Projecting Global Warming Potential of Production Systems for Tropical Perennial C4 Grasses Cultivated for Biofuel Feedstock in Hawaii.

Shifting from conventional sugarcane to ratoon-harvested Napier grass for biofuel production is likely to have multiple benefits including a diversified agricultural system, reduced irrigation water requirement, improved energy security, and more sustainable soil and water management practices.  Agricultural soils can serve as either a sink or a source for atmospheric carbon (C) and other greenhouse gases (GHG). This is particularly true for tropical soils where influences from climate and soil gradients are wide ranging. Current estimates of GHG flux from soil are often under or overestimated due to high variability in sample sites and inconsistencies in land use and vegetation type, making extrapolation to new study systems difficult. This work identified patterns of trace fluxes of carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) across two soil types and three species of warm season perennial C₄ grasses: Pennisetum purpureum (Napier grass), Panicum maximum (Guinea grass) and Saccharum officinarum (sugar cane) on the island of Maui in Hawaii.  Multiple static vented chambers were installed into replicate plots for each species; flux measurements were made during the growth, fertilization, and harvest cycles at set time intervals for one hour and analyzed by gas chromatography. Model simulations of soil C stock and GHG flux for the plantation area were made with the ALMANAC model. An average flux of 143 g m^-2 h^-1 and 155 g m^-2 h^-1 for P. maximum and P. purpureum respectively at full growth for CO₂ and 1.7 μg m^-2 h^-1and 0.3 μg m^-2 h^-1 for N₂O. Additionally, N₂O rates sampled after a typical fertilizer application were significantly greater than at full growth (p=0.0005) 25.2 μg m²h^-1 and 30.3 μg m²h^-1. With a global warming potential of 310 for N₂O, even short-term spikes following fertilizer application can cause long lasting effects of GHG emission from agricultural soils. Monitoring GHG emissions and soil carbon accumulation is integral to the environmental sustainability assessment of a potential biofuels production system; therefore further improvement in our ability to accurately project the global warming potential is critical.