Soil phosphorus responses to chronic nutrient fertilisation and seasonal drought in a humid lowland forest, Panama

M. J. Mirabello, Cornell University
J. B. Yavitt, Cornell University
M. Garcia, Smithsonian Tropical Research Institute
K. E. Harms, Smithsonian Tropical Research Institute
B. L. Turner, Smithsonian Tropical Research Institute
S. J. Wright, Smithsonian Tropical Research Institute

Abstract

We used the Hedley sequential fractionation scheme to assess phosphorus (P) chemistry of a strongly weathered soil from a humid lowland forest in Panama. Our analyses were part of a factorial experiment of nitrogen, P, and potassium addition, with nutrients added annually, i.e. a chronic input. The aim was to examine changes in soil P chemistry with 7 years of nutrient addition for soils collected in the wet season and the dry season. The majority of P occurred in fractions extracted by NaOH (24% of the total soil P) and hot concentrated HCl (58% of the total). Organic P (Po) was ∼54% of extractable P. Labile P, defined as Po plus inorganic P (Pi) extracted by NaHCO 3, was largely Po (84% of the NaHCO3- extractable P). Chronic P addition increased NaHCO3-extractable Po several-fold and NaOH-extractable Pi two-fold. Seasonal variation occurred for labile P and NaOH-extractable P, whereas occluded P did not vary throughout the study period. Extractable P was ∼15% higher in surface than subsurface soil. We added 350kgPha-1 during the 7-year period and recovered ∼55% by sequential extraction. According to biogeochemical theory, added P should show up in fractions with the shortest residence times, e.g. labile P. Our finding that added P accumulated in fractions with presumably long residence times, i.e. extracted by NaOH (bound) and hot concentrated HCl (occluded), suggests that greater attention be paid to the short-term dynamics of bound and occluded P in strongly weathered tropical forest soils. © 2013 CSIRO.