the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2025
Conceptualising carbon cycling pathways across different land-use types based on rates and ages of soil-respired CO2
Abstract. Soil carbon dioxide (CO2) efflux constitutes a major carbon (C) transfer from terrestrial ecosystems to the atmosphere, driven by numerous metabolic and allocation processes in the plant-soil system. Land use affects key components of C cycling pathways through vegetation type, C allocation, abiotic conditions, and management impacts on soil organic matter (SOM). However, systematic comparisons of these pathways among land uses remain scarce. We measured in situ respiration rates and C isotopic signatures (14C, 13C) of soil-respired CO2 and its autotrophic and heterotrophic sources during summer and winter at 16 sites across Switzerland, covering temperate and alpine grasslands, forests, croplands, and managed peatlands. Our findings revealed significant differences in the rates, ages, and sources of soil-respired CO₂ between land-use types, reflecting variations in C cycling dynamics. We propose that respiration rates and ages of soil-respired CO2 serve as comprehensive indicators to categorize C cycling into:
- High-throughput systems (temperate grasslands): High respiration rates of young (<10 years) CO2 in autotrophic and heterotrophic components reveal rapid C cycling.
- Retarding systems (alpine grasslands): Young (<10 years) in situ CO2 fluxes and dominance of autotrophic sources, but slow C cycling through SOM mainly due to cooler climatic
- Preserving systems (forests): Decadal-old CO2 reflects a delayed C transfer of assimilates back to the atmosphere through soil respiration.
- Destabilized C-depleted systems (croplands): Reduced C inputs and tillage lead to C depletion and to respiratory losses of older C (~650 years).
- Destabilized hotspots (managed peatlands): Release of ancient C (~3000 years) due to disturbances in natural C cycling by drainage.
Our results suggest that the relationship between rates and ages of soil-respired CO2 can serve as a robust indicator of C retention and destabilization along the trajectory from natural to anthropogenically disturbed systems on a global scale.
Competing interests: One of the co-authors (Frank Hagedorn) is part of the editorial board of Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-2267', Andreas Schindlbacher, 26 Jun 2025
Authors present a comprehensive study about soil respiration and its components across different ecosystems in Switzerland. Radiocarbon (14C) in respired CO2 was used to separate autotrophic and heterotrophic contributions and provided information about the age of the respired CO2 from both sources. Based on the fluxes, the age of respired C and the decomposability of SOC, the different ecosystems were categorized into five (soil) C cycling types. The manuscript is very well written and the study is highly valuable as it involved a whole set of different ecosystems in a replicated design. The interpretation of the results is straightforward. Maybe sometimes it’s a bit too straightforward.
Specific comments:
The descriptions of the sites could be improved. Figure 1 looks nice, but apart from showing the areal distribution of the sites, it does not contain any relevant information. I suggest moving Figure 1 to the supplementary material and replacing it in the main text with Table S1, which contains all the relevant site information. If basic soil data is available, such as C and N concentrations or stocks, this could also be presented in the table.
With regard to the cropland and disturbed peatland more information about land-use history needed to be introduced (and discussed). It is often referred that cropland on former grassland loses C, but it remains unclear when the three cropland sites were established and what was there before. The same for the disturbed peatland sites. The choice of the three forest sites needed to be explained. As there are “only” three or four replicates in each land-use class, their choice (site climate, soil type...) has quite an effect on the results. Seems that some forest sites experienced very dry conditions during summer-sampling. Is this just by chance, or are these forests generally particularly dry in summer (Pfrynwald is very dry)? This could be mentioned in th emethods and discussed in the discussion section. The drought effect is for instance impressivel in the CO2 fluxes, which were smaller during summer in two forests than during March though soil temperatures had been much higher in summer.
