Adel Youkhana, Department of Natural Resources and Environmental Management, University of Hawaii at Manoa, Honolulu, HI, Susan E. Crow, University of Hawaii Manoa, University of Hawaii at Manoa, Honolulu, HI, Manyowa Meki, Blackland Research & Extension Center, Texas Agrilife Research, Temple, TX, James R. Kiniry, USDA-ARS, Temple, TX, Richard M. Ogoshi, Department of Tropical Plants and Soil Sciences, University of Hawaii, Honolulu, HI and Mae Nakhata, Hawaiian Commercial and Sugar Company, Maui, HI
Although information on aboveground biomass and growth rates exists for biofuel plants grown across a range of environments and islands in Hawaii, there is a lack of information on belowground input quantity and quality and their impact on soil organic carbon (SOC) pool in spite of their importance. This is mainly due to the difficulty of evaluation and high variability of results. The mass, size, and distribution of roots are among the most important factors in determining belowground biomass, however root system depth of biofuel crops is a key parameter for water and mineral uptake but it is still very little known. Sugarcane and Energy cane as perennial biofuel grasses produce large amounts of aboveground and belowground biomass. Both C4 grasses can be harvested by ratooning (no-till) which leaves the roots and soil intact, undisturbed and accumulate SOC by improving soil aggregation, fertility with potential to increase soil C sequestration while simultaneously provides aboveground biomass for energy production. The objectives of this study were to determine belowground biomass and examine the quantity and quality of C input after one year cycle of ratooning energycane and two years cycle of sugarcane cultivation in Hawaii. The root system of each crop was excavated based on roots sampled from soil pits within 3 depths (0-40, 40-80 and 80-120 cm). Number of stalks and the maximum root depth for each crop were determined. Since ratooning removes most of aboveground biomass without incorporating residue into soil, one of the primary input of C to soil in the system is root turnover which is the key component of C cycling. It was hypothesized that the root decay of ratooning energycane and 2 years sugarcane varies with root origin and depth due to a negative relationship between root decay constant (k) and lignin concentration, greater amount of root lignin will result in slower decomposition, so additionally; a root decay experiment was conducted for each crop. The experiment was initiated in Feb 2013, and will continue as proposed for 9 months to determine root decomposition constant (k) using the litter bag method which will be collected randomly from each replicate at 1, 2, 4, 6, and 9 months. Decay rate and lignin content and quality will be measured. The preliminary results showed that belowground biomass of both crops was significantly linked to soil depth and most root biomass is found close to the surface and declines approximately with depth. Total root masses (living plus dead) was 0.459 kg m-2 for one cycle ratooning of energycane, dead roots was greater than live roots (70% to 30%). The proportions of dead roots based on soil depths (0-40, 40-80, and 120cm) were 71.59, 18.01, and 10.40% respectively. For sugarcane (2yrs cycle), total root masses was 0.311 kg m-2 and the proportions based on origin soil depths (0-40, 40-80 and 80-120 cm) were 70.06, 22.97, and 6.97 % respectively. Excavating the root system from soil pits should speed the development of our understanding of belowground biomass and soil C pools for sugarcane and ratooning energycane as biofuel crops.