| 12 Aug 2025
Amazonian Podzols – a carbon time bomb?
Abstract. It has recently been shown that the C stocks in Amazonian podzols are very large. They are much larger than was previously thought, particularly in the Bh horizon, which has been estimated to contain in excess of 10 Pg C for Amazonia alone. It is predicted that changes in the regional climate will result in a drier soil moisture regime, which may affect the C dynamics in these generally waterlogged soils. In order to determine the vulnerability to decomposition of the organic C contained in the Amazonian podzols as a result of environmental changes, we established a series of incubation experiments in which the effects of different environmental factors were measured. The direct effect of drier soil moisture regimes was tested by incubating undisturbed cores from the Bh horizon at a range of matric potentials. Contrary to what is usually found in soils, no significant difference in mineralisation was found among matric potentials, suggesting that other factors control microbial mineralisation of this organic C. In a second series of incubations, the effect of nitrogen additions, of anoxic conditions and of labile C substrate additions were also tested on undisturbed cores of the Bh horizon. Samples incubated under oxic conditions produced more than twice as much CO2 as samples incubated under anoxic conditions, whilst the mineralisation rates of samples incubated under oxic conditions with the addition of N increased more than four-fold relative to the anoxic samples. The addition of labile C did not have a significant effect on C mineralisation. An extrapolation of the data to the whole of the Amazonian podzols suggests that changes in environmental conditions could result in an increased C-CO2 flux to the atmosphere of up to 0.41 Pg C per annum. This is equivalent to 8 % of the current net global C flux to the atmosphere.
This study addresses the question of decomposability of organic matter in hydromorphic podzol soils from the Amazon basin. These soils are extremely interesting given their debated origin and vulnerability to environmental change. Through a set of incubations, the authors of this study found a strong response of C mineralization to oxygen and nitrogen additions, providing important information to improve our understanding of the main environmental controls that promote the stability of carbon in their Bh horizon.
In general, this is a very good study that merits publication in Biogeosciences. However, I do have a few major comments that prevent me to recommend publication in its current form.
*Major comments*
1. I strongly dislike the title of this study. The authors do not provide here any analysis of the explosiveness of this carbon reservoir and when such a 'bomb' explosion would occur. I know the term 'carbon time bomb' is used here metaphorically, but I do not find evidence in the results for such a metaphor. The study helps to understand the mechanisms for carbon stability in these soils, but this does not mean that there is a threat for a sudden increase in oxygen and nitrogen levels at 2 or 3 meters below the surface across the entire Amazon basin that would justify the metaphor of a carbon bomb. I think the title should emphasize the relevance of this study for understanding mechanisms, and do not trick readers with a suggestive title that is far from scientific rigor.
2. Similarly, the analysis of the contribution of these soils to total respiration globally based on an extrapolation of the incubation results are out of place. Even though the authors took care to conduct the incubations with undisturbed cores, the treatment themselves are highly artificial. Again, the treatments are very useful to understand mechanisms of carbon stability in these soils, and this is the main contribution of the study. But I do not see the point to extrapolate those incubation results to the entire Amazon. I think the authors should stick with the main contribution of their results to scientific understanding and avoid speculative analyses.
3. I was very surprised by the results presented at line 169 on the respiration from the E horizon. My own analyses of carbon content and respiration from this horizon had shown no detectable amounts of carbon and respiration in this horizon. So, I found strange that the authors report higher mineralization rates here. However, Figure 1 shows that total C in the E horizon is zero or close to zero, which means that when you compute the specific mineralization rates dividing the values by a number close to zero, the values artificially increase. It is well known that when you divide any number by a value close to zero, the result is a very large number. Or in mathematical jargon, the limit goes to infinity. Therefore, the specific mineralization rates presented in Figure 2B for the E horizon are an artifact due to the division by a very small total C value. This should be corrected and removed from the discussion.
*Minor comments*
- The authors cite the conceptual framework of Mayano et al a number of times to put the results in a larger context. This is very good, but I also would bring to the attention the conceptual framework of Davidson et al. (https://doi.org/10.1111/gcb.12718), which is very similar to that of Moyano, but expressed under a mathematical framework that is testable. I would recommend the authors to consider framing the study and results in the context of interactions between moisture and oxygen availability as in the DAMM model of Davidson. There are studies that have tested this framework experimentally manipulating oxygen and moisture levels simultaneously (https://doi.org/10.5194/bg-14-703-2017), and I think the present study adds important experimental support to the DAMM theoretical framework.
- Add the duration of the incubations to the caption of Figure 2.
- I also found very interesting the results of the addition of the mixture of carbon compounds because they do not provide any evidence for the priming effect. In general, I see a bias in the literature that generally gives a lot of emphasis to results that support the priming effect, but when results are not consistent with the priming hypothesis they are not mentioned. I think it's important to also bring this to the attention for a more balanced discussion on the relevance of priming. For these soils, it seems that priming is not a relevant mechanism.
- In the methods section, you mentioned that the added carbon mixture was labelled with 13C. Did you obtain any insights from the analysis of 13C in respiration or in the remaining soils after the incubation?