CONTROL OF PHOSPHORUS DYNAMICS BETWEEN SOIL, GROUNDWATER, AND STREAM COMPARTMENTS OF A MONTANE TROPICAL WATERSHED

Phosphorus (P) sorption is a process known to occur in groundwater, soils, and stream water. Much research has focused on better understanding the sorption capacity of sediments which has been found to be influenced by factors including: pH, redox potential, grain size, and sediment composition. However, the role of this process at the interfaces of soil, groundwater, and stream water compartments and how this influences P cycling between compartments is not well understood. A series of longitudinal transects from the Icacos River main stem and selected tributaries, soils data, and well data from the Icacos Watershed in the Luquillo Experimental Forest Critical Zone Observatory in Puerto Rico were used to compare phosphate (PO4) and other elements (Ca, Al, Fe, Mn) between the compartments to provide insight on processing of PO4 at the interfaces. The highly-weathered, fine-textured soils, and high frequency erosion events promote transfer of PO4 between soils and stream channels and groundwater inputs along stream banks and within channels are prevalent. Small differences between groundwater and main stem Icacos River concentrations of phosphate (1.67 ug/L), suggest P sorption does not play a strong role at this interface. However, large differences in PO4 concenctrations between the groundwater and the Icacos tributaries (6.78 ug/L) suggests P sorption plays a big role in the groundwater-stream water interface of tributaries. Further multivariate analysis of data suggests that calcium plays the biggest role in PO4 fixation via sorption in the main stem, and that aluminum plays the biggest role in PO4 fixation in the tributaries. Comparing soil PO4 concentrations and sorption capacity will help elucidate this compartment's role in PO4 cycling in the Icacos watershed. Better understanding of P processing and movement between these compartments contributes to our ability to create models, budgets, and efficient management practices for phosphorus.