Effect of low-temperature pyrolysis conditions on biochar for agricultural use
J. W. Gaskin, C. Steiner, K. Harris, K. C. Das, B. Bibens. 2008. Effect of low-temperature pyrolysis conditions on biochar for agricultural use. Transactions of the American Society of Agricultural and Biological Engineers, Vol. 51(6): 2061-2069.
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Abstract:
The removal of crop residues for bio-energy production reduces the formation of soil organic carbon (SOC) and therefore can have negative impacts on soil fertility. Pyrolysis (thermoconversion of biomass under anaerobic conditions) generates liquid or gaseous fuels and a char (biochar) recalcitrant against decomposition. Biochar can be used to increase SOC and cycle nutrients back into agricultural fields. In this case, crop residues can be used as a potential energy source as well as to sequester carbon (C) and improve soil quality. To evaluate the agronomic potential of biochar, we analyzed biochar produced from poultry litter, peanut hulls, and pine chips produced at 400°C and 500°C with or without steam activation.
The C content of the biochar ranged from 40% in the poultry litter (PL) biochar to 78% in the pine chip (PC) biochar. The total and Mehlich I extractable nutrient concentrations in the biochar were strongly influenced by feedstock. Feedstock nutrients (P, K, Ca, Mg) were concentrated in the biochar and were significantly higher in the biochars produced at 500°C. A large proportion of N was conserved in the biochar, ranging from 27.4% in the PL biochar to 89.6% in the PC biochar. The amount of N conserved was inversely proportional to the feedstock N concentration. The cation exchange capacity was significantly higher in biochar produced at lower temperature. The results indicate that, depending on feedstock, some biochars have potential to serve as nutrient sources as well as sequester C.
Introduction:
… There are several lines of evidence that charcoal plays an important role in soil fertility. Charcoal has been identified as an important soil constituent in fertile Chernozems (Schmitdt et al., 1999) and in anthropogenic enriched dark soil (Terra Preta) found throughout the lowland portion of the Amazon Basin (Glaser et al., 2000). Research on tropical soils indicates that charcoal amendments can increase and sustain soil fertility (Steiner et al., 2007). The beneficial effects appear to be related to alterations in soil physical, chemical, and biological properties, such as reduced acidity (Topoliantz et al., 2005), increased cation exchange capacity (CEC) (Cheng et al., 2008; Liang et al., 2006), enhanced nitrogen (N) retention (Lehmann et al., 2003; Steiner et al., 2008b), increased microbiological activity (Steiner et al., 2008a), and increased mycorrhizal associations (Warnock et al., 2007). …
Charcoals produced from wildfire or traditional charcoal production may have different chemical and physical characteristics from pyrolytic biochars created under specific conditions for energy production. …
After forest fires, on average, only 3% of the N in the biomass is found in ash, which contains black carbon or biochar (Giardina et al., 2000). …
Studies of wildfire effects on biomass composition indicate that N begins to volatilize at 200°C, and above 500°C half of the N in organic matter is lost to the atmosphere. …
Conclusions:
Pyrolytic biochar has the potential to be used in agricultural production to sequester carbon and serve as a fertilizer. Although pyrolysis conditions are known to affect the chemical and physical characteristics of biochar, at the relatively low pyrolysis temperatures used in this study, feedstock characteristics had the greatest influence on key agricultural characteristics. Carbon concentrations in the biochars decreased with increasing mineral content of the feedstock. Little DC was leachable from the fresh biochar. A high proportion of the feedstock N was conserved in the biochar; however, the N may not be plant available. Nutrients such as P, K, and Ca are extractable with a weak double acid extractant and may be plant available.
The higher pyrolysis temperature increased nutrient concentrations, except for N, but decreased CEC. Recent literature has shown that natural long-term oxidation of biochar in the soil increases the amount of negative charges on the biochar surface (Cheng et al., 2008). Development and optimization of pyrolysis and post-production treatments to increase CEC or available nutrients is important in order to increase the immediate benefits of biochar applications in agriculture.
