Effects of fertilisation on phosphorus pools in the volcanic soil of a managed tropical forest

Acacia koa forests benefit from phosphorus fertilisation, but it is unknown if fertilisation is a short or long term effect on P availability. Past research suggests that P cycling in soils with high P sorption capacity, such as Andisols, was through organic pathways. We studied leaf P and soil P fractions in a tropical forest Andisol for 3 years after fertilisation with triple super phosphate. Leaf P concentration and labile P remained high after fertilisation. Fertilisation had increased all the inorganic P fractions over the length of the study, while organic P fractions had not. The results suggested that the organic P fractions had a reduced role as a source of labile P after fertilisation. The size and dynamics of the sodium hydroxide- and hydrochloric acid-extractable P pools would suggest that either pool could be major sources of labile P. Because of the high level of poorly crystalline minerals in Andisols (allophone and imogolite), it would be expected that applied P would quickly lead to strong P sorption onto mineral surfaces and thus a rapid decline in P availability. We propose that the high organic matter present in these soils had masked some of the Al and Fe sorption sites, which allowed the sorption and desorption when large amounts of addition P were applied.