COS 228-5 - Fire exclusion, pyrogenic carbon, and ecosystem function: What have we lost?

Abstract: Elimination of fire from the vast majority of the Great Plains prairies and more than a century of forest management and active fire suppression as part of Euro-American settlement, have greatly terminated recurrent fires and their legacies from forest and grassland ecosystems of the western and Midwestern United States (US). A key legacy of fire is pyrogenic carbon (PyC), a spectrum of thermally altered organic materials formed during incomplete biomass burning events, which can directly influence soil properties and processes. PyC deposited during recurrent fire events serves as a significant contributor to the “slow C pool” and plays a significant role in dictating spatial and temporal diversity and soil function at a landscape scale. We conducted spatial, temporal assessment of historical biomass production and recurrent fire using a combination of biomass production and fire consumption data to determine the mass of PyC not produced due to fire exclusion. We found that recurrent fire in US Great Plains grassland ecosystems generated about 8.8 ± 3.9 Tg PyC yr^-1. Removal of these inputs as a result of fire exclusion over the last 125 years has likely resulted in reduction of grassland soil PyC inputs totaling between 963 Tg and 1,028 Tg of PyC. Once deposited in soil, PyC may remain intact for hundreds to thousands of years and thus influence long-term C storage in soils. PyC influences a range of soil physical, chemical, and biotic properties and to alter soil nutrient availability and nutrient cycling. However, sorption capacity of PyC in soil declines with time since formation. The impact of this loss of recurrent PyC input to the soil environment is not well understood. The use of prescribed fire treatments can re-introduce PyC to forest and grassland ecosystems, but the scale of what is added is small compared to the long-term loss of PyC. In agricultural settings, or where prescribed fire is infeasible, biochar applications may facilitate some of the critical ecosystem processes inadvertently lost following Settler Colonialism and the elimination of recurrent fire with contemporary land management strategies.