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

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 (P-o) was similar to 54% of extractable P. Labile P, defined as P-o plus inorganic P (P-i) extracted by NaHCO3, was largely P-o (84% of the NaHCO3-extractable P). Chronic P addition increased NaHCO3-extractable P-o several-fold and NaOH-extractable P-i 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 similar to 15% higher in surface than subsurface soil. We added 350 kg P ha(-1) during the 7-year period and recovered similar to 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.