Soluble reactive phosphorus (SRP) transport and retention in tropical, rain forest streams draining a volcanic landscape in Costa Rica: in situ SRP amendment to streams and laboratory studies

Soluble reactive phosphorus (SRP) transport/retention was determined in two rain forest streams (Salto, Pantano) draining La Selva Biological Station, Costa Rica. There, SRP levels can be naturally high due to groundwater enriched by geothermal activity within the surfically dormant volcanic landscape, and subsequently discharged at ambient temperature. Combined field and laboratory approaches simulated high but natural geothermal SRP input with the objective of estimating the magnitude of amended SRP retention within high and low SRP settings and determining the underlying mechanisms of SRP retention. First, we examined short-term SRP retention/transport using combined SRP-conservative tracer additions at high natural in situ concentrations. Second, we attempted to observe a DIN response during SRP amendment as an indicator of biological uptake. Third, we determined SRP release/retention using laboratory sediment assays under control and biologically inhibited conditions. Short-term in situ tracer-SRP additions indicated retention in both naturally high and low SRP reaches. Retention of added SRP mass in Upper Salto (low SRP) was 17% (7.5 mg-P m⁻² h⁻¹), and 20% (10.9 mg-P m⁻² h⁻¹) in Lower Salto (high SRP). No DIN response in either nitrate or ammonium was observed. Laboratory assays using fresh Lower Salto sediments indicated SRP release (15.4 ± 5.9 μg-P g dry wt.⁻¹ h⁻¹), when incubated in filter sterilized Salto water at ambient P concentration, but retention when incubated in filter sterilized river water amended to 2.0 mg SRP l⁻¹ (233.2 ± 5.8 μg-P g dry wt.⁻¹ h⁻¹). SRP uptake/release was similar in both control- and biocide-treated sediments indicating predominantly abiotic retention. High SRP retention even under biologically saturated conditions, absence of a DIN response to amendment, patterns of desorption following amendment, and similar patterns of retention and release under control and biologically inhibited conditions all indicated predominantly abiotic P flux.