Soil carbonate: Its measurement and formation from carbon dioxide in laboratory experiments

Pedogenic carbonate is the third largest pool of carbon in the carbon cycle. Interactions between pedogenic carbonate and atmospheric CO₂ are complex but have important implications for understanding carbon sequestration in arid regions. This dissertation had two objectives. First, to determine the most accurate and simplest method for measuring calcium carbonate by comparing six common methods. Second, to conduct lab experiments to simulate root respiration and carbonate formation to estimate the amount of atmospheric carbon dioxide channeled into the pedogenic carbonate pool.; For the first objective, each method was tested twice using known amounts of CaCO₃. Five methods are based on acid dissolution, the other method on dry combustion. Three methods measure CO₂ pressure, two methods absorb CO₂ in NaOH, and one method analyzes HCl consumption by titration. Based on accuracy, consistency, and simplicity the ranking order of the methods are the Hg manometer, pressure transducer under vacuum, dry combustion, acid titration, pressure transducer at ambient conditions, and the Kozlovskii's method.; For the second objective, three kinds of experiment were conducted. One used a “mechanical root” to emulate a root in dry soil with an air stone supplying carbon dioxide as soil respiration. The second experiment involved the bubbling of CO₂ into Ca-solutions. The third kind of experiment involved desiccation of Ca-solutions having various Ca 2+ concentrations, pH, and CO₂ flow rates. Results revealed that the mechanical root, which pumped CO₂ into a mixture of silica sand and CaCl₂, did not produce CaCO₃ because the initial pH was too low (pH = 6.9). However further experiments showed that under conditions where HCO−₃ and Ca2+ were amply supplied, CaCO₃ can form at pH values as low as pH 5.5. Bubbling CO₂ through Ca-solutions lowers pH and provides HCO− ₃ for the precipitation of CaCO₃. The desiccation experiments, designed to simulate a drying soil, showed that as Ca-containing solutions desiccated, Ca2+ concentrations increased, but pH remained roughly the same. Dessication experiments also showed that increasing amounts of Ca2+ produced increasing amounts of CaCO₃. However, increasing pH values and CO₂ flow rates did not show a clear trend to increasing amounts of CaCO₃ formation.*; *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation).