Linking Soil and Water Conservation Practices to Greenhouse Gas Flux and Fine Root Dynamics: A Comparison of Sugarcane and Napier Grass Grown for Bioenergy Production.

As biofuel production expands in the Hawaiian Islands it becomes critical to better understand the impact that these crops will have on their surrounding environment. In general, agricultural soils can serve as either a sink or a source for atmospheric carbon (C) and other biogenic gases. This is particularly true for tropical soils where influences from management practices, crop selection, and climate are wide ranging. Globally, water limitation is a major factor influencing the potential productivity of biofuel crops and the sustainability of these agricultural systems. This project aims to identify patterns in the flux of carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) under sugarcane (Saccharum officinarum) and napier grass (Pennisetum purpureum) in response to deficit irrigation treatments. Multiple static vented chambers were installed within replicate plots under each irrigation treatment: 100%, 75%, and 50% of current plantation practice. Gas samples were extracted by hand during the growth, fertilization and harvest cycles and analyzed by gas chromatography. Soil CO₂ emissions displayed sensitivity to soil moisture and above-ground plant growth but did not differ between the treatments; the highest fluxes were found in relation to napier grass. CH₄ flux displayed a negative trend for both species under all treatments, which suggests that these upland agricultural soils act as a weak sinks for atmospheric CH₄. N₂O emissions were found to be highly dependent on soil moisture, especially following N fertilization events. We found that on average, N₂O flux within the irrigation row was approximately 45% higher than fluxes found between rows in the days following fertilization. This data will aid in the selection of feedstock species grown for bioenergy in Hawaii, which will minimize agricultural sources of GHG production and maximize water and nutrient use efficiency across these systems.