Fluctuating Water Table Effects On Phosphorus Release and Availability From a Florida Spodosol.

Spodosols in Florida exhibit a unique hydrology with fluctuating water table that can often reach the surface Ap horizon during the summer months. We evaluated the effects of fluctuating water table on P fluxes and availability from a typical Florida Spodosol. Treatments consisted of three applied P rates (0, 5 and 10 kg ha^-1) arranged in a completely randomized design. Five suction cup lysimeters were installed in each plot at depths of 15, 30, 60, 90, and 150 cm. The shallower two lysimeters (15 and 30 cm depths) were located above the spodic (Bh) horizon, whereas the remaining lysimeters (60, 90 and 150 cm) were below the Bh horizon. A pressure transducer was installed at the center of the experimental site to monitor changes in water table depth. Two anion exchange membranes (2 x 6 cm) were buried in each plot at 15 cm depth to estimate in-situ P availability over the growing season. The membranes were collected bi-weekly, extracted to determine soluble P, and new membranes installed. During the 2-yr study, regardless of the applied P rate, soluble P extracted by lysimeters above the Bh horizon increased as water tables rose in the months of August and September. In both years, P concentration extracted by lysimeters below the Bh horizon was not affected by P additions and remained relatively constant (0.02 mg L^-1) during the entire growing season. Treatments that received the greatest P rate (10 kg P ha^-1) showed greater available P in the early part of the growing season (June), but there was no treatment effect on P availability when water table rose in August. Our results showed that the fluctuating water table conditions experienced during the summer causes upward flux of P from the Bh horizon, which increase soil P bioavailability.