Biochar N Content, Pools and Aromaticity as Affected by Feedstock and Pyrolysis Temperature

Forms and availability of nitrogen (N) in biochars are influenced by both feedstock and pyrolysis temperature. We conducted an assessment to examine how feedstock composition and pyrolysis temperature impact crucial properties and nitrogen (N) content and distributions within biochars. We subjected feedstocks with 0.2% to 8.9% N to pyrolysis at 300 °C and 750 °C. Biochars derived from different feedstock types (n = 11) were assessed for pH, electrical conductivity (EC), total C, water-soluble C (WSC) content, yield, total and available N content, as well as for C and N retention. We examined the aromaticity of the biochars by analyzing the ratio of specific FTIR spectra bands. As the pyrolysis temperature increased, we observed a decrease in yield, total N content, and N in the forms of N-NH₄⁺ and N-NO₃⁻. Concurrently, with the intensification of the charring process, the WSC content experienced a sharp reduction. Most biochars experienced an increase in carbon (C) content along with a notable decrease in total nitrogen (N) content as the pyrolysis process intensified. As charring conditions intensified, available N forms, such as N-NH₄⁺ and N-NO₃⁻, were lost. Biochars obtained from pine bark, eucalyptus sawdust, sugarcane bagasse, and bamboo exhibited high C content, low N content, and C:N ratios exceeding 100:1. The losses of C and N were independent of each other, with N chemical species demonstrating higher volatility compared to C compounds. At a temperature of 300 °C, the extent of N loss during pyrolysis depended on the feedstock, and the N content in the resulting biochar could be predicted based on the N content in the original feedstock. Pyrolysis at 300 °C retained a substantial amount of N in biochars derived from chicken manure, castor oil cake, chitosan, and shrimp carcass. This preserved nitrogen can serve as a valuable nitrogen source for crop applications.