Tropical Biofuel Crops Can Store More Soil Carbon Than They Release

Sugarcane like this can be grown for biofuels in ways that store carbon in the soil, helping fight climate change. Amornchaijj/Shutterstock

Let's start the year off with some good news on climate change, shall we? Two biofuel crops can, under the right conditions, store enough carbon in the soil to outweigh what is lost in production, making a real contribution to the fight against global warming.

A paper in PLOS One reports a carbon balance when sugarcane and napiergrass were grown in Hawaii using the zero-tillage method, which minimizes the loss of soil carbon. The air above the fields were constantly monitored for gas release and the soil carbon was tested through the two-year trial. Using both conventional irrigation and far more restricted watering, three times as much carbon was stored in the soil as released into the air – up to 17 tonnes per hectare (8 tons per acre) per year.

In theory, biofuels represent a much more environmentally friendly way of powering the world than fossil fuels. Plants and algae draw carbon dioxide out of the atmosphere, and even though this gets released when the fuel is burned, the cycle is theoretically neutral.

However, the reality is much more complex. We burn almost as much fuel growing and processing the corn to make ethanol than we get out. Some biofuel crops are so bad for the environment they've discredited the entire biofuel industry. The missing piece measuring the value of the better options has been what happens to soil carbon, potentially a bigger factor than the fossil fuels avoided.

“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," said co-author Dr Susan Crow of the University of Hawaii in a statement. Sugarcane and napiergrass were chosen because both are C4 plants, which usually represent much better carbon dioxide harvesters than their C3 equivalents.

Giving the plants only 50 percent of the usual watering actually increased the amount of carbon stored, but reduced the crop yield, potentially making projects less economically viable. Emissions of gasses such as methane and nitrous oxide, far more potent planet warmers than carbon dioxide, were too low to make much difference, provided fertilization was done with care. Napiergrass may not be as sweet as sugarcane, but it provided a better fuel yield.

Nevertheless, many questions remain unanswered. Aside from whether the results would be the same in places with a less tropical climate and without Hawaii's volcanic soils, Crow acknowledged that the study only looked at the first two years of crop production. There is a risk that soil storage may reduce over time.

The work was done on soils that had been farmed for sugarcane for a century, but where low sugar prices caused neighboring farmers to shift to alternatives.

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