Chemical and Isotopic Thresholds in Charring: Implications for the Interpretation of Charcoal Mass and Isotopic Data

Charcoal plays a significant role in the long-term carbon cycle and its use as a soil amendment is becoming a viable carbon sequestration strategy (biochar). One challenge in this research area has been comparing results between studies in part due to the diversity of lab and field production conditions. Although the highest treatment temperature (HTT) is often used to describe pyrolysis conditions, several studies have shown that length of time at the highest temperature can also cause changes to the physicochemical qualities of charcoal and ignoring this effect may introduce inter-comparison problems. Addressing this issue becomes especially important in the discussion of optimizing biochar for soil remediation and carbon sequestration, and in discussions of charcoal use in reconstructing past fire regimes, as increasing time at temperature may cause changes in charcoal properties similar to the changes caused by increasing HTT. Here we introduce a formal definition of charring intensity (CI) to more accurately characterize pyrolysis, and we document variation in this property with pyrolysis temperature and reaction duration. We found two types of responses to CI: either a linear or a threshold relationship. We show that a threshold exists where %C, %N and δ15N begin exhibiting large changes, and this CI threshold co-occurred with an increase in charcoal aromaticity. Mass yield decreased linearly with charring intensity and carbon isotopes did not change from original biomass values in our controlled laboratory experiments. Analysis of these data shows that pyrolysis parameters should be defined in the literature as a combination of temperature and duration conditions, and that biomass that has undergone pyrolysis may be influencing soil organic nitrogen. Additionally, the lack of alteration in carbon isotopes across our matrix supports the efficacy of using pyrolyzed material for archaeological reconstructions.