Letter to the Editor on 'Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle'.

Abstract

Santín et al. (2015) report the conversion of different boreal forest biomass pools to pyrogenic organic matter (PyOM) during a forest fire and suggest that ~100 Tg C yr−1 may be converted to PyOM in boreal forests globally. They further suggest that PyOM formation represents a missing C sink. The phrase ‘missing C sink’ derives from a lack of closure in the atmospheric C budget. Approximately ⅓ of the CO2 emitted to the atmosphere via burning of fossil fuels and land use change cannot be accounted for after oceanic uptake and atmospheric accumulations are tallied (Schlesinger & Bernhardt, 2013). To resolve the C budget, we must identify processes that (i) draw a meaningful quantity of CO2 out of the atmosphere and (ii) have increased their rate of CO2 capture as CO2 emissions from fossil fuels and land use change became important features of Earth's C cycle. The second criterion, perhaps less intuitive than the first, is critical: because atmospheric [CO2] was relatively constant for over 10 000 years of the Holocene, identifying a modern C sink does nothing to resolve Earth's C budget if we cannot also demonstrate that the sink did not exist, or functioned at a reduced rate, prior to the industrial revolution. The formation of PyOM does not meet either criterion. First, the formation of PyOM, regardless of the rate, does not remove CO2 from the atmosphere. When fire generates PyOM, regardless of the persistence of the resulting material, the process represents a transformation of one type of fixed C (biomass) into another (PyOM). As such, there is no direct effect of PyOM generation on atmospheric CO2. Second, PyOM has been forming in boreal forest soils for 1000s of years during which atmospheric CO2 was stable. Although the area of boreal forest subjected to burning apparently has increased in recent decades (Kasischke et al., 2010), there is no evidence that the increased burned area translates to enhanced rates of CO2 capture from the atmosphere via forest productivity. Some studies suggest that PyOM may promote short-term decay of soil organic matter (SOM) (e.g. Cely et al., 2014; Maestrini et al., 2014), and enhanced concentrations of PyOM in soil also could influence a forest's C budget by promoting the retention of SOM (e.g. Whitman et al., 2014). However, neither of these phenomena constitutes a C sink. Even if PyOM promotes SOM retention and thus mitigates soil CO2 emissions, it does not remove CO2 from the atmosphere. The formation of PyOM deserves concerted study because of its potential influence on SOM cycling. However, the formation of PyOM does not remove CO2 from the atmosphere, even if more of it has formed in recent decades, and PyOM is not a missing C sink for atmospheric CO2.