The stability of Pyrogenic Organic Matter is dependent upon its wood source and charring temperature

Abstract:

Fire is a major mediator of carbon (C) cycling in forests and can result in the formation of pyrogenic organic matter (PyOM). The biological reactivity of PyOM is largely dependent upon the physiochemical characteristics of source material and production temperature. As a result, PyOM can persist up to centennial time scales after deposition while simultaneously enhancing or suppressing the mineralization of native soil C (NSC).

To investigate the interactive effects of PyOM source and production temperature on NSC, we added ¹³C-enriched red maple (RM) or jack pine (JP) pyrolyzed at 200, 300, 450 or 600°C to a low C (0.5%;), near-surface soil (0–20 cm-depth) at 60% water holding capacity and 11% of native soil C and then incubated the samples in the dark at 25⁰C for 6 months.

We found that PyOM mineralization rates decreased with increasing pyrolysis temperature for either species while NSC mineralization was suppressed across all treatments with the largest decrease observed in JP 300⁰C. RM mineralization rates were consistently greater (∼5 to ∼25%) than for JP <600⁰C during the first 17 days. Mean residence time (MRT) of PyOM-C was significantly higher in PyOM >300°C resulting in MRT of ~300 – 550 y compared to 8–30 y in lower temperature PyOM (0-300 °C). RM exhibited significantly lower MRT in 300°C PyOM than JP corresponding to differences observed in PyOM and NSC mineralization rates. The modelled active and slow pools of PyOM-C mineralized decreased with increasing combustion temperature. JP 300°C had a 20% smaller active pool when compared to RM suggesting that for both species, 300⁰C - 450⁰C represented a thermal transition point which was most evident in jack pine.

These results highlight how differences in PyOM physiochemical characteristics linked to a species thermal transformation threshold may be a predictor in determining its biological reactivity in soil.