Stable soil phosphorus transformations following long-term litter manipulation in a lowland tropical forest

Productivity in tropical forests is limited by phosphorus (P) availability, but much of the P in tropical soils occurs in stable forms of limited biological availability. Phosphorus availability is therefore maintained in part by recycling from litterfall, which is susceptible to alterations through environmental change. However, the extent to which litterfall influences more stable forms of soil P remains unclear. We quantified the chemical nature of P in soils from a long-term litter manipulation experiment in lowland tropical forest in Panama, in which soils were subject to a decade of litter removal or addition. We used sequential fractionation to separate soil P into operationally defined inorganic and organic pools of contrasting stability. Labile inorganic P extracted in 0.5 M NaHCO 3 increased three-fold with litter addition, but there was no change in labile organic P, nor in moderately labile P pools extracted in 0.1 M NaOH. Surprisingly, residual (unextracted) P and stable organic P extracted in 0.25 M NaOH + 0.05 M EDTA increased strongly with litter addition and declined with litter removal. However, the distribution of P fractions as a proportion of the total P remained relatively constant. These findings show that long-term change in litterfall influences both available and stable forms of soil P in tropical forests, even though the relative distribution of P forms remains largely unchanged. The large pool of stable P in strongly weathered tropical soils has the potential to support greater long-term productivity as forests exploit the modern CO 2 -enriched atmosphere. • Long-term litter manipulation in a lowland tropical forest changed labile and stable forms of soil phosphorus. • Stable organic and residual phosphorus increased with litter addition and declined with litter removal. • Stable phosphorus is dynamic and could increase in tropical forest productivity in response to rising atmospheric CO 2 .