The use of 'winter' and 'summer' as sampling times is generally a bit strange to me. While July–August is clearly summer, sampling in March does not represent typical winter conditions (20 March onwards is astronomical spring, and the whole of March is spring under the meteorological definition!). I therefore suggest referring to the sampling dates as 'July/August' and 'March' sampling throughout the text, rather than 'summer' and 'winter'. CO₂ fluxes were quite high in March, which is not typical of real winter, and all the snow had melted, which is also not typical of a Swiss winter. This issue has been discussed to some extent (much warmer air temperatures than soil temperatures in March, etc.). So why not drop the term 'winter' as a definition?
Another methodological issues is the clipping of all vegetation in the chambers already 1-2 weeks prior to measurement. Do you believe that this has no effect on autotrophic soil respiration, especially in the grasslands? I am not so sure. The reasoning for this design needed to be worked out here and explained why this approach was chosen.
Though I am not specialized in 14C modelling, the distinction between autotrophic and heterotrophic components is reasonable and the methodoligical attempts to account for CO2 from carbonates are impressive. There still remain some general methodological issues, such as e.g. that sieving and physical disruption couold make protected SOC accessigle to decomposers and affect the heterotrophic CO2 efflux (and age of respired C). In the cropland soil, picking all the roots out might cause some bias too. Since the cropland sites are harvested two times a year, there should be some dead decomposing roots in the soil. I would consider the CO2 from this source heterotrophic. Decomposing fresh roots would have a very recent 14C signature, similar to that of “autotrophic respiration”. By removing all roots from the soil sample only “older” C is left for the assessment of heterotrophically respired C. This might be a reason why the 14C data suggests such extremely high contribution of autotrophic respiration (>90%) from cropland (even during winter). However, this is just an idea. If you think it makes sense, you could include a discussion of such methodological constraints.
In the methods 2.6 a whole chapter describes the analysis of 14C in soil organic carbon. However, these data are nowhere presented or used in the manuscript, except for estimating the contribution of carbonate C to CO2. The SOC 14C values would be very interesting to the reader of this work – C transit times could be calculated and used to back up the theoretical framework.
Maybe this is normal, but is there any reason why the delta 14C in atmospheric CO2 was much more depleted in in winter than in summer? (Page 13 L290)
Seems decomposability of SOC in the different soil and humus layers was analyzed by incubating at field soil moisture at 22°C (Page 7 L180onwards). This might be problematic because very dry soil might not respond similarly to the change in temperature to 22° as soil with sufficient water supply - hence one would end up with lower “decomposability” in dry soil (this maybe the reason for the very low decomposability of the organic layer at Pfrynwald, decomposability was high in the other two forests, Figure 5).
Figure 6 croplands – why is the summer measurement at Reckenholz completely different when compared to all the other cropland measurements. Is there any explanation?
In general, I sometimes found the use of the terms 'destabilisation' and 'degradation' unclear. For example, on page 17, line 360, it says that 'destabilised C-depleted systems, where reduced C inputs result in the depletion of recent C material in SOC stocks', but no SOC-14C data is presented here, so how can this conclusion be drawn?
I very much like the grafical abstract, but I am not sure if the other conceptual figure 7 is fully conclusive. E.g. Increases "Vulnerability" really if the age of the respired heterotrophic C in CO2 increases? What if all the young C is decomposed? - would then the soil be less "vulnerable"?
Citation: https://doi.org/10.5194/egusphere-2025-2267-RC1 -
RC2: 'Comment on egusphere-2025-2267', Andres Tangarife-Escobar, 07 Jul 2025
Overview
The authors are investigating how different land-use types affect soil carbon cycling by comparing respiration rates and carbon isotopic signatures (¹⁴C and ¹³C) of bulk soil and soil-respired CO₂ across a wide range of landscapes representing grasslands, forests, croplands, and peatlands. By analyzing both the quantity and age of CO₂ released from autotrophic and heterotrophic sources during summer and winter, they identify distinct carbon cycling regimes ranging from rapid, recent carbon turnover in temperate grasslands to the release of millennia-old carbon in disturbed peatlands. This study is highly relevant to understand how land use and management impact carbon retention and loss at the ecosystem level and deepens on the important question of carbon stabilization timescales. This research has direct relevance for carbon cycling timescales modeling, climate change mitigation, and land-use policy development.
