Biofuel crops in Hawaiʻi may successfully sequester carbon in soil. (photo credit: Theo Crazzolara, Flickr)

Two potential biofuel crops in Hawaiʻi—sugarcane and napiergrass—may sequester more carbon in soil than is lost to the atmosphere, according to a study published January 4, 2017 in the open-access journal PLOS ONE by Meghan Pawlowski from University of Hawaiʻi at Mānoa College of Tropical Agriculture and Human Resources and colleagues.

From a climate change perspective, replacing fossil fuel with biofuel makes sense only if the latter has a smaller greenhouse gas footprint. Sugarcane and napiergrass are promising biofuel crops because, like other tropical C4 grasses, they have a large carbon-storing root biomass that could offset carbon-dioxide fluxes occurring during cultivation.

To test this, Pawlowski and colleagues monitored conventional sugarcane and non-tilled napiergrass crops in Hawaiʻi over two years, measuring the above- and below-ground biomass and assessing the greenhouse gas flux. In addition, these thirsty crops were grown with either conventional or deficit irrigation, which is half that of the current commercial practice.

The researchers found that by the end of the two-year study, both crops had successfully sequestered more carbon in the soil than was lost from the soil surface—the largest component of the greenhouse gases in this case. For example, soil in the sugarcane plots had three times as much carbon as was lost to the atmosphere. Deficit irrigation boosted soil carbon sequestration but also reduced yield, though this tradeoff was less pronounced for napiergrass, which can be harvested more than once a year. The authors suggest that, with zero tillage conservation management, sugarcane and napiergrass biofuel feedstocks could therefore sequester carbon in soil. However, they caution that further study is necessary to determine whether this will continue over the long term and which crop may be best.

Susan Crow, co-author and assistant professor in the Department of Natural Resources and Environmental Management notes, “A common misperception persists that biofuels are non-viable because of inefficiencies and net carbon losses that negate the potential for climate change mitigation. These results show that in the right system, coupled with the right crop and management, biofuels can be an important contributor to sustainable renewable energy portfolios.”

This Post Has 10 Comments
  1. This is great news. The carbon sequestering of these two grasses is exciting to hear about for assisting us to have a clean environment and control toxicity to the atmosphere. Maybe if this can be implemented and utilized in a mass way in the Hawaiian islands we will see the effects here soon! The burning of the bio fuels concerns me though as carbon pollution. That factor may offset benefits of the carbon sequestering. I look forward to seeing more studies completed on the actual effects to the carbon footprint after burning is fully incorporated into equations for the final carbon footprint of using these two grasses as bio fuels.

  2. Burning fuel of any origin releases carbon dioxide to the atmosphere; the benefit of biofuels is the replacement of fossil resources with renewable biomass. This is the carbon-neutral component to biofuels (the carbon emitted was recently fixed through photosynthesis), but as pointed out, the trick is to have enough sequestered to more than cover the incurred carbon costs of production, conversion, and transportation. The magnitude of soil carbon sequestration in the study gives us hope that we can achieve the ideal net carbon benefit of the whole system. Looking forward, as part of a renewable energy portfolio for Hawaii biomass-derived fuel or energy can become even more beneficial as the external energy requirements of the system are replaced by other forms of renewables such as wind and solar.

    1. I appreciate your detailed reply ! Thank you.
      Would you know, can you provide information on my following question:

      How is the system on Kauai where they grow the Albezia Tree on island, its speed in growth considered faster than other crops. and then the use of it for the bio fuel source comparing to these fuels you are working as carbon footprint and also the aspect of sequestering?

  3. The sugarcane root system which sequesters the carbon, will release the carbon as carbon dioxide and methane when the root decomposes. The root will begin to decompose as soon as the crop is harvested or dies.

    1. We’ve found in similar systems on Oahu that decaying roots predominantly stay as soil carbon and are not lost to the atmosphere at carbon dioxide. Root decay products and the soil microbes become stabilized in the soil and are sequestered, as long as the management practice is sustainable. In this study, the soil is protected from losses because it is a zero-tillage system. This is the critical factor to facilitating soil carbon sequestration.

      1. I know that zero tillage is providing better quality production, higher production volume, less pest attacks, and taste of the food crops is improved, but this is additional good news that a ‘no tillage policy’ also plays a role in the carbon footprint by helping that the decaying roots are not allowing escaping carbon into the atmosphere when no tillage is followed! I never considered that before!

  4. Hawaii doesn’t need biofuel crops, it needs food. People are starving out here. Food crops also reduce the carbon and the urban development footprint, create jobs, and provide the community with a service. If everyone could eat locally and spend very little because the land was used for food and the communities were strong, why would we need to drive three hours through traffic into town to earn bread in our biofueled or electric cars?

    1. Thank you to everyone for the comments!
      Yes, food production is extraordinarily important as well. So much agricultural land in Hawaii is fallow, abandoned and/or being converted to development. Getting the land back into agriculture to improve food and energy independence in Hawaii is a common goal.

      1. Very good reply to the food comment. We could use more people who want to be farmers in Hawaii to help use the fallow planting areas.

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