|PAWLOWSKI, MEGHAN - University Of Hawaii|
|CROW, SUSAN - University Of Hawaii|
|MEKI, MANYOWA - Texas Agrilife Research|
|TAYLOR, ANDREW - University Of Hawaii|
|OGOSHI, RICHARD - University Of Hawaii|
|YOUKHANA, ADEL - University Of Hawaii|
|NAKAHATA, MAE - Hawaiian Commercial And Sugar Company|
Submitted to: PLoS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2016
Publication Date: 1/1/2017
Publication URL: http://handle.nal.usda.gov/10113/5695380
Citation: Pawlowski, M.N., Crow, S.E., Meki, M.N., Kiniry, J.R., Taylor, A.D., Ogoshi, R., Youkhana, A., Nakahata, M. 2017. Field-based estimates of global warming potential in bioenergy systems of Hawaii: Crop choice and deficit irrigation. PLoS One. 12(1):e0168510. doi:01.1371/journal.pone/0168510.
Interpretive Summary: Replacing fossil fuel with biofuel is environmentally friendly only if the greenhouse gases are reduced. When replacing annual crops with perennial grasses, changes in greenhouse gas production and soil carbon are critical for the impacts of the environment. Here, we compared greenhouse gas production, plant yield, root biomass, and soil carbon for two tropical, perennial grass biofuels: conventional sugarcane and repeatedly-harvested napiergrass. We evaluated two irrigation levels, 100% of normal irrigation and with a 50% reduction. Trends in greenhouse gas production followed farming events such as the repeated harvests of napiergrass and fertilization. Yet, greenhouse gas production was dominated by carbon dioxide. Efficient water management, including buried irrigation lines reduced nitrogen losses that often occur when replacing fossil fuels with bioenergy crops. From soil used for sugarcane for the last century, soil carbon and root biomass increased rapidly following cultivation. Soil carbon change over the two-year crop cycle was three-fold greater than the yearly CO2 flux measured from the soil surface. Reduced irrigation reduced yields but increased soil carbon accumulation as more plant resources were produced belowground. Repeatly-harvested napiergrass did not increase greenhouse warming potential compared to sugarcane and has the advantage of multiple harvests per year and less negative effects of reduced irrigation on yield.
Technical Abstract: Replacing fossil fuel with biofuel is environmentally viable only if the net greenhouse gas (GHG) footprint of the system is reduced. The effects of replacing annual arable crops with perennial bioenergy feedstocks on net GHG production and soil carbon (C) stock are critical to the system-level balance. Here, we compared GHG flux, crop yield, root biomass, and soil C stock under two potential tropical, perennial grass biofuel feedstocks: conventional sugarcane and ratoon-harvested napiergrass. Evaluations were conducted at two irrigation levels, 100% of plantation application and at a 50% deficit. Peaks and troughs of GHG emission followed agronomic events such as ratoon harvest of napiergrass and fertilization. Yet, net GHG flux was dominated by carbon dioxide (CO¬2), as methane was oxidized and nitrous oxide (N2O) emission was very low even following fertilization. Efficient water management, including buried irrigation lines and fertilization, mitigated the gaseous nitrogen (N) losses that often negate the benefit from replacing fossil fuels with bioenergy. From soil intensively managed for a century in sugarcane, soil C stock and root biomass increased rapidly following cultivation. The net soil C change over the two-year crop cycle was three-fold greater than the annualized CO2 flux measured from the soil surface. Deficit irrigation reduced yields, but increased soil C accumulation as proportionately more photosynthetic resources were allocated belowground. Ratoon-harvested napiergrass did not increase net greenhouse warming potential (GWP) compared to sugarcane and has the advantage of multiple ratoon harvests per year and less negative effects of deficit irrigation to yield.