The world is facing an energy crisis. The greenhouse effect, caused by the burning of fossil fuels, has aroused serious concerns among scientists and politicians. Some have turned to the hope seemingly offered by biological sources of energy. But are they really the solution? One type of biomass, “energy grasses”, which are grown specifically for power generation, have been widely touted and have received widespread media attention. But the claims about energy grass crops have been exaggerated, and the reality is not as miraculous as it first sounds.
Let’s start by taking a look at the nature of biomass energy, the first type of fuel ever used by humans, which is in fact solar energy that has been collected and stored in greenery as carbohydrates. In a very wide sense, biomass could include plants, animals and microorganisms – even coal, oil and natural gas were formed from the biomass of earlier geological eras. But nowadays biomass energy is taken to mean plant oils, straw, wood chips, bark, branches and algae.
Half a million different plant species store solar energy through photosynthesis. A practical source of energy, however, should be fast-growing, easy to harvest and transport, as well as able to photosynthesise efficiently with a large leaf area. There are three ways that plants can store this energy: the C3, C4 and CAM photosynthesis pathways. C4 is the most efficient, and it is found in sugarcane, corn and sorghum.
Tropical rain forests are the world’s biggest natural biomass producers, creating 35 tonnes of biomass per hectare every year. But artificial conditions, such as high-density planting with fertilisers and watering, can increase yields further. Shandong Agricultural University has managed to produce an annual yield of 66 tonnes of biomass per hectare of maize and wheat. Any energy grass would need to utilise the highly efficientC4 photosynthesis pathway, and would involve large quantities of fertiliser and water in order to grow quickly and densely.
According to media reports, Fujian Agriculture and Forestry University’s energy grass crop has been treated to coexist with beneficial bacteria, and brings a harvest of 105 tonnes of biomass per hectare; enough to generate as much electricity as three to four tonnes of coal.
This type of grass has been referred to as “bacteria grass”, though this means little to a scientist. Judging from reports, it is likely to be a plant utilising the C4 photosynthesis process and inoculated with some kind of bacteria that improves its absorption of nutrients. In the natural world, there are a wide range of relationships, competitive and symbiotic, between organisms. The fungus found on the roots of legumes, which fixes nitrogen from the soil, is a classic example of symbiosis. There are few examples of grasses and bacteria coexisting in this way, but artificial inoculation could create one. Increasing absorption of nutrients would make no difference to the efficiency of photosynthesis, however, which means the claims of such large yields seem unlikely when compared with the previous record of 66 tonnes. Perhaps such yields could be found in tropical regions, if one planted three crops a year and used large amounts of fertiliser and machinery. But the energy inputs required in the fertiliser and other work would offset the original goal of replacing fossil fuels.
Unlike agricultural crops, energy grass is quick growing; it can be harvested repeatedly; and it can be cultivated on otherwise unproductive land, which means it will not compete with food crops for arable land. However, energy grass suffers the same disadvantages as the straw produced from agricultural crops: sources are geographically scattered and harvesting costs are high. Without transportation infrastructure, electricity, water, fertiliser and machinery, such large harvests would be difficult to achieve on unproductive land. Moreover, “unproductive land” is actually home to natural vegetation and important for biodiversity. China’s ecosystems are already facing large-scale degradation; we cannot sacrifice the environment for profit.
Experts from China Agricultural University produced a list of potential candidates that was more scientific than the exaggerated marketing claims I have mentioned. Their suggestions for “energy grasses” include annual, biannual or perennial grass and shrubs, especially tall plants such as sweet sorghum, willow and miscanthus, which are tolerant to arid and saline conditions and can grow in unproductive and mountainous regions. This inspired Beijing’s municipal government to launch plantations producing ethanol in the districts of Changping and Daxing. Even so, results in arid and semi-arid regions will be limited by environmental factors, and are unlikely to live up to the hopes of the experts.
Grass crops may be able to help relieve the energy crisis to a certain extent, but its potential failings need more scrutiny. Large-scale planting could still end up competing with food crops, especially if there are profits to be made, and this could have a serious impact on food security.
At the same time, we are ignoring the 700 million tonnes of straw produced in China every year, which is currently burned off in the fields, wasting energy and creating air pollution. There is no point not using this straw, while planting energy grass crops elsewhere. Scientists and governments must exercise caution when approaching the claims about energy grass crops.
Jiang Gaoming is a professor and Ph.D. tutor at the Chinese Academy of Sciences’ Institute of Botany. He is also vice secretary-general of China Society of Biological Conservation and board member of China Environmental Culture Promotion Association. He is known for his concepts of "urban vegetation" and allowing damaged ecosystems to recover naturally.