Water table effect on phosphorus solubility in biosolids-amended soils

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.