Bioenergy and Biochar – Two concepts brought together for sustainable land use?
Increasing carbon storage in soils through biochar and producing bioenergy from perennial plants can be a powerful means to mitigate climate change. Understanding soil microbial processes is crucial to achieve improved soil fertility, biodiversity, and carbon sequestration.
It is well known that the world’s forests play a major role in regulating our climate. An estimated 17% of global greenhouse gas emissions stem from land use and deforestation. Less public awareness exists about the fact that carbon stored in the soil by far exceeds the carbon in the wood of trees. Together, both soil and plants constitute a carbon reservoir approximately 2.7 times as large as the atmosphere. Natural landscapes therefore contribute significantly to stabilizing the climate by storing carbon that may be emitted into the atmosphere under intense agricultural use. Scientific and technical efforts have emerged to enhance the climate benefits and yield highly productive landscapes. Adding biochar to soils and growing short-rotation coppice plantations for bioenergy production may achieve both goals simultaneously.
Biochar is a type of charcoal that is produced by heating plant material such as wood and crop residues in the absence of oxygen. The carbon in biochar is very stable and does not decompose like untreated plant material—a process by which much of the contained carbon is released into the atmosphere. Adding biochar to soils can potentially increase the fertility and water storage capacity of agricultural lands, and contribute to higher crop yields while reducing emissions. Short-rotation coppice (SRC) is a plantation technique by which trees are cut every 3-5 years and resprout from their trunks. This leads to fast growth and high levels of wood production that can be used as a feedstock for bioenergy. In a recently published article in Bioenergy, researchers from the Centre for Ecology & Hydrology, Lancaster University, and Heriot-Watt University analyzed the impacts of biochar and bioenergy production on a range of living organisms in the soil and related ecosystem processes. These impacts play an important role in determining the suitability of biochar and bioenergy plantations to enhance carbon storage.
The researchers found that the application of biochar to the soil can have varying effects on insects, worms, fungi, and bacteria. Generally, biochar has been found to increase the abundance of soil microorganisms across ecosystems. Through its porous structure, biochar can lead to increased soil nutrient retention and water storage. Biochar increases the soil carbon content, and its neutral to alkaline pH can help improve highly acidic soils. On the other hand, the researchers caution that changed soil structure and chemistry could adversely affect animals in the soil and the productivity of certain plants. The researchers obtained a less ambiguous picture about bioenergy production systems. Replacing annual crops such as wheat and corn with perennial grasses such as miscanthus and switchgrass, or with willows and poplars in SRC, affects soil biodiversity positively. Perennial plants have stronger and deeper roots and do not require annual tillage. Unlike annual disturbances from harvest and tillage as in annual crop systems, the researchers found the multi-year harvest cycles of perennial plants promote higher diversity of plants and soil organisms. Perennial bioenergy systems such as SRC can therefore lead to increased carbon sequestration and lower emissions than annual crops.
The most potential for biochar and perennial bioenergy plants to promote biodiversity and carbon sequestration may be realized on ex-arable lands, the researchers suggest. Areas that have been heavily used for agriculture and whose fertility has declined may be well suitable for extensive bioenergy production. Applying biochar to the soil requires one-time tillage, which impacts soil structure and organisms, but can help improve soil properties once in place. Combining biochar with SRC can result in a low-disturbance production regime since poplar and willow SRC are typically managed without chemical inputs such as mineral fertilizer. This is instrumental to achieve stable soils that are rich in structure and biodiversity and constitute a durable carbon reservoir.
Bioenergy production from SRC and perennial grasses can turn otherwise unsuitable areas into productive landscapes while not constituting a competition to food production. Biochar may be an effective means to increase carbon storage and other soil qualities, however the uncertainties around the impacts of biochar on soil microbiology warrant long-term studies. Given that policies create the enabling conditions, both bioenergy and biochar can potentially play a role in developing land use systems that achieve multiple ecosystem services. Further research is needed to help understand how biodiversity and soil conservation can be combined with economically viable food and bioenergy production.