QUANTIFYING CO2 SEQUESTRATION BY ENHANCED ROCK WEATHERING IN TROPICAL AGROECOSYSTEMS: AN EXPERIMENTAL, EMPIRICAL AND GEOSPATIAL APPROACH

Tropical agroecosystems show great potential to sequester CO₂ by enhanced rock weathering (ERW) of powdered mafic rocks applied to fields. This study seeks to quantify rate of CO₂ sequestration by ERW in the humid tropics (1) by experimental weathering of basaltic to andesitic powders in humid tropical conditions, and (2) from weathering rates determined from a Holocene tropical soil chronosequence where parent material is andesitic sediments. Experimentally determined weathering rates of andesitic basalts from Costa Rica (Arenal and Barva) indicate potential sequestration of 2.0 to 4.5 tonnes CO₂/ha/yr, and the chronosequence indicates a similar rate of 2.0 t CO₂/ha/yr. USGS basalt standard BHVO-1 yields a rate of 18.1 t/ha/yr, influenced by finer grain size and higher volcanic glass content compared to the others. These rates were quantified by measuring cations released by weathering and reaction stoichiometry, e.g. Ca-plagioclase + 2 CO₂ + 3 H₂O = Ca²⁺ + 2 bicarbonate + kaolinite. XRD was used to quantify mineral assemblages, XRF and ICPMS to measure composition, and SEM-EDS to observe chemical weathering of mineral grains. The weathering experiment consists of 1 mm of powdered basaltic powder applied atop 12 mm of Ultisol in a 10-cm diameter reactor at 35 ^oC. To simulate effect of organic acids,100-mL aliquots of a dilute solution of oxalic acid (15 mmol/L in carbonated water) were rained onto soils 8 times over a 2-week period to simulate quantity of rain in humid tropics. Solutions that were displaced by subsequent aliquots were collected and analyzed by ICPMS for cation concentrations, yielding the 2.0-4.5 tonnes/ha/yr rate for Costa Rican basaltic-andesites presented above. At this rate, annual applications of powdered basalt seems feasible. CO₂ sequestration potential of Costa Rica by ERW was estimated for a scenario where 50 % of lowland kaolinitic soils (mainly Ultisols, frequently farmed) received 1 mm ERW applications at a CO2 sequestration rate of 3.25 t/ha/yr. Given these values, Costa Rica could sequester 60-70 % of their annual CO₂ emissions via ERW, potentially more if widely adopted. This estimate excludes national parks, higher elevations, dry tropics, and non-lateritic soil types. Ongoing work includes modeling rates and a more-detailed spatial analysis from regional to global scale.