Interspecific and intraspecific variations in root phosphatase activity among tropical tree species with different soil phosphorus associations

Abstract

Variation in soil phosphorus (P) availability promotes niche differentiation among tropical tree species, yet the traits that underpin specialization on low-P and high-P soils (hereafter low-P and high-P species) remain poorly understood. Here, we examined interspecific and intraspecific variation in three types of root phosphatase enzymes and morphological traits among neotropical tree species in Panama with different habitat associations. We collected fine roots from 51 individual trees of four congeneric pairs of low-P and high-P neotropical tree species in the genera Cordia, Hirtella, Inga and Protium in forests on moderate to low-P soils. We determined root morphological traits (specific root length, diameter and root tissue density) and the root surface activities of phosphomonoesterase (PME), phosphodiesterase (PDE) and phytase (PHY) enzymes, which are synthesized to release inorganic orthophosphate from soil organic P. Soil P availability was determined by measuring resin-extractable P concentration for soils collected from the base of each tree. Low-P species allocated more resources to produce enzymes that decompose more complex forms of P, as indicated by greater PHY activity and greater PHY:PME and PDE:PME ratios at a given soil P availability. A principal component analysis of fine-root traits showed a greater Euclidian distance among individuals of low-P species than among those of high-P species, supporting the hypothesis that fine-root traits vary more among low-P species than among high-P species. Synthesis. These results suggest that the specialization of tropical tree species to low-P soil involves investment in the acquisition of complex soil organic phosphates such as phosphodiesters and phytic acid. This is possibly related to root trait divergence and indicates that variation in P acquisition strategies among tropical tree species could contribute to resource partitioning on low-P soils.