and water-soluble P using 0.01-M KCl solution. Bulk density and digital imaging were conducted using a multisensor core logger. Clay-sized phyllosilicates were identified using a computer-controlled X-ray diffractometer. Saturated hydraulic conductivity was measured using the slug-out technique, and the Hvorslev method was used to evaluate results. Results and discussion Unobliterated stratification was observed within the wetland in contrast to the pedologically formed A, E, and B horizons within the upland. The presence of clay horizons composed of smectite and kaolinite was dominant below 120 cm, posing potential hydrological and chemical implications, such as low conductivity and higher P sorption capacity. Saturated hydraulic conductivity of the soils was 1.22 m day⁻¹ (±0.4). Diagnostic features such as increase in bulk density due to compaction, and the “red-edge effect” were useful in correlating the effect of cattle activity and hydrology on soil profiles. Soil organic matter content (38-48%) and total P (100-600 mg kg⁻¹) concentrations were highest and well-correlated (r ² = 0.81) at the surface. However, a significant amount of P was also present in the deeper horizons associated with clay. Conclusions The deeper clay horizons account for up to 25% of P per hectare of the entire soil profile. These estimates are nontrivial and need to be accounted for while dealing with belowground P budgets, especially because subsurface lateral flows are dominant within the region.