How much fire-derived carbon is transported vertically through the soil and what controls its mobility?

Wildfires produce a fire-derived carbon (C) residue known as pyrogenic carbon (PyC). This residue is one of the largest and most persistent terrestrial organic C pools with residence times over 1000 years. Large quantities of PyC can remain on the site where the fire occurs or can be transferred along the landscape and eventually enter the soil where it represents a major component of the total soil organic carbon (SOC). The drivers explaining the vertical PyC mobility in soils and the quantities it represents remain largely unknown, which limits our understanding of the fate of PyC in the terrestrial systems. Here, we used a manipulative soil column experimental approach in order to study the proportion of transportable PyC and its interaction with the soil mineral phase and non-fire-derived SOC during its vertical transport. To do this, we applied highly ¹³C-labelled ryegrass biochar (as a PyC proxy) on the upper layer of small soil columns (7 cm length, 2.5 cm diameter), and traced the fire-derived C in the soil and in the soluble fractions sequentially extracted during a percolation of 600 pore volumes under saturated conditions (equivalent to 18 years continues rainfall). We studied a combination of ranging soil texture (sandy loam and sand), SOC content (0.3-3.0%) and weathering state (age) of the ryegrass biochar (artificial oxidation with H₂O₂) to identify drivers controlling the vertical mobility of fire-derived C. We hypothesized that: i) Significant proportions of fire-derived C are transported through the soil column over the whole experimental period but decrease with time, ii) fire-derived C can be retained and stabilized within the soil column and influence the mobility of non-fire-derived SOC and iii) weathered fire-derived C is more mobile than fresh fire-derived C which may control its long-term fate in soils.