Overall comments
I found the manuscript enjoyable to read and well-written, with a clear presentation of ideas. It offers a valuable contribution by advancing the application of radiocarbon measurements to investigate soil carbon cycling—an approach highly relevant to the broader biogeosciences community.
Nonetheless, to more effectively position this study within the broader context of current research, the manuscript would benefit from a deeper integration with the existing literature on the age of soil organic carbon (SOC) and respired CO₂. Specifically, comparing the findings with those of previous studies would strengthen the interpretation. A more explicit discussion of how bulk SOC and respired CO₂ radiocarbon measurements align or differ across ecosystems would enhance the relevance and impact of the conclusions. Additionally, incorporating more recent and thematically aligned references would clarify how this study extends, supports, or challenges established understanding in the field.
For example, you might integrate the findings of the following recently published articles into your introduction and discussion:
https://doi.org/10.1029/2024JG008191
https://doi.org/10.5194/bg-21-1277-2024https://doi.org/10.5194/soil-10-467-2024
https://www.nature.com/articles/s41598-021-85506-w
https://doi.org/10.5194/bg-10-7999-2013
https://doi.org/10.1111/gcb.17320
https://www.nature.com/articles/s41561-020-0596-z
Abstract
Pretty clear and succinct. Thanks!
However, the rationale behind the “retarding system” seems to be counterintuitive. The interpretation that more bomb-derived Δ¹⁴CO₂ in heterotrophic respiration at higher elevations reflects a slowing of C cycling appears counterintuitive. Enrichment in bomb ¹⁴C generally indicates faster cycling of relatively recent carbon. To robustly support the claim of retarded carbon turnover at higher elevation, it would be helpful to refer to bulk SOC Δ¹⁴C values, which are expected to become more ¹⁴C-depleted (older) with elevation. Clarifying this distinction between respired and bulk signatures would strengthen the argument.
Graphical abstract:
The graphical abstract looks to me a bit counterintuitive when observing that the direction in which SOC decomposability increases, is where there is more C retention. A high SOC decomposability indicates “a large fraction of readily available organic matter” (L86-88), with subsequent short persistence and low C accumulation, right? Maybe some clarification is needed. The graphical abstract should be very explicit without room for misinterpretation. Also, why is C stabilization and C retention in opposite extremes of the Y axis? Don’t they point out to actually the same effect? Or what does belong to the X and Y axis?
Introduction
L45: The definition of "root respiration" in the manuscript is conceptually unclear and potentially misleading. By including heterotrophic respiration driven by root exudates under this term, the distinction between autotrophic and heterotrophic processes is blurred. I recommend replacing "root respiration" with "rhizosphere respiration" and clearly distinguishing the contributions of plant and microbial respiration to avoid misinterpretation.
L63: This article might be of interest to introduce the allocation of C in the organic layer and other pools in a forest, analyzed from the radiocarbon perspective https://doi.org/10.1029/2024JG008191.
L71: “e.g.” can be deleted in this and the subsequent reference parenthesis?
L76: The concept of transit time, while seemingly central to this study, is not new. Its original formulation can be traced back to Eriksson (1971) and Bolin and Rodhe (1973). It would be appropriate to acknowledge these foundational works to provide historical context. That said, it is also scientifically valuable (as you did) to see this concept being re-applied to current questions in terrestrial carbon cycling. That said, its re-application to contemporary questions in terrestrial carbon cycling, as done here, is both timely and valuable. However, although the concept is introduced in this paragraph, it is not revisited or further developed in the methods, results, or discussion. It would be helpful to understand whether there was a specific reason for not integrating this concept more explicitly throughout the analysis.
