- Author: Yoni Cooperman
Sequestering Carbon in the Soil Using Biochar
Soils store three times more carbon than exists in the atmosphere. Plants absorb atmospheric carbon during photosynthesis, so the return of plant residues into the soil contributing to soil carbon. While much of this carbon ultimately returns to the atmosphere as soil microbes decompose carbon based plant biomass and release carbon dioxide, soil carbon stores can increase if the rate of carbon inputs exceeds the rate of microbial decomposition. Carbon sequestration refers to this process of storing carbon in soil organic matter and thus removing carbon dioxide from the atmosphere.
Biochar is produced from burning organic material at high temperatures with little to no oxygen availability. The potential of utilizing biochar to sequester carbon in the soil has received considerable research attention in recent years as part of efforts to develop climate smart agricultural practices. As the majority of biochar is carbon (70-80%) it can potentially contribute more carbon than plant residue (approximately 40% carbon) of similar mass. Furthermore, around 60% of this biochar organic carbon is of high stability and therefore resists decomposition more-so than plant material that has not be processed into biochar. That being said, many questions remain as to the effectiveness of biochar application in sequestering carbon.
(For more information about biochar, check out our recent blog post)
Persistence of Biochar Carbon in Soil
While biochar does contain high levels of carbon, there remains uncertainty as to how long that carbon will persist in the soil following application. The inherent characteristics of the biochar--as dictated by feedstock and pyrolysis conditions--interact with climatic conditions such a precipitation and temperature to influence how long biochar carbon remains stored in the soil. Recent studies suggest that shorter pyrolysis times and higher pyrolysis temperatures make for more recalcitrant biochar (i.e. it persists for longer periods in the soil). However, there are trade-offs involved as these pyrolysis conditions produce less biochar per unit feedstock. As is so often the case, soil texture plays a key role in determining the persistence of biochar carbon. Biochar becomes stabilized in the soil by interacting with soil particles. Clay particles have more surface area for biochar to interact with and are therefore more effective at stabilizing biochar.
The Priming Effect
A number of studies have observed an increase in the rate of organic matter decomposition following biochar application. This so-called “priming effect” complicates any efforts to sequester carbon as this increase in microbial activity could result in decomposition rates exceeding carbon input rates (see figure above). While the exact mechanism responsible for this effect has not been conclusively identified, it may result from the stimulation of microbial activity as microbes utilize carbon and nitrogen present in biochar.
Biochar remains a hot topic with regards to increasing soil carbon stores and helping fight climate change. However, many questions remain before definitive conclusions about what conditions allow for biochar to positively contribute to soil carbon sequestration.
Sources
Ontl, T. A. & Schulte, L. A. (2012) Soil Carbon Storage. Nature Education Knowledge 3(10):35
Lal, R. (2016). Biochar and Soil Carbon Sequestration. Agricultural and Environmental Applications of Biochar: Advances and Barriers. M. Guo, Z. He and S. M. Uchimiya. Madison, WI, Soil Science Society of America, Inc.: 175-198.
Stewart, C. E., et al. (2013). "Co-generated fast pyrolysis biochar mitigates green-house gas emissions and increases carbon sequestration in temperate soils." GCB Bioenergy 5(2): 153-164.
Yang, F., et al. (2016). "The Interfacial Behavior between Biochar and Soil Minerals and Its Effect on Biochar Stability." Environmental Science & Technology 50(5): 2264-2271.
- Author: Yoni Cooperman
- Contributor: Deirdre Griffin
In the on-going quest to develop sustainable agricultural practices, growers are looking for new and inventive technologies. In this blog post, we'll focus on biochar, one such technology that has been a focus of intense research in recent years. Biochar is produced by burning organic material at extreme temperatures as high as 1600° F with little to no oxygen available. Oftentimes biochar is a by-product of energy production, but it can also be produced solely to be used as a soil amendment.
There's a few reasons growers might incorporate biochar into their cropping systems. Biochars' high surface area allows it to act as a reservoir of water while increasing the retention of nutrients such as calcium, magnesium, and ammonium. This is especially useful in more sandy soils with low cation exchange capacity. Biochar can also serve as a liming agent to increase soil pH, which increases nutrient availability in acidic soils. Additionally, biochars with high ash content can contain calcium and potassium that plants can use. Biochar inputs are also high in carbon. Stay tuned to this blog for another post highlighting the potential for biochar to increase soil carbon storage.
Feedstock – the organic material used to produce biochar – varies widely. Common feedstocks include wood chips, nut shells, and grasses. In California nut shells stand as a potentially useful source of feedstock due to the large nut industry. Biochar can also be produced from manures. Both feedstock and production temperature influence how biochar will behave in the soil. Dr. Sanjai Parikh's lab at the University of California, Davis has developed a biochar database that includes both of these characteristics.
Initial interest in biochar stemmed from the study of the Terra Preta soils in South America. These generally low fertility, acidic oxisols were able to sustain higher productivity than nearby non-Terra Preta soils while also accumulating organic matter. One of the reasons for this productivity was the addition of charcoal by indigenous farmers thousands of years ago. The hope was to mimic this in a modern agricultural setting.
Like most agricultural practices, biochars present some challenges for effective integration into a cropping system. Like compost or manure, it can be difficult to predict when nutrients from biochar will become plant available or how a char will interact with a particular soil. Different soil types require different rates of biochar application. For example, a clay loam would require more biochar to increase pH when compared to a sandy soil as a result of the clay loam's higher buffering capacity (see figure below).
UC Davis Soils and Biogeochemistry graduate student Deirdre Griffin is researching how soil microbes respond to biochar additions. She explains that “while biochars can sometimes serve as a source of labile carbon to spur microbial activity, some chars can give off inhibitory compounds that may reduce microbial activity.” In particular, she is looking at whether biochars with high sorption capacity (i.e. the ability to hold on to compounds in the soil) can interfere with signaling between legumes and soil bacteria that fix nitrogen and make it available to plants. She is careful to note that “others have found biochars to increase nodulation in legumes.”
All in all, “the leaders in the field recognize that while there are many benefits of biochar, there can also be negative impacts…There was a burst of [research] excitement followed by some backlash, and now things are starting to even out.” Biochar can serve as a tool for sustainable production systems, but it isn't appropriate for every situation. Continued research will illuminate what types of biochar are suitable for different soils.
