Leaf Litter Leachate Reduces Soil Phosphate Sorption in Lowland Tropical Forest Soils.

In lowland tropical forests, dissolved organic carbon (DOC) in leaf litter leachate may promote phosphate availability through competition for soil sorption sites. Using phosphorus-32 as a tracer, we investigated the effect of litter leachate from five tree species (Albizia gauchabele, Anacardium excelsum, Cecropia peltata, Castilla elastica, Ficus insipida) on phosphate sorption in three tropical forest soils from the Republic of Panama. Soils differed in total carbon content due to six years of litter manipulation treatments. Sorption of phosphorus-32 and DOC from litter leachate (extracted for 4 hours in a 1:30 litter/solution ratio) was evaluated over a 1-hour period with a simulated throughfall solution as a control. Solutions were spiked with five phosphate levels (0-8 ug P ml^-1) to account for covariance of phosphorus-32 sorption with initial phosphate concentration. Phosphate sorption was high for all soils (>98%), but sorption was markedly greater for soil with the lowest carbon content (litter removal treatment). For this soil, leachate from all species decreased phosphate sorption compared to the throughfall solution. In contrast, only three species decreased phosphate sorption for the soil with the highest carbon concentration. Total sorption of DOC during the experiment was significantly correlated with phosphorus-32 remaining in supernatant, but only when A. excelsum was excluded from the analysis – leachate from this species showed the highest absolute sorption of DOC but had little effect on phosphate sorption. Further experiments on additional tropical soils showed that some low molecular weight organic acids (LMWOA), specifically citrate and oxalate, strongly reduced phosphorus-32 sorption, yet tannic acid had no effect even though more than 80% of the initial DOC was sorbed. In summary, litter leachate can decrease phosphate sorption to soil which may contribute to phosphorus availability in tropical forests. However, this appears to be regulated by differences in chemical composition of DOC in the leachate.