L74-100: The references cited in this section are important early contributions; however, a more comprehensive and updated literature review would strengthen the background, especially considering the significant methodological and conceptual advances in the field over the past three decades and the diversity of researchers emerging from different groups. Including more recent work would better reflect the dynamic evolution of approaches used to study carbon cycling and persistence using radiocarbon measurements. This would not only highlight the continued relevance of the field but also situate the present study more clearly within ongoing scientific discourse.
L91: This article might be of interest https://doi.org/10.5194/soil-10-467-2024 for comparing the effects of land-use in SOC stability in croplands and managed ecosystems.
L101-107: Please define what is young and what is old in terms of timescales. Younger than X or older than X? Also, I wonder if the five hypotheses could be summarized within an underlying mechanism that explains the transit time (age of respired CO2) across land-use types? Just a thought/suggestion that can be useful for better synthesis.
Materials and methods
2.1 Study sites
Please consider providing more detailed information on the study sites, especially for the managed peatlands. Since key parts of the discussion rely on mechanisms such as seasonal changes in water table depth, SOC content, and site-specific management practices, a clearer description of these variables is essential for the reader to fully understand the interpretation of seasonal Δ¹⁴CO₂ variability.
For instance, site-level context would also assist in evaluating how representative the findings are, and how much variability exists among peatland responses, at least for the sampled seasons. This is particularly relevant since the manuscript proposes generalizable categories of carbon cycling (stabilizing vs. destabilized systems).
L112: Introduce LULUCF abbreviation by writing it complete.
Figure 1: In the caption: “Five” instead of “six”?
L132-151: As the height of the PVC frames might be different when inserted into the ground. Please clarify if the respiration rates where normalized by the actual headspace volume of the chamber. Also, indicate how many replicates of CO2 fluxes for each chamber were taken.
L159: Please be more specific on the volume estimation method.
L185: The use of a uniform Q₁₀ value of 2.4 to adjust respiration rates across all sites assumes a relatively high and consistent temperature sensitivity of microbial respiration. Given the diverse ecosystems studied (alpine grasslands, forests, peatlands), this assumption may not hold universally. Temperature sensitivity of SOC decomposition is known to vary with substrate quality, moisture availability, microbial community composition, etc. Could you elaborate on the justification for applying a single Q₁₀ value?
Moreover, are all sites assumed to be primarily temperature-limited in their respiration response, or could other limiting factors (substrate or water availability) affect the comparability of basal respiration as a proxy for SOC decomposability across sites? The uncertainty of this assumption deserves further discussion.
L209: Maybe not necessary to specify the characteristics of the device once again if it was already introduced in the previous paragraph.
L281: The statement regarding differences in Δ¹⁴C of total soil respiration between summer and winter across temperate grassland, alpine grassland, and forest ecosystems appears somewhat strong given the actual magnitude of those differences. While the p-value may indicate statistical significance, the practical or ecological significance is less clear. To aid interpretation, it would be helpful to include mean values directly on the box plots. Additionally, the phrasing of this statement could be revised to avoid overgeneralization and better reflect the nuance in the data.
L295: This is a particularly interesting observation: “values in croplands and managed peatlands increased from very depleted values at 0–5 cm to less depleted values at 5–10 cm soil depth.” Such pattern suggests a counterintuitive vertical trend in ¹⁴C values, potentially pointing to surface inputs of older carbon or distinct microbial processing dynamics across layers. It would be valuable to elaborate more on the possible mechanisms behind this in the Discussion.
L327: The finding that forests exhibited the lowest average autotrophic contribution (~40%) and were the only land-use type with dominant heterotrophic respiration is intriguing. Given that forests are generally characterized by high productivity, substantial root biomass, and continuous carbon allocation belowground, one might expect a relatively high autotrophic contribution to total soil respiration. Could you please elaborate on the underlying mechanisms that might explain this pattern? For instance, could this be related to deeper rooting systems with less activity near the soil surface (where respiration was measured), slower root turnover, seasonal carbon allocation dynamics, or differences in microbial decomposition relative to root respiration? Clarifying this could help resolve what appears to be a counterintuitive result.
