Managing carbon in soils, feedstocks, and conversion pathways of tropical agricultural production systems for bioenergy

Abstract

Achieving statewide 100% renewable energy and net carbon storage by 2045 in Hawaii will require exploration of all possible sustainable energy sources. This dissertation focused on one important component of Hawaii’s potential clean energy portfolio—biofuels produced from tropical perennial C4 grasses. Accurate system-level analyses of these tropical agricultural production systems were lacking information in critical areas, which created barriers to their full consideration for inclusion in the portfolio. To address this issue, several key data sets were collected, including: 1) physicochemical predictors of soil organic carbon under long-term C4 grass production, 2) carbon and energy conversion efficiency of several bioenergy grasses through two conversion pathways, and 3) lignin structural controls on conversion to energy. Belowground, aggregate stability, surface charge, and mineral-root interactions protected soil carbon from losses despite intensive agriculture practices. Aboveground, highly variable amounts of energy were recoverable across species and varieties of C4 grasses, with lignin ratios showing strong prediction of energy variance. Taken together, the data provides important system constraints that must be maintained to keep agricultural production of these grasses for bioenergy carbon neutral, allowing the first data-driven development of carbon and energy neutral production systems.