Empirical Correlation between Electrical Conductivity and Nitrogen Content in Biochar as Influenced by Pyrolysis Temperature

Abstract

Much progress has been made in understanding the conditions of biochar production related to biochar properties and carbon (C). Still, very little knowledge has been gained regarding the effects on nitrogen (N), one of the most critical nutrients affected by pyrolysis temperature (PT). Analysis of N in biochar is costly, and alternative methods should be developed to estimate the N content in biochar quickly under different pyrolysis conditions. We hypothesized that there was 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 significant 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 predict the N in biochar easily. A more extensive set of biochar samples and pyrolysis conditions should be tested to validate this approach.