DEFINITION: Biochar is black carbon produced from biomass sources [i.e., wood chips, plant residues, manure or other agricultural waste products] for the purpose of transforming the biomass carbon into a more stable form (carbon sequestration). Black carbon is the name of the range of solid residual products resulting from the chemical and/or thermal conversion of any carbon containing material (e.g., fossil fuels and biomass) (Jones et al., 1997). Biochar does not refer to a singular product with a given set of chemical and physical characteristics. Rather, biochar spans the spectrum of black carbon forms (Spokas, 2010) and it is chemically and physically unique as a function of the feedstock, creation process (pyrolysis unit), cooling, and storage conditions.
PURPOSE: The main purpose for the creation of biochar is for carbon sequestration. Biochar is speculated to have been used as a soil supplement thousands of years ago in the Amazon basin, where regions of fertile soil called "Terra Preta'" (dark earth) were created by indigenous people. Anthropologists hypothesize that inhabitants of the region produced biochar by practicing 'slash and char' management on vegetation to improve soil fertility and crop yields (Mann, 2005). Biochar application to soil has been the assumed end use for the created biochar. Even though biochar can be used in other purposes as long as the biochar is not used for fuel or energy. The burning of biochar would not achieve the goal of carbon sequestration since this would allow the carbon to return to the atmospheric pool where it originated before being fixed in plants by photosynthesis.
HOW DOES THIS PRACTICE WORK: In the Amazon basin, biochar along with debris from cooking-fires and kitchen waste products were hypothesized to be mixed into the soil (Oxisols) resulting in enrichment of soil organic carbon (SOC), other nutrients such as nitrogen (N), phosphorus (P), and potassium (K) as well as increasing the soils cation exchange capacity (CEC) (Denevan, 1996; Glaser et al., 2002; Liang et al., 2006). Over time, this practice is believed to result in the conversion of the unfertile, red-color soil into the darker-colored soils capable of producing crop yields sufficient to feed the Amazonian inhabitants (Mann, 2005). In fact, thousands of years since their creation, these Terra Preta soils are still desired today since they have high fertility characteristics compared to other soils in the region. However, we do not fully understand the fundamental mechanisms responsible or the overall contribution of the black carbon in this observed soil fertility improvement. Nevertheless, biochars are being manufactured to replicatethe Terra Preta condition in worn-out or over-managed soils for restoring their crop productivity potentials. Unfortunately, there is not 'a-one-size-fits-all' biochar for soils (Novak and Busscher, 2011). The addition of biochar to soil has the potential of increasing soil carbon, soil nutrient content, and plant productivity. On the other hand, biochar additions have been observed to alter microbial populations and negatively impact plant growth. Thus, for a biochar to deliver a benefit, it is important to understand how biochar quality (physical and chemical properties) is influenced by the choice of feedstock and the pyrolysis conditions used in its manufacturer (Antal and Gr?nli, 2003; Lehmann and Joseph, 2009; Novak et al., 2009).
WHERE THIS PRACTICE APPLIES AND ITS LIMITATIONS: For biochar to serve a beneficial role in revitalizing nutrient impoverish soils, there should be a noted increase in the quantity of plant available nutrients and its nutrition retention capacity. However, improving soil fertility and crop yield is an intricate task, involving a complex balance of biotic and abiotic processes. The exact mechanisms of the observed impacts on plant and soil microbial system are not yet fully understood.
EFFECTIVENESS: When examining the literature, there is large variability in the response of agronomic crop yield to biochar additions [negative to >2 fold improvements] (Spokas et al., 2012). From examining past studies, there is the suggestion that wood based biochar and biochar with higher nutrient contents (e.g., poultry and dairy manure biochar) have a higher likelihood of producing a positive yield improvement than other biochar source materials (Spokas et al., 2012). However, care must be taken when extrapolating results from past historical literature due to the skewed positive bias, since the publication of positive results is typically favored (M?ller and Jennions, 2001).
COST OF ESTABLISHING AND PUTTING PRACTICE IN PLACE: The economics for direct field application of biochar are speculative, and with optimistic assumptions (e.g. crop yield benefits, carbon sequestration credits, and fertilizer offsets) only marginally economically viable given the absence of a biochar market and limited number of production scale biomass pyrolysis plants.
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Spokas, K.A., Cantrell, K.B., Novak, J.M., Archer, D.W., Ippolito, J.A., Collins, H.P., Boateng, A.A., Lima, I.M., Lamb, M.C., McAloon, A.J., Lentz, O.D., Nichols, K.A. 2012. Biochar: A synthesis of its agronomic impact beyond carbon sequestration. Journal of Environmental Quality. 41(4):973-989.
Field experiments are ongoing in Rosemount, MN as part of the USDA-ARS Biochar and Pyrolysis Iniative.