Pyrogenic carbon redistribution in the landscape: example of a small, cultivated temperate watershed

Abstract

Naturally occuring pyrogenic carbon (PyC) is produced during wildfires under oxygen limiting conditions. After a fire event, PyC is fragmented, dissolved and transported at the soil surface1,2 and/or downward into the soil3,4⁠. PyC represents on average 15 % of organic carbon in soils and sediments5⁠.Its residence time in soil ranges from 50 to 1000 years6⁠,which makes it the most persistent form of organic carbon in soils. However, at the mouth of the world’s largest rivers, PyC is on average 16,000 years old. This difference is probably due to isolated measurements of turnover time in surface soil horizons which does not take into consideration transport and accumulation processes happening at the landscape scale. We make the following hypothesis : (i) PyC accumulates at depth in soil and in lowland and hill-foot positions, and (ii) PyC in accumulation zones is significantly older than PyC from other sites/depths.We studied the dynamics of PyC in a well characterized, 120 ha watershed in Brittany, France (ORE AgrHys). We collected soil cores at different topographic positions along three transects and quantified PyC using standard (chemo-thermal oxidation, hydrogen pyrolysis) and novel (Rock-Eval thermal analysis) analytical methods. We also measured the 14C ages of the PyC fraction. We show that relative to total SOC, PyC is preferentially redistributed to depth and that the subsoil (30 to 60 cm) represents about a third of the total soil PyC stock. We do not observe accumulation of PyC at the hill-foot except where superficial erosion products are retained before reaching the stream. We discuss the potential sources and redistribution mechanisms of PyC in the area over the last 10000 years.1. Bellè, S.-L. et al. Biogeosciences Discuss. 1–35 (2021)2. Rumpel, C., Ba, A., Darboux, F., Chaplot, V. & Planchon, O. Geoderma 154, 131–137 (2009).3. Soucémarianadin, L. et al. Soil Biol. Biochem. 133, 12–15 (2019).4. Schiedung, M., Bellè, S. L., Sigmund, G., Kalbitz, K. & Abiven, S. Biogeosciences 17, 6457–6474 (2020).5. Reisser, M., Purves, R. S., Schmidt, M. W. I. & Abiven, S. Front. Earth Sci. 4, 1–14 (2016).6. Singh, N., Abiven, S., Torn, M. S. & Schmidt, M. W. I. Biogeosciences 9, 2847–2857 (2012).