The Interacting Controls of Pyrolysis Temperature and Plant Taxa on the Reactivity of Pyrogenic Organic Matter and Its Influence on Soil Carbon in a Northern Michigan Forest Soil.

Pyrogenic organic matter (PyOM) produced during forest fires is an important component of soil C cycling in boreal-temperate forest ecotones influencing both the rate of stable C input to soils and the microbial interaction with soil organic carbon (SOC) pools. We have an incomplete understanding of how projected changes to forest plant taxa dominance and fire frequency and intensity will interact to affect SOC dynamics. To uniquely determine PyOM-C and native soil C (NSC) turnover rates, we followed the fate of ¹³C-enriched wood or PyOM (200, 300, 450, or 600°C) derived from red maple (RM) or jack pine (JP) wood in soil from a forest in northern Michigan. We found that pyrolysis temperature-controlled PyOM physiochemical properties influenced, with threshold dynamics, PyOM stability resulting in mean residence times (MRT) of up to 450 years, confirming that most PyOM (<600°C) turns over on the century, not millennial time scale. Water leachable C, carbohydrate and non-lignin phenol content correlated positively with early PyOM-C mineralization for both taxa, but the pyrolysis temperature at which this interaction was most pronounced differed with taxa. The overall mineralization rate of the NSC was suppressed with addition of both wood and PyOM from both taxa, with a 3 to 40% reduction across treatments. For both taxa the T300 PyOM, the temperature associated with initial aromatization and carbohydrate conversion, induced the greatest suppression in NSC mineralization as well as the largest increase in soil oxidative enzyme activity. Additionally, the MRT of the fast cycling C pool of NSC was lower (1.4 – 1.6 d) for soils with RM PyOM than for JP PyOM (2.9 – 8.3 d). Our results show that while the first order control on PyOM stability and its interaction with NSC is pyrolysis temperature in this soil, wood taxa influenced both PyOM-C and NSC C MRT.