Our Experiment

Trop-Soil is a nutrient addition experiment being conducted at EARTH University Forest Reserve in the Caribbean lowlands of Costa Rica. The purpose of this experiment is to explore how nitrogen (N) and phosphorus (P) additions influence biological processes, such as net primary productivity (NPP), decomposition of organic matter, and emissions of greenhouse gases from the forest soil.

Experimental Design

In May 2007, we established 24 30 x 30 m plots and randomly assigned them three fertilizer treatments or a control in a complete block design (n = 6). In addition to the six control plots, the three treatments include +P (47 kg ha-1yr-1 of P as triple super phosphate), +N (100 kg ha-1yr-1 of N applied as ammonium nitrate and urea), and +NP (N and P added together in quantities as in +N and +P plots). Fertilizer is broadcast by hand twice per year on the surface of the 900 m2 plots. All measurements are restricted only to the central 400 m2 of each plot (20 x 20 m) to reduce edge effects, and all plots are at least 50 m apart to avoid between-plot contamination. Each plot includes at least one Gavilan tree (Pentaclethra macroloba) and one canopy Socratea palm (Socratea exohrriza), two of the most common species in this forest.

We have measured multiple parameters in these plots including tree growth, litterfall productivity, root biomass and growth, and foliar, litter, and root chemistry. We have also measured soil parameters such as pH and concentrations of various elements (e.g., nitrate, ammonium, phosphate, base cations). We are currently conducting measurements of greenhouse gas emissions from forest floors, which include carbon dioxide, methane and nitrous oxide.