A geochemical equilibrium model was used to quantify Dissolved Organic Carbon (DOC) electron donors during aerobic respiration and denitrification in a tropical stream riparian zone of the Luquillo Experimental Forest, Puerto Rico. DOC electron donors were measured across three general redox zones (Oxic: slope, Transitional: slope-riparian interface and Anoxic: riparian-floodplain) of the Icacos watershed. Model results suggest that nitrate and oxygen are completely reduced after approximately 10.1 mg/L of DOC have reacted with an initial ground water solution. In order to reach the observed mean oxygen concentration of 3.79 mg/L in the Oxic zone from the modeled equilibrium oxygen concentration of 9.46 mg/L, approximately 5.33 mg/L of DOC need to be oxidized. Additionally, 2.06 mg/L of DOC are oxidized in order to reach the observed mean oxygen concentration of 1.6 mg/L in the Transitional zone. In order to reach the observed mean Anoxic zone oxygen concentration of 1.27 mg/L from the observed mean Transitional zone oxygen concentration, an additional 0.309 mg/L of DOC are oxidized. From modeled equilibrium concentrations of oxygen (9.46 mg/L), approximately 8.8 mg/L of DOC are oxidized by oxygen before nitrate becomes more thermodynamically favorable as the electron acceptor and begins decreasing in concentration. Model simulations suggest that 1.19 mg/L of DOC reduce the observed mean nitrate concentration of 0.47 mg/L found in the Oxic zone to the lowest observed mean nitrate viii concentration of 0.01mg/L found in the Transitional zone. Differences between the observed DOC concentrations in the field and the modeled DOC concentrations needed to reach zone levels of oxygen and nitrate suggest that field reported values for DOC electron donors could represent residual or unused electron donors. Results also indicate that between 8.68 mg/L and 10.7 mg/L of DOC oxidation, 0.42 mg/L of dissolved N2 are produced, HCO3 increases from 0.33 mg/L to 2.64 mg/L and CO2 concentrations decrease from 13.8 mg/L to 13.7 mg/L before continuing to increase. This pronounced interval of DOC oxidation at which denitrification occurs and beyond which CO2 continues increasing suggests a specific range at which denitrifiers metabolize versus a larger range at which a general heterotrophic population metabolizes.
Jiménez R.A., (2011): Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification. Master's Capstone and Thesis. University of Pennsylvania.
This Paper/Book acknowledges NSF CZO grant support.