Tuesday, November 14, 2006 - 11:00 AM

Pasture Water Management for Reduced Phosphorus Loading in the Lake Okeechobee Watershed.

Patrick Bohlen, MacArthur Agro-Ecology Research Ctr, 550 Buck Island Ranch Rd, Lake Placid, FL 33852, Odi Villapando, South Florida Water Mgmt District, Lake Okeechobee Division MSC 4430, South FL Water Mgt Dist 3301 Gun Club Road, West Palm Beach, FL 33406, Ken Campbell, Univ of Florida, 105 Frazier Rogers Hall, PO Box 110570, Gainesville, FL 33852, and John Capece, Southern Data Streams, 132 North Lee St., LaBelle, FL 33935.

Improved pastures on beef cattle ranches are the biggest source of phosphorus (P) in the watershed draining into Lake Okeechobee, a critical link in the greater Kissimmee-Okeechobee-Everglades ecosystem. We used an array of eight 20-ha experimental pastures to test the idea that retaining water in drainage ditches could reduce P runoff, as predicted by hydrologic models. In one block of 4 pastures we installed water detention structures and set the riser boards approximately 15 cm below pasture surface; no such structures were intalled in the adjacent block of four pastures, which served as controls. Flumes at the outlet of each pasture were used to monitor total amounts of surface runoff and autosamplers collected surface water for nutrient analysis (TP, TKN, NH4+, NO3-). Total water runoff from pastures with water detention structures was 9% less than the runoff from pastures without structures. However, the concentration of total P was 38% greater in runoff from pastures with water control structures. Consequently, there was a 39% increase in total P loads from pastures with water retention structures compared to pastures without structures, contrary to our original predictions. The concentration of Mehlich-1 P in surface soil was nearly two times greater in pastures with water control structures, indicating that the greater P loads in surface runoff were caused by increased soil P release in pastures where surface runoff was detained. Results from the first year of our study indicate that detention of surface runoff in drainage ditches may not be an effective method for reducing P loads from pastures in this region. Continued collection of data from these sites and from other sites with different hydrologic conditions will be needed to determine if pasture water management can be a useful approach for reducing P loads in this region.