Ignacie Tumushime, Eric J. Jokela, and Jason G. Vogel

The end of rotation effects of fertilization and weed control on C, N, and P accumulation and retention was examined for a managed loblolly pine (Pinus taeda L.) forest in central Florida. The objective of this study was to determine the effect of fertilization with different nitrogen and phosphorus fertilizers (diammonium phosphate (F_DAP; 250 kg N ha^-1, 90 kg P ha^-1); triple superphosphate (F_TSP; 200 kg N ha^-1, 90 kg P ha^-1), weed control (W) and the combined application of these treatments (F_DAPW or F_TSPW) on C accumulation, and N and P accumulation and retention for a managed forest. The soils at the site are classified as sandy, siliceous, hyperthermic Ultic Alaquods. The forest was harvested at age 25 years and tree and understory biomass sampled. The forest floor and four soil depth intervals (0-10 cm, 10-20 cm, 20-50 cm, and 50-100 cm) were also sampled. F_DAPW and F_TSPW significantly increased total aboveground biomass 20% and 18%, respectively, above the control. These combined treatments also had greater accumulation of N and P accumulation than the control in plant tissues, but not for the forest floor. For plant tissues, the type of fertilizer was not significant for any treatment although nutrient concentrations were increased by all fertilizer combinations.  Soil extractions of NH₄⁺ and NO₃^- (KCl) and PO₄^3- (Mehlich III) indicated few significant differences among treatment except for PO₄^3-, which was significantly greater in the F_DAPW than the F_TSPW treatments in the 20-50 cm soil layer. This suggests greater PO₄^3- movement to deeper depths with the use of DAP than TSP, which might indicate varying levels of P retention. Ongoing mass balance analysis is being used to determine how intensive silvicultural treatments will affect N and P retention at this site.