Root functional traits and microbial communities modulate phosphorus acquisition. However, the mechanisms driving root and microbial function are not clear, especially how they relate to model improvement. Our overall goal is to understand the interactions between root traits and microbial communities in order to improve the representation of nutrient controls over forest productivity. Three sites: Rio Icacos (ICA), El Verde Ridge (EVR), and El Verde Valley (EVV), in Luquillo Forest in Puerto Rico were chosen to represent a range of phosphorus availability as measured by the resin P method. Fine-root chemical and morphological traits, root phosphatase, and microbial community composition were compared between dominant tree species. Root cores were collected to determine root and nutrient depth distribution. Root architecture, morphology, and chemical composition were measured in roots traced from the base of each tree. A colorimetric assay using para-nitrophenyl phosphate (pNPP) as a substrate was used to measure root phosphatase activity of first and second order roots from dominant tree species in ICA, EVR, and EVV. Microbial phosphatase activity was also measured using pNPP from isolated bacteria in EVR and EVV. Lastly, 16S rRNA was sequenced from rhizosphere soil in trees within EVR and EVV to assess bacterial community composition.
Resin P values were 0.20 μg g-1 soil for ICA, 0.40 μg g-1 soil for EVR, and 0.60 μg g-1 soil for EVV. In EVR and EVV, the dominant tree species are Dacryodes excelsa and Prestoea montana. D. excelsa is prominent within the litter layer, whereas P. montana is evenly distributed down the soil profile (0-30 cm). Phosphorus in roots range from 1.2 – 4.5 mg g-1 root with D. excelsa having the highest phosphorus concentration. Root phosphatase activity in both EVR and EVV was highest in D. excelsa, averaging 10 mmole pNP g-1 root h-1 compared to 1 - 4 mmole pNP g-1 root h1 in P. montana. Averaging root phosphatase activity in ICA, EVR, and EVV showed a strong negative correlation between resin P and root phosphatase. Microbial community composition indicated little variation between EVR and EVV with most of the bacterial community from the two phyla: Proteobacteria and Acidobacteria. Microbial isolate phosphatase activity was highest in isolates from D. excelsa than from P. montana. These results suggest a connection between root characteristics and root and microbial phosphatase activity. Understanding these trait relationships may shed light on combinations that determine phosphorus acquisition in future environments.