Variation in soil phosphorus (P) promotes niche differentiation among tropical tree species, yet the morphological and physiological traits that underpin the specialization of species to low-P and high-P soils (hereafter low- and high-P species) remain poorly understood. We tested the hypotheses that fine-roots of low-P species exhibit greater phosphatase activity (phosphomonoesterase, phosphodiesterase, and phytase), specific root length (root length per root weight), and mycorrhizal infection rate, compared with high-P species.

We collected fine roots (< 2 mm diameter) from 51 individual trees of four congeneric pairs of low and high-P neotropical tree species in the genera Cordia, Hirtella, Inga, and Protium, growing on medium to low-P soils. Bulk soil was collected from the base of each tree to quantify resin-extractable soil P concentration as an indicator of P availability.

Phytase activity was greater at lower soil P availability, yet phosphomonoesterase and phosphodiesterase did not vary with soil P availability. In line with the hypothesis, low-P species allocate more resources to enzymes that decompose more recalcitrant forms of P, as indicated by greater phytase activity, and greater phytase:monoesterase and diesterase:monoesterase ratios at a given resin P availability. Overall, these results suggest that the specialization of tropical tree species to low-P soils significantly alters P acquisition mechanisms.