Mineralization of Nitrogen and Phosphorus from Soil Organic Matter in Southern Pine Plantations

Sustained weed control and/or annual applications of fertilizer have accelerated stand development of young slash pine ( Pinus elliottii Engelm. var. elliottii ) and loblolly pine ( P. taeda L.) growing on Spodosols in Florida. Productivity of these plantations will depend largely on the extent to which nutrients are recycled through soil organic matter derived from decomposition of fresh residues. We attempted to sample this newly formed soil organic matter and measured the effects of weed control and fertilizer treatments on N and P mineralization under laboratory conditions (42‐d aerobic incubation). Sequential in situ containments using interbed soil were used to provide information on field rates of mineralization. The laboratory study demonstrated that specific N mineralization was little affected by cultural treatment. In contrast, specific P mineralization was consistently increased by fertilizer application, both in the laboratory and in situ. This suggests that, in organic residues, the general composition of P substrates was altered by fertilizer application, while organic‐N substrates remained unaffected. The effect of weed control inhibited specific N and P mineralization in the laboratory, indicating that organic residues of understory vegetation are a better supply of available N and P than are organic residues of pine. In fertilized plots and for P, annual rates of in situ mineralization were large relative to demand (uptake) by vegetation. The recycling of P through organic residues of fertilized plots has the potential to enhance long‐term productivity beyond the immediate benefits derived from fertilizer uptake.