Abstract
Pyrogenic carbon (PyC) constitutes an important pool of soil organic matter, particularly for its reactivity and because of its assumed long residence times in soil. In the past, research on the dynamics of PyC in the soil system has focused on quantifying stock and mean residence time of PyC in soil, as well as determining both PyC stabilization mechanisms and loss pathways. Much of this research has focused on decomposition as the most important loss pathway for PyC from soil. However, the low density of PyC and its high concentration on the soil surface after fire indicates that a significant proportion of PyC formed or deposited on the soil surface is likely laterally transported away from the site of production by wind and water erosion. Here, we present a synthesis of available data and literature to compare the magnitude of the water-driven erosional PyC flux with other important loss pathways, including leaching and decomposition, of PyC from soil. Furthermore, we use a simple first-order kinetic model of soil PyC dynamics to assess the effect of erosion and deposition on residence time of PyC in eroding landscapes. Current reports of PyC mean residence time (MRT) range from 250 to 660 years. Using a specific example-based model system, we find that ignoring the role of erosion may lead to the under- or over-estimation of PyC MRT on the centennial time scale. Furthermore, we find that, depending on the specific landform positions, timescales considered, and initial concentrations of PyC in soil, ignoring the role of erosion in distributing PyC across a landscape can lead to discrepancies in PyC concentrations on the order of several hundred g PyC m-2. Erosion is an important PyC flux that can act as a significant control on the stock and residence time of PyC in the soil system.
Highlights
Fires and Production of Pyrogenic CarbonFire is a major environmental perturbation and driver of biogeochemical processes across a diversity of landscapes worldwide
The synthesis of published results and the model presented here illustrates how not accounting for integrated dynamic decomposition and geomorphological processes can lead to significant errors in our current understanding of pyrogenic carbon (PyC) dynamics in the terrestrial ecosystem
Not accounting for post-fire erosion could lead to significant differences in projected stocks and turnover times of PyC within the soil, depending on specific ecosystem properties such as erosion and decomposition rates
Summary
Fire is a major environmental perturbation and driver of biogeochemical processes across a diversity of landscapes worldwide. PyC can become stabilized or lost from soil through similar processes that control the fate of bulk or non-pyrogenic carbon, including biotic and abiotic decomposition, leaching, and erosion (Rumpel et al, 2006; Major et al, 2010; Zimmerman, 2010). The presence of PyC in litter and surface soil can increase the rates of bulk erosion, as fire-affected biomass tends to have lower density, compared to uncharred biomass and litter, making it easier to transport by both water- (Rumpel et al, 2006) and winddriven (Beyers et al, 2005; Shakesby, 2011) erosional processes. Soot and smaller PyC constituents can be rapidly transported post-fire (Figure 3), depending on local wind and precipitation conditions, such that dry climates are TABLE 1 | Examples of decomposition rates measured in laboratory and field studies, as converted to percent mass loss of PyC per year
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