Nitrogen and Phosphorus Release from Decomposing Needles of Southern Pine Plantations

Abstract

AbstractFertilizer 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.