Assessing the Potential Carbon Sequestration of Hawai'i's Soils Used for Bioenergy Feedstock Production: An Investigation of Soil Fractionation Methods.

The success of biofuel as an alternative energy depends on the sustainability of the entire system. To this end, it is important to consider carbon (C) balance and sequestration. Methods used to assess this typically begin with physical separation of the soil into various fractions that represent C pools with turnover times ranging from years to millennia. However, the ash-derived Andisols of Hawai'i have unique properties due to their mineralogy, high oxide content and high aggregation. These soils have not been thoroughly investigated despite the fact that they are heavily used for agriculture and forestry and may support important feedstock crops for biofuel production. Here, our objective was to compare and contrast three commonly used and accepted methods for a variety of agriculturally important soils in Hawai'i; and to create a new method that works well with Hawai'i's Andisols. It was found from mass balance calculations that most of the mass in the soil (97 % +0) occurred in the dense organo-mineral fractions, however, the lighter fractions (0.81 % and 11%, depending on method) contained the larger proportion of C (17% and 49%, depending on method), and it has been shown that it is these fractions that are most sensitive to land-use change. More occluded organic matter (OM) (0.46 % +0) was recovered than free OM (0.35 % +0), suggesting that aggregation plays a large role in the process.  This role as well as the interactions between OM, minerals, and oxides must be understood in order to facilitate future studies on ecosystem C balance and soil C sequestration. To this end, a new method was created that combines three methods and targets these interactions.  This method first separates the soil into three size classes:  > 250 μm, 53-250  μm, and < 53 μm.  The next step involves separating the free light fraction (OM not protected within aggregates) and the occluded light fraction (OM protected within aggregates) within each of the size classes.  The dense fraction is separated by density, with different densities representing different minerals and oxides that may be present in the soil.