Above and Belowground Biomass Dynamics Under Ratoon Harvest Practices for Biofuel Feedstock Production in Hawaii.

The use of renewable biomass for biofuel production presents a promising alternative to fossil fuels for sustainable energy production by minimizing the net of greenhouse gas flux. Tropical, perennial C₄ grasses such as sugarcane, energycane and napier grass produce large amounts of both above-and belowground biomass. These grasses can be harvested by ratooning (no till), which leaves the roots and soil undisturbed and have the potential to rapidly accumulate soil organic carbon while simultaneously providing biofuel feedstock. The objectives of this study were to 1) estimate above-and belowground biomass and C accumulation for sugarcane, energycane and napier cultivated on the island of Maui, 2) develop optimal allometric relationships to predict aboveground biomass and C production and 3) determine root death and decay dynamics after harvesting. Aboveground biomass was quantified using standard plant growth protocols while root biomass was determined volumetrically from excavated soil pits by depth: 0-40, 40-80 and 80-120 cm. Root decay constant (k) was determined using a litterbag experiment. The one year above-and belowground biomass and soil C were ranked as follows: energycane > sugarcane > napier. For all allometric equations, a simple power model provided the optimal prediction of aboveground biomass and its C accumulation. Stalk diameter and dewlap height were good predictors for aboveground biomass. The total root mass of all crops decreased with soil depth. The dead versus live roots percentage for ratooning energycane and napier were 70 to 30% and 11 to 89% and for 2year sugarcane was 41 to 59%. The maximum root:shoot ratio was 0.14 for napier. Decay constants were different at marginal significance across species and soil depths, ranging from 0.118 to 0.040 and napier grass had statistically greater (k). The root turnover results confirm the rapid flush of new shoots in the napier ratooning system compared to sugarcane and energycane. Given the many benefits of soil C in the sustainable management of soil fertility, quantifying above-belowground biomass and determining root turnover are important to understanding C dynamics in any biofuel cropping system.