A glaring uncertainty in terrestrial ecosystem models is the response of tropical forests to altered climatic regimes such as the increased severity and duration of droughts. Models predicting drought mortality typically exclude root and microbial functions that may alleviate water stress, to the detriment of acquiring phosphorus, typically a limiting nutrient in tropical forests. We conducted a greenhouse study using the facultative CAM tree species, Clusia pratensis to explore how drought may impact phosphorus acquisition via changes in root and microbial functional traits. Bacteria isolated from the rhizosphere of tropical trees were selected based on measured activity in four areas: phosphorus solubilization, phosphatase activity, IAA production, and drought tolerance. The full constructed community consists of eight bacterial isolates with the highest measured activities. A total of 80 plants were inoculated with one of three treatments, which differed in the bacterial isolates included. We expect that plants inoculated with the full constructed community will exhibit reduced levels of lipid peroxidation in response to drought and decreased levels of root phosphatase activity in response to phosphorus limitation. Understanding changes to root and microbial functional traits improves the comprehension of belowground mechanisms that may improve the accuracy of modeling tropical forests.
Results/Conclusions
We chose C. pratensis because it reliably switches from C3 to CAM photosynthesis under drought conditions, providing a unique opportunity to explore how photosynthesis relates to root and bacterial functional traits critical to phosphorus acquisition and drought tolerance. Preliminary drought trials showed the expected decline of C3 photosynthesis during the day, followed by increased activity in the evening. We then selected bacterial isolates with high phosphorus solubilization and phosphatase activity. Phosphorus solubilization isolates, VT46 and RP53, solubilized 101.68 μg P ml-1 and 35.16 μg P ml-1 respectively. RT90 (0.176 μmol pNP ml-1) and RP35 (0.149 μmol pNP ml-1) had the highest phosphatase activity of the 100 isolates tested. We then selected drought tolerant bacteria using polyethylene glycol, VT37 (OD600 = 0.48 ) and VT13 (OD600 = 0.55 ). The last two bacterial community members were isolates RT91 and VP93, which produced 0.051 ug IAA ml-1 and 0.044 ug IAA. Further measurements will include sequencing these isolates, relating changes in their composition to physical and physiological measurements of root traits, and connecting both root and bacterial function to indicators of drought tolerance and phosphorus acquisition.