PHOSPHORUS, NITROGEN, AND CARBON FRACTIONS IN LITTER AND SOIL OF SOUTHERN PINE PLANTATIONS

The long-term productivity of intensively managed loblolly (Pinus taeda L.) and slash pine (P. elliottii Engelm. var. elliottii) plantations will be determined largely by the effect of silvicultural practices on subsequent release of N and P from litter and soil. This study characterized labile P, N, and organic-C fractions in the Oi, Oe1, and Oe2 horizons of needle litter and the A horizon of soil from plots receiving weed control, fertilizer treatments, or both. Because fertilizer had been placed in localized bands around the base of each tree, samples could be collected away from these areas and analyzed without interference from residual fertilizer. Samples were analyzed for inorganic P (P(i)), organic P (P(o)), organic N (N(o)), sugars, and phenols in a sequential-extraction procedure of cold, then hot, trichloroacetic acid (TCA). Inorganic P was about 40% of total P in Oi horizon needles of reference (untreated) plots, and about 75% of total P in fertilized plots. In loblolly pine, addition of fertilizer caused the concentration of P in Oi horizon needles to about double, this increase being accounted for by accumulation of P(i). Total P decreased between the Oi and Oe1 horizons of litter, indicating a large net release of P during the early stages of decomposition. This depletion in total P was wholly accounted for by depletion of P(i). Only about 20% of the total N in Oi horizon needles was labile (soluble in TCA), and N fractions were unaffected by cultural treatments. Total N concentration increased during decomposition, mostly through progressive accumulation of the residual (insoluble) N fraction.