title: Field-Based Estimates of Global Warming Potential in Bioenergy Systems of Hawaii: Crop Choice and Deficit Irrigation.
abstract: Replacing fossil fuel with biofuel is environmentally viable from a climate change perspective 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, zero-tillage 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 (CO2), as methane was oxidized and nitrous oxide (N2O) emission was very low even following fertilization. High N2O fluxes that frequently negate other greenhouse gas benefits that come from replacing fossil fuels with agronomic forms of bioenergy were mitigated by efficient water and fertilizer management, including direct injection of fertilizer into buried irrigation lines. From soil intensively cultivated for a century in sugarcane, soil C stock and root biomass increased rapidly following cultivation in grasses selected for robust root systems and drought tolerance. The net soil C increase over the two-year crop cycle was three-fold greater than the annualized soil surface CO2 flux. Deficit irrigation reduced yield, but increased soil C accumulation as proportionately more photosynthetic resources were allocated belowground. In the first two years of cultivation 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.
authors.affiliation: Department of Natural Resources and Environmental Management, University of Hawaii Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Meghan N
authors.initials: MN
authors.lastname: Pawlowski
authors.affiliation: Department of Natural Resources and Environmental Management, University of Hawaii Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Susan E
authors.identifier: http://orcid.org/0000-0001-8735-1580
authors.initials: SE
authors.lastname: Crow
authors.affiliation: Texas A&M AgriLife Blackland Research and Extension Center, Temple, Texas, United States of America.
authors.forename: Manyowa N
authors.initials: MN
authors.lastname: Meki
authors.affiliation: United States Department of Agriculture-Agricultural Research Service Grassland Soil and Water Research Laboratory, Temple, Texas, United States of America.
authors.forename: James R
authors.initials: JR
authors.lastname: Kiniry
authors.affiliation: Department of Biology, University of Hawaii Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Taylor
authors.affiliation: Department of Tropical Plant and Soil Sciences, University of Hawaii Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Richard
authors.initials: R
authors.lastname: Ogoshi
authors.affiliation: Department of Tropical Plant and Soil Sciences, University of Hawaii Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Adel
authors.initials: A
authors.lastname: Youkhana
authors.affiliation: Hawaiian Commercial & Sugar, Puunene, Hawaii, United States of America.
authors.forename: Mae
authors.initials: M
authors.lastname: Nakahata
mesh_terms: D057914:Agricultural Irrigation; D056804:Biofuels; D018533:Biomass; D002244:Carbon / Q000032:analysis; D018556:Crops, Agricultural / Q000502:physiology*; D005308:Fertilizers / Q000032:analysis; D005740:Gases / Q000032:analysis; D057232:Global Warming; D017752:Greenhouse Effect; D006254:Hawaii; D009609:Nitrous Oxide / Q000032:analysis; D018517:Plant Roots / Q000254:growth & development; D012987:Soil; D013045:Species Specificity; D013696:Temperature; D013997:Time Factors; D014867:Water
chemical_list: D056804:Biofuels; D005308:Fertilizers; D005740:Gases; D012987:Soil; D014867:Water; D002244:Carbon; D009609:Nitrous Oxide