Soil Properties and Tree Productivity Following a Decade of Nutrient Addition to a Lowland Tropical Rain Forest.

We report changes in soil properties and tree productivity following 10 years of factorial combinations of nitrogen, phosphorus, and potassium to a lowland tropical rain forest in central Panama. The soil is an Oxisol (Typic Eutrudox) developed on Miocene basalt and supports mature (> 200 year old) secondary forest of high diversity. The site receives approximately 2300 mm of annual rainfall, with a strong four month dry season between January and April. Fertilizer has been applied since 1998 to 40 x 40 m plots at annual rates of 125 kg N/ha, 50 kg P/ha, and 50 kg K/ha. Based on monthly measurements over an annual cycle in 2006–2007, fertilization has approximately doubled the concentrations of inorganic nitrogen and extractable potassium compared to control plots, while extractable phosphate has increased from < 1 mg P/kg in control plots to 30 mg P/kg in phosphorus addition plots. Nitrogen addition reduced soil pH and base saturation, presumably because nitrogen was added as coated urea. The relative growth rate of trees declined significantly between 1997–2000 and 2000–2008, a trend reported recently for tropical forests worldwide. This decline was ameliorated by nitrogen plus potassium addition for trees < 10 cm diameter at breast height, and by phosphorus addition for larger diameter trees. Fine root growth declined in response to potassium addition, while total fine litterfall increased in response to phosphorus addition. These results provide the most comprehensive assessment to date of the nutrient status of a lowland tropical rain forest and indicate that no single nutrient limits tree productivity.