Feedstock choice, pyrolysis temperature and type influence biochar characteristics: a comprehensive meta-data analysis review

Authors: IPPOLITO, JAMES - Colorado State University; CUI, LIQIAN - Yancheng Institute Of Technology; KAMMANN, CLAUDIA - Hochschule Geisenheim University; WRAGE-MONNIG, NICOLE - University Of Rostock; ESTAVILLO, JOSE - University Of Basque Country; FUERTES-MENDIZABAL, TERESA - University Of Basque Country; CAYUELA, MARIA LUZ - Universidad De Murcia; Sigua, Gilbert; Novak, Jeffrey; Spokas, Kurt; BORCHARD, NILS - Natural Resources Institute Finland (LUKE)

Submitted to: Biochar
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2020
Publication Date: 9/28/2020
Citation: Ippolito, J.A., Cui, L., Kammann, C., Wrage-Monnig, N., Estavillo, J.M., Fuertes-Mendizabal, T., Cayuela, M., Sigua, G.C., Novak, J.M., Spokas, K.A., Borchard, N. 2020. Feedstock choice, pyrolysis temperature and type influence biochar characteristics: a comprehensive meta-data analysis review. Biochar. 2:421-438. https://doi.org/10.1007/s42773-020-00067-x.
DOI: https://doi.org/10.1007/s42773-020-00067-x

Interpretive Summary: Understanding the influences that pyrolysis type, pyrolysis temperature, and initial feedstocks have on final biochar properties can help researchers and practitioners create biochars to meet agricultural environmental demands. Based on ~5,400 published articles and over 50,000 individual observations, this project makes inferences to further our understanding of biochar physicochemical properties from the broad to specific and minute perspective. As compared to fast pyrolysis, slow pyrolysis leads to biochars containing greater specific surface area (SSA), calcium carbonate equivalent (CCE), ash content, available iron and nitrate concentrations. Pyrolysis temperature influences biochar stability, with temperatures >500 oC generally leading to longer-term half-lives (>1,000 years). This, in combination with greater pyrolysis temperatures promoting more stable carbon (C) structures, greater SSA, and potential improvements in soil aeration, percolation, infiltration, and overall structure, potentially suggest that greater pyrolysis temperatures may lead to long-term soil improvements and C storage. Perhaps the most important influence on final biochar properties is feedstock choice. Wood-based feedstocks typically led to biochars containing the greatest SSA as compared to other feedstocks; this, in combination with pyrolysis temperature could greatly influence soil improvements. Crop-, grass-, and manures/biosolids-based feedstocks led to biochars containing elevated cation exchange capacity (CEC) as compared to wood-based biochars, which could affect nutrient sorption following land application. Based on the complete dataset collected, it appears possible to predict some plant-available biochar nutrients simply from total nutrient analysis. The collected data showed that we could reasonably predict 1) available nitrogen (N) from softwood, corn, pig manure, and cattle manure biochars, 2) available phosphorus (P) from corn, wheat, and rice straw/husk biochars, and 3) available potassium (K) from hardwood, softwood, and wheat derived biochars. This latter information could be useful when creating designer biochars for specific nutrient applications, simply by blending several feedstocks together. Based on this information, future research should test whether the available nutrient predictive functions, in combination with created mixed feedstock biochars, would hold true when placed within nutrient-poor soils.

Technical Abstract: Various studies have established that feedstock choice, pyrolysis temperature, and pyrolysis type influence final biochar physicochemical characteristics. However, overarching analyses of pre-biochar creation choices and correlations to biochar characteristics is severely lacking. Thus, the objective of this work is to help researchers, biochar-stakeholders and practitioners make more well-informed choices in terms of how these three major parameters influence the final biochar product. Utilizing approximately 5400 peer-reviewed journal articles and over 50,800 individual data points, herein we elucidate the selections that influence final biochar physical and chemical properties, total nutrient content, and perhaps more importantly tools one can use to predict biochar’s nutrient availability. Based on the large dataset collected, it appears that pyrolysis type (fast or slow) plays a minor role in biochar physico- (inorganic) chemical characteristics; few differences were evident between production styles. Pyrolysis temperature, however, affects biochar’s longevity, with pyrolysis temperatures greater (>) 500 degrees centigrade (oC) generally leading to longer-term (i.e., >1000 year) half-lives. Greater pyrolysis temperatures also led to biochars containing greater overall carbon (C) and specific surface area (SSA), which could promote soil physico-chemical improvements. However, based on the collected data, it appears that feedstock selection has the largest influence on biochar properties. Specific surface area is greatest in wood-based biochars, which in combination with pyrolysis temperature could likely promote greater changes in soil physical characteristics over other feedstock-based biochars. Crop- and other grass-based biochars appear to have cation exchange capacities greater than other biochars, which in combination with pyrolysis temperature could potentially lead to longer-term changes in soil nutrient retention. The collected data also suggest that one can reasonably predict the availability of various biochar nutrients (e.g., nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium, iron, and copper) based on feedstock choice and total nutrient content. Results can be used to create designer biochars to help solve environmental issues and supply a variety of plant-available nutrients for crop growth.