NITROGEN AND PHOSPHORUS RELEASE FROM DECOMPOSING NEEDLES OF SOUTHERN PINE PLANTATIONS

Fertilizer application and weed-control treatments have increased biomass accumulation of loblolly (Pinus taeda L.) and slash pines (P. elliottii Engelm. var. elliottii) in north-central Florida. While the effects of these treatments have been immediate, the long-term influences on rates of nutrient cycling are unknown. This study measured rates of mass loss and changes in N and P contents in Oi horizon needles decomposing in the laboratory, and in Oi, Oe1, and Oe2 horizon needles decomposing in situ (litterbag study). The quality of organic-matter substrates in needles (determined by laboratory incubation) was increased by both weed-control and fertilizer treatments (P < 0.05), and litter quality of loblolly pine was greater than that of slash pine. However, these effects were not manifest in the field study. Across all treatments, an average of 31% of the original organic matter was lost after 1 yr of decomposition in situ. Generally, N was immobilized into Oi horizon needles but was released in modest amounts (< 15% yr-1) from Oe1 and Oe2 horizon needles. In contrast, substantial release of P from Oi horizon needles (> 35% yr-1) was followed by smaller release from Oe1 and Oe2 horizon needles (< 15% yr-1). More P was cycled per unit of organic matter in fertilized than nonfertilized plots, and loblolly pine cycled more P than slash pine. Changes in total P content of needles during decomposition were accounted for by changes in the pool of inorganic P. The rapid recycling of P in fertilized plots has the potential to enhance long-term productivity beyond the immediately beneficial effects derived from fertilizer uptake.