Abstract
Abstract. Pyrogenic carbon (PyC) is produced by the incomplete combustion of vegetation during wildfires and is a major and persistent pool of the global carbon (C) cycle. However, its redistribution in the landscape after fires remains largely unknown. Therefore, we conducted rainfall simulation experiments on 0.25 m2 plots with two distinct Swiss forest soils (Cambisol (clay loam) and Luvisol (sandy silt)). We applied PyC produced from wood (Picea abies) labeled under FACE conditions and C4 grass (Miscanthus sinensis) to the soil surface to study PyC redistribution by runoff and splash and the vertical mobility of PyC in a 10 cm unsaturated soil column based on the differences in δ13C of soils and PyC. We assessed the effect of soil texture, slope angle and PyC characteristics (feedstock and particle size) on the mobility of PyC during 30 min of intense rainfall (102 mm h−1). Our results highlight that PyC is highly mobile. Surface runoff transported between 0.2 % and 36.0 % of the total added PyC. Erosion by splash further redistributed 10.3 % to 25.3 % of the added PyC. Soil type had a substantial impact on the redistribution of PyC by both runoff and splash: on average, we recovered 10.5 % of the added PyC in runoff and splashed material for the clay-rich Cambisol and 61.3 % of the added PyC for the sandy silt Luvisol combined. PyC feedstock had a clear but contrasting effect on PyC redistribution: relocation in the runoff and splashed material was greater for wood PyC (43.4 % of total added PyC) than grass PyC (28.4 %). However, more wood PyC (11.5 %; fraction of organic C derived from the PyC) remained where it was initially applied compared to grass PyC (7.4 %). The results further suggest that the effect of PyC characteristics on its mobility can be highly variable and depend not only on the material from which it was derived, but also on other factors (e.g., particle size, porosity, density). In particular, the mobility of PyC was almost twice as large for fine-grained PyC (< 63 µm) than for coarse PyC (63 µm–2 mm). Vertical mobility of PyC up to 10 cm depth was greater in the clay-rich, well-aggregated Cambisol but limited in the physically instable Luvisol, likely due to quick aggregate breakdown and surface sealing. The addition of PyC to the surface of the studied soils further induced changes in the export of native soil organic carbon (nSOC) after the 30 min rainfall event. Our study shows that large quantities of PyC can be redistributed by water erosion over short timescales and that the mobility of PyC depends to a great extent on the response of soils to rainfall. Moreover, the addition and redistribution of PyC affects the export of nSOC and thus the C budget of fire-affected soils and catchments.
Highlights
Wildfires burn 345 to 464 Mha, i.e., 4 % of vegetated land area, annually and are a major disturbance for terrestrial ecosystems (Giglio et al, 2013; Randerson et al, 2012)
We found more wood Pyrogenic carbon (PyC) than grass PyC on the soil surface (0–1 cm) in the upslope position where PyC was applied for both soils (fraction of organic carbon (OC) derived from the PyC in percent: 11.5 ± 1.5 % for wood PyC and 7.4 ± 1.0 % for grass PyC; Fig. 6a–b), indicating that wood PyC either remained where initially applied or moved outside the 0.25 m2 plot, while grass PyC was redistributed more within the plot
We show that PyC was highly mobile and quickly transported and relocated on 0.25 m2 plots during 30 min of intense rainfall
Summary
Wildfires burn 345 to 464 Mha, i.e., 4 % of vegetated land area, annually and are a major disturbance for terrestrial ecosystems (Giglio et al, 2013; Randerson et al, 2012). PyC is environmentally more resistant than its unburnt precursors and can remain in terrestrial and aquatic ecosystems for decades to millennia (Abiven and Santín, 2019; Coppola and Druffel, 2016; Santín et al, 2016). It is one of the largest and oldest C pools on Earth (Bird et al, 2015) and globally accounts for around 15 % of organic carbon (OC) in soils (Reisser et al, 2016). We currently have little knowledge about the fate of PyC in the landscape, and the dominant processes that lead to its mobilization, degradation and stabilization at different landscape positions and at the interface between land and ocean still need to be clarified (Abiven and Santín, 2019; Abney and Berhe, 2018; Masiello and Berhe, 2020)
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