| 26 Jun 2025
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 PyCCTO 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. PyCHyPy 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 PyCHyPy 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 PyCCTO stock. PyCHyPy proportion in SOC increased with depth and reached an average of 11±3.3 % at 50–60 cm depth. PyCHyPy 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 PyCHyPy 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.
General comments
This study investigates controls on the erosion and stability of pyrogenic across eroding hillslopes in the Brittany region of France with the aim of identifying controls on PyC accumulation across this landscape. By the use of combined methods for quantifying PyC across landscapes, they show increased PyC stocks at deeper soil depths and at some, but not all downslope landform positions.
The authors should consider that they have not measured rates of erosion or slope directly along the transect. This may confound some of their results, as slope can directly affect erosion rates. This manuscript could be improved by including some discussion and/or analysis of data that includes the slope of their landform positions.
Specific comments
Line 81-2. This study (Abney et al 2017) does not give full evidence that the PyC was further eroded, although this is a possibility. It also may have been buried by subsequent erosion events, leached down the soil profile, and/or decomposed.
Paragraph starting line 93. This paragraph is a little bit hard to follow. It would be improved by starting with the overall aims of the study, then the hypotheses and methodological approaches.
Section 2.2. This section would benefit from some more details – are the same number of samples taken from each transect? How many samples total and how many per transect?
For hypothesis 2b, I think this could be more specific, as we would hypothesize that all soil carbon at depth should be older than surface carbon. Perhaps the authors would hypothesize that PyC at depth is older than bulk soil C at depth (which is supported later in results)?
Figure 3 – This addresses hypothesis 2a, but perhaps it would be illustrative to also conduct this analysis with slope. As some upslope landform positions (i.e. far from the toeslope) could be relatively flat and have low erosion rates, or even be depositional environments along the slope where one might hypothesize PyC would remain in place or accumulate compared with more erosion prone (i.e., steep) slopes.
Line 434. It would be better to say that this hypothesis is refuted for this specific site. I don’t think there is sufficient statistical power (n=3 transects) to make this broad statement even for this general region, as there are many soil classifications along the transects.
Technical comments
Line 34-5, Bird et al 2015 review supports this statement. Reference: Bird, M. I., Wynn, J. G., Saiz, G., Wurster, C. M., & McBeath, A. (2015). The pyrogenic carbon cycle. Annual Review of Earth and Planetary Sciences, 43(1), 273-298.
Line 111 – There are several abbreviations here that are not spelled out previously (ORE AgrHys and OZCAR).
Line 112 – “Slopes are at most 5%.” What is the range of slope, or average? This description could benefit from more details, if available.
Line 114 – It would help to identify the time frame of the last glacial period.
Figure 1. The transects in inset A should be labelled in either the legend or caption for this figure.
Line 127. It is not clear, but I assume the last 10,000 years is the most recent unglaciated period? I recommend editing for clarity.