Do biofuels really sequester carbon? Hawaii research says yes.

Biofuel is a kind of stepchild in the renewable energy discussions—on the grounds that it may not really reduce carbon emissions.

Is biofuel a pipe dream? Is it really superior to burning fossil fuels?

New Hawai`i research indicates that certain grassy biofuels do better than most people believed. 

Sure, some of the CO2 stored in the plants gets released when they are converted to energy, but some of it also stays in the soil—sequestered out of the atmosphere.

A group of researchers conducted two years of testing using sugar cane and napiergrass, under the title, Field-Based Estimates of Global Warming Potential in Bioenergy Systems of Hawaii: Crop Choice and Deficit Irrigation. 

Their work was published in PLOS One. The authors are Meghan N. Pawlowski, Susan E. Crow, Manyowa N. Meki, James R. Kiniry, Andrew D. Taylor, Richard Ogoshi, Adel Youkhana, and Mae Nakahata. The two lead authors are with the Department of Natural Resources and Environmental Management, University of Hawaii Mānoa.

They worked with napiergrass that was repeatedly harvested, and with sugar grown at Maui’s HC&S plantation—which ironically has now gone out of sugar production.

The crops were selected for having big, robust root systems, and being somewhat drought tolerant. And they worked with two irrigation systems—giving the crop 100 percent of what a plantation would normally provide, and depriving the plants of water with 50 percent irrigation.

“Deficit irrigation reduced yield, but increased soil (carbon) accumulation as proportionately more photosynthetic resources were allocated belowground,” the authors wrote.

But it’s all in how you manage the crop. 

“If inappropriately managed, the production of biofuel feedstocks could be a net contributor to greenhouse gas … emissions,” the authors wrote.

One key is that if you want to increase soil organic carbon (SOC), you harvest the crop above the soil line and leave the root mass in the ground. You keep harvesting the tops (ratoon harvesting, this is called) off the same root system.

Most sugar operations in Hawai`i in recent decades have pulled the roots out on a regular basis, reducing the carbon in the soil. A different cultivation plan would increase soil carbon, the authors found.

“Our results demonstrate the potential to sequester SOC in both of the sugarcane and napiergrass feedstock scenarios if conservation management practices, such as ratoon harvests and reduced tillage operations, are implemented,” they wrote.

(Here’s a 2012 Science Daily piece on using napiergrass as a biofuel. https://www.sciencedaily.com/releases/2012/09/120927142524.htm)

© Jan TenBruggencate 2017