Subsoils, but not toeslopes, store millennia-old PyC in a gently sloping catchment under temperate climate after centuries of fire suppression

Abstract. Pyrogenic carbon (PyC) is the carbonaceous solid residue of incomplete combustion of biomass. It is a continuum of condensed and aromatic molecules. PyC persists for longer in soils relative to non-PyC organic carbon. However, residence time varies depending on the method used to estimate it. The time and spatial scales investigated are not always adapted to the long-residence time and vertical and lateral mobility of PyC in the soil profile and the landscape. In addition, agricultural land-use and shallow slopes are under-represented in the PyC literature.

We measured the concentrations and stocks of PyC down to 60 cm along three toposequences in a small agricultural catchment with shallow slopes and homogeneous soil parent material in the west of France. We used two methods (chemo-thermal oxidation – CTO and hydropyrolysis – HyPy) of PyC quantification that cover the intermediate to highly condensed part of the PyC continuum, and also measured the radiocarbon values in both total SOC and the PyC fraction. There was no or little PyC inputs to the catchment in the last 150 years which gave us access to the resultant, long term PyC distribution in the landscape. In particular, we aimed to investigate whether the vertical and horizontal distribution of PyC were similar or differed from SOC and whether they were affected by the gradient of soil evolution along the slope.

Topographic position was not the main driver of PyC stocks in this landscape. The stock of PyC_CTO averaged 2.5±0.22 t ha^-1 across topographic positions and was only slightly larger in a Solimovic Cambisol at the toeslope (3.3±0.26 t ha⁻¹), likely formed following changes in erosion dynamics with land-use. Contrary to previous reports, erosion redistributed already aged PyC without enrichment or depletion. Future studies should assess whether erosion modalities and age and quality of PyC affect its fate during erosion events. PyC_HyPy concentrations in the topsoil decreased from upslope (median = 1.6, IQR = 0.22 gC kg⁻¹ soil) to downslope positions (median = 1.10, IQR = 0.40 gC kg⁻¹ soil), which we attribute to PyC_HyPy dissolution following the destabilization of mineral associations with iron oxides in the water-table affected portion of the transects. The subsoil (30–60 cm) represented between 37 and 51 % of the PyC_CTO stock. PyC_HyPy proportion in SOC increased with depth and reached an average of 11±3.3 % at 50–60 cm depth. PyC_HyPy had an uncalibrated radiocarbon age of 2520 to 9600 years BP at this depth, significantly older than bulk SOC at the same depth and than PyC_HyPy at 0–10 cm. These results confirm the long persistence of PyC in soils and point to a slow advection of PyC towards the soil depth under the pedoclimatic conditions of our study area. Identifying the proportion of PyC produced which is quickly transported away from the watershed and that which remains and is stabilized in soils for millennia after a fire is an important knowledge gap that still needs to be investigated to close the terrestrial PyC budget.