Variability in potential to exploit different soil organic phosphorus compounds among tropical montane tree species

We hypothesized that tropical plant species with different mycorrhizal associations reduce competition for soil phosphorus (P) by specializing to exploit different soil organic P compounds. We assayed the activity of root/mycorrhizal phosphatase enzymes of four tree species with contrasting root symbiotic relationships - arbuscular mycorrhizal (AM) (angiosperm and conifer), ectomycorrhizal (EM) and non-mycorrhizal - collected from one of three soil sites within a montane tropical forest. We also measured growth and foliar P of these seedlings in an experiment with P provided exclusively as inorganic orthophosphate, a simple phosphomonoester (glucose phosphate), a phosphodiester (RNA), phytate (the sodium salt of myo-inositol hexakisphosphate) or a no-P control. The EM tree species expressed twice the phosphomonoesterase activity as the AM tree species, but had similar phosphodiesterase activity. The non-mycorrhizal Proteaceae tree had markedly greater activity of both enzymes than the mycorrhizal tree species, with root clusters expressing greater phosphomonoesterase activity than fine roots. Both the mycorrhizal and non-mycorrhizal tree species contained significantly greater foliar P than in no-P controls when limited to inorganic phosphate, glucose phosphate and RNA. The EM species did not perform better than the AM tree species when limited to organic P in any form. In contrast, the non-mycorrhizal Proteaceae tree was the only species capable of exploiting phytate, with nearly three times the leaf area and more than twice the foliar P of the no-P control. Our results suggest that AM and EM tree species exploit similar forms of P, despite differences in phosphomonoesterase activity. In contrast, the mycorrhizal tree species and non-mycorrhizal Proteaceae appear to differ in their ability to exploit phytate. We conclude that resource partitioning of soil P plays a coarse but potentially ecologically important role in fostering the coexistence of tree species in tropical montane forests.