Insights from a Long-term, Factorial Nitrogen, Phosphorus, and Potassium Addition Experiment Conducted in a Mature Lowland Tropical Forest in the Barro Colorado Nature Monument

We initiated a factorial nitrogen-phosphorus-potassium addition experiment in old-growth forest growing on an infertile Oxisol in 1998. The experiment provides clear evidence that each added nutrient limits multiple plant and ecosystem functions. All three added nutrients limit tissue nutrient concentrations, allocation to roots, and seedling growth. Phosphorus also limits soil microbial biomass, stand-level fine litter productivity, reproductive effort by trees, and soil and litter invertebrate abundance. Potassium also limits stomatal function and soil and litter invertebrate abundance. The experiment provides no evidence for diversity declines associated with eutrophication. Rather, added nutrients increased soil microbial diversity and soil and litter invertebrate diversity. Going forward, we anticipate additional lagged responses by long-lived tree species adapted to infertile soil and a shift in tree species composition to species adapted to more fertile soils. We plan further studies to explore how tropical forests maintain high productivity despite impoverished soils.