Water table effect on phosphorus solubility in biosolids‐amended soils

Abstract

The predominantly coarse texture of Florida Spodosols, limited P‐holding capacity, and fluctuating water table present major challenges for management of P, particularly in agricultural areas receiving biosolids. This study evaluated the impacts of water table level on P fate in Spodosols with contrasting P‐management histories. Treatments were a combination of soils with different biosolids histories (control [no biosolids], intermediate and high [biosolids‐P loads of 567 and 706 kg P ha−1, respectively]) and two moisture regimens (drained and flooded [water table at the depth of spodic horizon or 2 cm above soil surface, respectively]). Soil samples (A, E, and B horizons) were packed into PVC columns (96 cm tall, 11 cm in diameter), mimicking the soil profile. Leachate and pore water samples were collected at 2‐week intervals for 20 weeks. Regardless of treatment, proportion of P leached was low (∼0.2% total P mass). This was due to the significant P storage provided by the spodic horizon. Despite elevated soil P levels, Al and Fe added with biosolids reduced P solubility by as much as 55% compared with the control. Although flooding increased P leaching due to the reductive dissolution and hydrolysis of Al‐P and Fe‐P compounds, moisture regimen effects were generally negligible and inconsistent. Soil P storage capacity has been used as a risk assessment tool to determine biosolids‐P applications; however, our data demonstrated that additional field studies are needed to validate this concept, particularly its applicability and interpretation in biosolids‐amended soils that contain appreciable amounts of Fe‐ and Al‐organic complexes.