Retention of phosphorus in highly weathered soils under a lowland Amazonian forest ecosystem

The low available phosphorus ( P) pools typical of highly weathered tropical forest soils are thought to result from a combination of export of phosphorus via erosion and leaching as well as chemical reactions resulting in physically and chemically protected P compounds. Despite the low apparent P availability, these soils support some of the highest terrestrial net primary productivity globally. We followed different P fractions after P additions to two soil types, sandy loam and sandy clay, over 1 year in a lowland Amazonian forest. Of all the soil P fractions measured, only the NaHCO3 and NaOH extractable fractions showed a significant increase following P additions, and this occurred only in sandy clays (+ 56.9 +/- 15.1 kg ha(-1) and + 2.8 +/- 1.5 kg ha(-1), respectively). Our results indicate that intermediate rather than recalcitrant pools are the dominant fate of added P over an annual timescale even in fine-textured soils. Fine root and forest floor P pools increased more in the sandy loams following P additions suggesting a larger biotic P sink in these soils. Leaching of inorganic P from the surface soils was an unexpected and significant fate of added P in both soil types ( 9 +/- 3% in the sandy loams, 2 +/- 1% in the sandy clays). Significantly more of the added P was retained in the sandy clay soils than in the sandy loams ( 69 +/- 20% versus 33 +/- 7%) over the 1-year period.