Multi-Rotation Effects of Fertilization and Competition Control on Tree Growth and C, N, and P Dynamics in a Loblolly Pine Plantation.

Multi-rotation effects of fertilization, weed control and the combined application of these treatments on C, N and P dynamics and growth were examined for a loblolly pine (Pinus taeda L.) plantation in north-central Florida. In the first rotation, we assessed 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 the end of rotation C, N and P accumulation. At the beginning of the second rotation, we determined the carry-over (CO) effect on tree growth and nutrient dynamics for the weed control (COW), and DAP fertilized (COF and COFW) treatments, and also for re-fertilized F_TSP and F_TSPW plots. The soils at the site are classified as sandy, siliceous, hyperthermic Ultic Alaquods. Forest floor samples were collected at the end of 25 year rotation and soils sampled at depth intervals of 0-10, 10-20, 20-50 and 50-100 cm at the end of rotation and at the start of the second rotation.

In the first rotation, F_DAPW and F_TSPW significantly increased total aboveground biomass by 20% and 18%, respectively, above the control. F_TSP significantly increased N accumulation in the forest floor (ƿ=0.021) but decreased it in the soil (20-50 cm) (ƿ=0.038). F_DAP also increased P accumulation in the forest floor (Oi, ƿ<.0001; Oe+Oa, ƿ=0.010) but W decreased it (Oe+Oa, ƿ=0.091). F_TSP increased P accumulation in the forest floor (Oi, ƿ=0.072; Oe+Oa, ƿ=0.002).

In the second rotation, the COW had a negative (ƿ<.0001) and re-fertilization a positive effect (ƿ<.0001) on tree growth. COW and COF had greater N mineralization in the 0-10 cm (ƿ=0.008) and the 20-50 cm (ƿ=0.059) depths. Re-fertilized F_TSP (ƿ=0.048) and F_TSPW (ƿ=0.083) had greater N mineralization in the 10-20 cm depth than the control. For P mineralization, F_DAP (ƿ=0.091) and W (ƿ=0.095) had negative effect in 20-50cm depth and the retreated F_TSP had a negative effect in 0-10 cm depth (ƿ=0.040).