Discussion
L343-350: This synthesis provides a strong and concise summary of the main findings. However, this section would benefit from a clearer contrast with existing studies to better position its contributions within the wider body of research. Several of your findings are in line with, or complementary to, previous work, especially regarding carbon age dynamics in grasslands, peatlands, and forests.
Figure 7: In the figure, SOC decomposability and in situ soil CO₂ efflux are positioned at opposite ends of the conceptual spectrum. Could the authors clarify why these two variables, while related, are treated as contrasting rather than complementary indicators of carbon cycling dynamics? This distinction may be counterintuitive, as one might expect systems with high SOC decomposability to also exhibit elevated CO₂ efflux, depending on environmental constraints. This distinction would help readers better interpret the position of each land-use system within the diagram.
L385 and 424: Omit parenthesis when citing the author directly inside the sentence. Apply this change consistently along the manuscript.
L441-442: Does it need to be an “advanced” degradation? Or could it be that the water table decrease enhances immediate mineralization of previously-locked old C stocks? Would you please add some lines on this idea?
Section 4.6
The discussion of seasonal controls on respiration appears internally inconsistent. In line 459 states that “autotrophic respiration is rather driven by plant phenology than by temperature,” yet phenology itself is strongly modulated by temperature and photoperiod. A few sentences later the text mentions the 9 °C air–soil temperature offset in March to explain enhanced autotrophic activity, implicitly re‑introducing temperature as a key driver.
In addition, the Q₁₀ value of 2.4 across all land-use types assumes that soil respiration is always strongly controlled by temperature. However, if autotrophic respiration (from roots and associated organisms) is not mainly driven by temperature—while heterotrophic respiration (from microbes decomposing organic matter) is—then adjusting all respiration rates with the same temperature response could be misleading. This approach might distort the seasonal balance between autotrophic and heterotrophic respiration, which in turn could affect the interpretation of the Δ¹⁴CO₂ values, since these depend on correctly estimating the contribution of each respiration source. I suggest clarifying:
- Mechanistic rationale – reconcile why phenology can override temperature for roots in winter, yet a temperature‑based Q₁₀ correction is still appropriate for the overall flux. For example, is the assumption that heterotrophic respiration is temperature‑limited while autotrophic respiration is controlled by recent photosynthate supply?
- Implications for Δ¹⁴CO₂ patterns – discuss whether the Q₁₀ scaling might underestimate or overestimate winter heterotrophic contributions, and how that affects the conclusion that autotrophic respiration dominates in March.
Addressing these points would strengthen the argument and resolve the apparent contradiction between phenology‑driven and temperature‑driven controls on seasonal soil respiration.
Conclusions
L497 and L28: The classification of forests as "preserving systems" that stabilize carbon could be reconsidered or further nuanced. While the presence of decadal-old CO₂ and dominant heterotrophic respiration may suggest delayed C release, the low SOC stocks across the soil profile and few data points evidencing old Δ¹⁴C signatures raise questions about whether forests in this study actively stabilize carbon, or rather reflect low belowground C allocation with partial stabilization of what is retained. It may be more accurate to describe these systems as exhibiting limited input and slow turnover, rather than implying strong carbon preservation capacity. A brief clarification in the conclusions could help avoid overgeneralization.
Supplementary material
The readers would benefit from having a label with mean values for S3, S4 and S5, since the differences can be very subtle. For example, between total soil respiration (filled circles), weighted heterotrophic respiration (open squares) and autotrophic respiration (open triangles) in S5.
Figure S3: Please add some explanation on why the atmospheric background in croplands is so negative compared to the other land-use types. This is specially intriguing.
Citation: https://doi.org/10.5194/egusphere-2025-2267-RC2
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