Nutrient Dynamics in Slash Pine Plantation Ecosystems

Changes in the distribution and cycling of N, P, K, Mg, and Ca were investigated in plantations of splash pine (Pinus elliottii var. elliottii) using an age sequence of replicated stands 2, 5, 8, 14, 18, 26 and 34 yr old. All stands had a similar history of management and occurred within 20 km on somewhat poorly drained Ultic Haplaquod soils in northern Florida. Because of the rapid colonization of these sites by grasses, plant nutrient uptake was already high at age 2 yr. Thereafter, uptake of N, P, K, and Mg increased <50% to a maximum at 8 yr, around the time that maximum leaf area for these stands is reached. Ca uptake increased 130% over this time. Uptake declined after age 8 yr for all elements. Nutrient return in litterfall remained relatively constant after 5 yr. Canopy leaching, estimated as the difference between bulk precipitation and throughfall, was negligible for N and P, but highly significant for all three cations. K leaching from the canopy was greatest and amounted to °60% of litterfall in mature stands. Internal redistribution of mobile elements (N, P, K) in the vegetation was negligible until the pine canopy closed, but then increased greatly. From 2 through 26 yr, vegetation nutrient contents increased much more than nutrient uptake rates. The surface soil of the youngest stands appeared to be enriched by material added during site preparation. The nutrient content of the Bh horizon increased through the 8th yr, indicating a downward movement of the added material. Soil contents (to 100 cm depth) of total N and double—acid extractable P and cations then decreased over time, with the greatest decline during the first 18 yr. Forest floor contents increased at a constant rate, with no plateau indicated through 34 yr. Annual precipitation inputs of nutrients varied from 0.3 kg/ha for P to 13.5 kg/ha for N, with cation values of 2.6—5.0 kg/ha. Annual ecosystem balances of N, P, K, and Ca appeared slightly positive, while the content of Mg appeared to decline. Nutrient use efficiencies (net primary production ° uptake) increased greatly with time and were very high in older stands, especially for P. However, it appears that mineralization processes in these very nutrient—poor systems and the small atmospheric inputs cannot supply the annual nutrient requirements of mature stands, especially for P; this leads to a cessation of vegetative biomass increment around age 26 yr.