Phosphorus Fertilizer Fate 27 Years After Exceptionally High Establishment Rates in a Loblolly Pine Stand in the Western Gulf Coastal Plain.

Fertilizer phosphorus (P) is retained relatively well on most forest sites and has been shown to remain available well into a second rotation in southern pine stands as well as other plantation systems.  In 1982, a study was installed to determine the optimal P fertilizer rate for loblolly pine bioenergy plantations on a P-deficient silt loam soil in the western Gulf coastal plain.  Four P fertilizer rates were applied at the time of study establishment that represent much higher than normal fertilizer rates: 0, 81, 162, and 324 kg P ha^-1, respectively.  The objectives of this study were to determine the fate and potential availability of the fertilizer P after 26 years.  In 2008, the soils were sampled at five depths (0-10, 10-20, 20-50, 50-75 and 75-100 cm, respectively) and soil P was fractionated into labile, weakly held, strongly held, and occluded using a modified Hedley fractionation.  Aboveground biomass (pine and understory), coarse woody debris, and forest floor P were estimated or determined to elucidate the final fate of the P fertilizer.  Total fertilizer recovery in soil, plant biomass, coarse woody debris, and forest floor ranged from 86% in the 81 kg/ha P treatment to 40% in the 324 kg ha^-1 P treatment. If the stands were harvested at this point and 100% of pine biomass P was removed, 38, 77, and 96 kg/ha fertilizer P would remain present in the 81, 162, and 324 kg ha^-1 P treatments, respectively. Soil P was significantly greater only in the surface 20 cm of the fertilized soils; no additional fertilizer P was found deeper than 20 cm in any fraction.  Fertilizer P was primarily contained in the labile organic, weakly held inorganic, weakly held organic, and residual pools.  Available (Mehlich I) P was low in the surface 20 cm of the unfertilized plots (2.99 kg ha^-1) and was significantly greater on the 162 and 324 kg/ha plots (9.62 and 8.81 kg ha^-1, respectively). This study indicates that one-time fertilizer rates higher than 81 kg ha^-1 P do not efficiently retain fertilizer after a single rotation as compared to lower fertilizer rates, which, as expected, retained much of the fertilizer P in relatively labile or easily accessible fractions in the surface soil.