Biological and Geochemical Sinks for Phosphorus in Soil from a Wet Tropical Forest

In many tropical and volcanic soils, phosphorus (P) availability is strongly influenced by geochemical sorption, which binds P to soil minerals. The aim of this study was to determine whether biological demand or soil sorption strength was the primary control over phosphate availability and retention in a wet tropical soil with high sorption capacity and low P availability. We added ³²PO₄ to soil from the upper two horizons and assessed the ability of soil microbes to immobilize the added phosphate in the presence of strong sorption. We added phosphate at two concentrations, one representing background turnover that adds low concentrations of P to the soil solution, and the other representing nutrient pulses that can add fairly high fluxes of P to the soil solution. Sorption and microbial immobilization were rapid for both concentrations, consuming most added P within 30 min. Thus, little P remained in the soil solution or extractable pools, which are considered more available to plants. Although soil sorption strength was almost identical for the two horizons, immobilization of tracer P was approximately three times greater in the upper horizon, where most microbial activity was located. This result suggests that microbial demand controlled how P was partitioned into biological versus geochemical sinks. Further evidence for microbial control is suggested by the movement of tracer P from the sorbed pool into the microbial pool when demand was stimulated by the addition of carbon (C). We also explored how increased nitrogen (N) and P availability changed P dynamics in this nutrient poor soil. In contrast to the unfertilized soil, long-term N and P fertilization substantially reduced biological control over inorganic P. P fertilization saturated the soils, overwhelming biological P demand, whereas N fertilization appeared to increase available P through reduced P sorption. Where biological demand for P is high and P becomes available in the soil solution, microbes may play an important role in controlling P partitioning into biological versus geochemical sinks even in soils that have high sorption capacity.