the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2025
Rates of soil organic carbon loss from rainforest to pasture conversion at a deforestation hotspot in the Amazon basin
Abstract. Deforestation in the Amazon basin drives critical losses of soil organic carbon stocks (SOCs), but the impacts of pasture conversion in tropical hotspots of deforestation remain poorly quantified in soil depths and time. Using the chronosequence approach, we sampled mature forests and pastures from 1 to 30 years old to address variations in soil bulk density (SBD), soil organic carbon concentration (SOCc) and stocks (SOCs). Mass spectrometry of the isotope ratio (IRMS) was used to obtain δ13C values and estimate the input of forest and pasture SOC to the total pool. The rainforest soil (0–30 cm) stored 83.328 Mg C ha-1, after 30 years of pasture conversion, SOCs declined by 21 % (66.061 Mg C ha-1), while SBD increased 15 % (1.215 to 1.397 g cm-3). Soil carbon turnover was depth dependent: forest-derived SOCs loss rate at 10–20 cm (0.112 year-1) was twice as fast in the upper horizon (0–10 cm; 0.063 year-1); simultaneously, pasture-derived SOCs gain was faster in the upper 10 cm (0.055 % year-1) reflecting the change from forests deep roots to a pastures shallower system. After 30 years of pasture conversion, forest-derived SOC still represented 19.6 % of the total pool in the 0–10 cm soil horizon, highlighting the important effect of previous vegetation.
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RC1: 'Comment on egusphere-2025-2959', Anonymous Referee #1, 30 Jun 2025
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In this manuscript the authors present a relative straightforward study conducted in South East Colombia in which they use the chronosequence approach to assess changes in soil bulk density, soil C concentration, soil C stocks and origin of soil C based on 13C isotope analysis. They show that the soil C stocks declined over time, but surprisingly forest-derived soil C stocks losses were faster in the 10-20 cm depth interval than in the 0-10cm depth interval whereas the pasture derived soil C stocks increased faster in the 0-10 cm depth interval, which they explain with the change from deep roots in the forest to shallow roots in the pastures.
The strength of this study is that it was conducted in the field, based on a careful site selection with sufficient replication. It was conducted in an understudied area of the Amazon basin and as such it is important that these data become available to the scientific community. However, there are some critical points, that I think the authors should address before I can recommend publication.
-As is mentioned by the authors in the discussion (l. 202) soil texture can have a major influence on decomposition rates. Unfortunately, soil texture is never mentioned and may have had a strong influence on the results. The authors mention that soil were Ferralsols or Acricols (l. 75). Whereas these soils all have in common that they are highly weathered with a low CEC they may have substantial difference in soil texture, especially since Acrisols have undergone clay translocation. The best solution for this problem is if you can analyze back-up sample for soil texture (sand-silt-clay). At a minimum you should convince the reviewers that there were no significant differences in soil texture among the sampling sites.
-To assess forest- and pasture-derived C normally you also need an 13C values of the ‘endmembers’ (forest litter and pasture litter). If I am not mistaken, this is not mentioned. Please give the ‘pure’ signatures of C3 and C4 that you used and explain how you assessed them.
-In your assessment you assume that the forest was replaced with a 100% C4 pasture. However, in my experience, pure C4 pastures do not exist, especially in frontier areas like where you worked. So the C-input in the pasture, will be at least partly be from C3 plants and this contribution may even increase with pasture age if a pasture degrades and C3 bushes start to grow. I think this may be the main reason why you actually found slower forest-derived SOCc loss in the top 10 cm compared to the 10-20 cm depth interval: you assume that all C3 carbon in the top 10cm was forest-derived, but part of it is probably from C3 herbs or bushes in the pastures. There is no simply way to fix this, but you could do an analysis in which you make assumptions about the C3 carbon input and how this may have affected your results.
-I noticed in Figure 1 that there are irregular yellow areas that have no pasture age. Could you explain what these areas are?
-I did not see the original data (maybe I missed them). However, I think it is very important that these data will become available without restrictions and I encourage you to include them in an appendix or in a data repository. I know that you write that they will be made available upon request, however, it is possible that the first author cannot be reached in some years which would make the data unavailable.
Citation: https://doi.org/10.5194/egusphere-2025-2959-RC1
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