Effects of Drought on the Chemistry and Isotopic Composition of Soil Waters in Tropical Forests of Puerto Rico

Abstract:

As part of the U.S. Department of Energy, Office of Science, Next Generation Ecosystem Experiments-Tropics we have been monitoring soil water chemistry and stable isotopes at three sites (Sabana, El Verde, and Icacos) in the El Yunque National Forest in eastern Puerto Rico. One goal is to investigate how intra- and inter-site differences (e.g., bedrock and vegetation types, and catchment position) affect soil water biogeochemistry and hydrology. We are also interested in quantifying variability in geochemistry and isotopes over time. Soil waters are collected quarterly at multiple locations at each site using macro-Rhizon samplers. Using syringes attached to the Rhizon samplers we are able to vacuum extract waters directly from the soil with minimal disturbance. Samplers are installed at 10- and 30-cm depths at each location (a few samplers are installed at 70 cm at Icacos). Samples are analyzed for anions, major cations, and stable isotopes (δ¹⁸O and δ²H). Monitoring began in March, 2016 and since the network was established dry conditions have prevailed, and drought conditions were quite strong in 2015 prior to this study. The prolonged drought thus provides an opportunity to examine how pore water chemistries and isotope compositions are affected and we are particularly interested in how nutrient concentrations vary over time. This information will be useful for developing conceptual and mathematical models to understand impacts of drought on tropical forests especially because drought frequency and severity is expected to increase. In this presentation we will discuss how a large suite of biogeochemical parameters have varied through the current drought and how conditions at the three sites serve to modify drought effects.