Empirical correlation between electrical conductivity and nitrogen content in biochar as influenced by pyrolysis temperature

Authors: DE MORAIS, EVERTON GERALDO - Federal University Of Lavras; SILVA, CARLOS ALBERTO - Federal University Of Lavras; Gao, Suduan; MELO, LEONIDAS C A - Federal University Of Lavras; LAGO, BRUNO COCCO - Federal University Of Lavras; TEODORA, J.C. - Federal University Of Lavras; GUILHERME, L R G - Federal University Of Lavras

Submitted to: Nitrogen
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2024
Publication Date: 4/9/2024
Citation: de Morais, E., Silva, C., Gao, S., Melo, L., Lago, B., Teodora, J., Guilherme, L. 2024. Empirical correlation between electrical conductivity and nitrogen content in biochar as influenced by pyrolysis temperature. Nitrogen. 5(2):288-300. https://doi.org/10.3390/nitrogen5020019.
DOI: https://doi.org/10.3390/nitrogen5020019

Interpretive Summary: Despite many studies on biochar’s agronomic benefits as a soil amendment, the effects of production condition on nitrogen (N) content in biochar and means to predict it are not clear. This study determined the correlation between biochar N content and its electrical conductivity (EC) as affected by pyrolysis temperature and the ability of EC that can be easily measured to predict N content in biochar. Biochar products derived from coffee husk and chicken manure produced at increasing PT (300 to 750°C) were used in this study. The results show that increasing pyrolysis temperature caused greater N loss during pyrolysis and consequently reduced total N content in biochar. The highest N loss (82%) or lowest total N content (1.2 g kg-1) were found in chicken manure biochar pyrolyzed at 750°C. The lowest N loss during pyrolysis was at 300°C for both feedstock biochars. A negative correlation between EC and total N, and a positive correlation with N loss, were found across the wide range of pyrolysis temperature investigated for both biochar products. These relationships enabled the use of EC to predict N content in biochar affected by feedstock and pyrolysis temperature.

Technical Abstract: Much progress has been made in understanding the conditions of biochar production related to biochar properties and carbon (C), but very little knowledge has been gained regarding the effects on nitrogen (N), one of the most important nutrients affected by pyrolysis temperature (PT). Analysis of N in biochar is costly and alternative methods should be developed to easily estimate the N content in biochar under different pyrolysis conditions. We hypothesized that there is a correlation between biochar N content and its electrical conductivity (EC). We aimed to evaluate total N and the effect of PT through the correlation with EC, a parameter that can be easily measured. Biochar products derived from coffee husk (CH) and chicken manure (CM) produced at increasing PT (300 to 750°C) were used for the study and measured for total N and EC. The increase in PT caused greater N loss, consequently reducing total N content in biochars, with the highest loss (82%) and lowest total N content (1.2 g kg-1) found in CM biochar pyrolyzed at 750 °C. The lowest N loss (21% for CH biochar and 36% for CM biochar) was observed at a PT of 300 °C. A negative correlation between EC and total N, and a positive correlation with N loss, were found in both biochar products across the wide range of PT investigated. To preserve the N content in biochars, the PT should not exceed 400 °C. Our results indicate that EC is a fast and accurate biochar proxy attribute capable of predicting the N content and its loss in coffee husk and chicken manure-derived biochars, as the pyrolysis temperature increased from 300°C to 750 °C, and could be used as an alternative to easily predict the N in biochar. A larger set of biochar samples and pyrolysis con-ditions should be tested to validate this approach.