the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2025
Vulnerability of carbon in subalpine soils in the face of warmer temperatures
Abstract. Alpine and subalpine soils are significant reservoirs of labile carbon (C) and are highly sensitive to warming, yet the mechanistic interactions between temperature and organic inputs are poorly understood. A one-year laboratory incubation was conducted with mineral surface soils from a subalpine pasture and an adjacent coniferous forest site. Soil samples were incubated in closed jars at three different temperatures: Current growing season temperature (12.5 °C), and two increased temperature treatments (16.5 °C and 20.5 °C). To assess decomposition differences between litter and native soil organic matter (SOM), 13C-labelled plant litter was added to a subset of the jars. CO2 production, δ13C partitioning, and phospholipid fatty acid (PLFA) profiles were used to quantify soil organic matter (SOM) and litter decomposition, and to assess microbial dynamics. Warming increased total CO2 respiration by 15–37 % in pasture and 12–33 % in forest soils, with strongest stimulation in litter-amended soils. Positive priming of native soil organic matter (SOM) peaked within one week (up to +77 % over controls) and declined to near zero after one month. Cumulative litter-induced respiration (LIR) was highest at 16.5 °C (+6–10 % vs. 12.5 °C) in both soils, coinciding with maximum microbial biomass; 20.5 °C reduced microbial biomass by up to 25 % and accelerated 13C label loss. The response of pasture soils was more rapid and pronounced compared to forest soils, which exhibited slower, more sustained responses. PLFA profiles revealed warming-induced declines in Gram+ and Gram- bacteria and increased cyclopropyl markers at high temperature. These findings show that even moderate warming can accelerate C loss from subalpine soils, with vegetation history and microbial traits modulating both rate and timing of decomposition.
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Status: open (until 04 Jan 2026)
- RC1: 'Comment on egusphere-2025-5429', Anonymous Referee #1, 15 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5429', Jérémy Puissant, 23 Dec 2025
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This manuscript is well described, detailed, and clearly written. I enjoyed reading it, and it represents a valuable contribution to the field by providing mechanistic insights into how warming influences carbon losses from subalpine soils and how microbial processes may modulate the timing of these responses. Several points would benefit from clarification before publication. In particular, given the limited site replication and the nature of the incubation experiment, some conclusions—especially those related to vegetation effects and temperature optima—could be interpreted more cautiously. I therefore suggest adding a short “limitations of the study” paragraph to clarify the scope and generality of the findings, especially with respect to points 3–5 below. The aim of these comments is not to downplay the results, but rather to more precisely define what this study contributes to the field and to clearly state its limitations, so that its mechanistic insights can be interpreted appropriately and provide a robust basis for future work. The comments below focus on several key points that would benefit from detailed clarification and response, as they influence the interpretation and scope of the conclusions.
Major comments
1-Soil moisture control and incubation conditions
Additional details on soil moisture control would help ensure that the higher litter-induced respiration (LIR) observed at 16.5 °C compared to 20.5 °C reflects biological responses rather than methodological effects.
Please clarify: How soil moisture was controlled throughout the experiment; The target soil moisture used (gravimetric water content or %WHC); How often soil moisture was checked and adjusted; Whether differences in vapor pressure deficit among temperature treatments were considered; Whether a pre-incubation period was used to stabilize respiration before applying temperature treatments and litter addition; Whether soils were air-dried and rewetted or kept field-moist and stored cold prior to incubation (and for how long).
These details are important for interpreting temperature effects on respiration.
2-Link between hypotheses and discussion
The Introduction presents three main research questions/hypotheses, but these are not always explicitly revisited in the Discussion. Structuring the Discussion more clearly around these hypotheses, or explicitly stating whether each is supported, would improve readability. Priming is mentioned in two hypotheses but is only briefly discussed, despite being a central component of the results. A more explicit treatment of priming in the Discussion would strengthen the manuscript.
3-Duration and realism of the incubation experiment
A one-year incubation at constant temperatures represents a strong treatment for subalpine soils. The 12.5 °C treatment corresponds to growing-season temperatures, whereas these ecosystems normally experience long periods of cold temperatures and snow cover. It would be helpful to clarify that this experiment is primarily intended for mechanistic or hypothesis testing, rather than for direct simulation of field conditions, and to discuss how the results should be interpreted in relation to natural climate scenarios.
4-Interpretation of litter-induced respiration (LIR)
Cumulative litter-induced respiration (LIR = R(L+) − R(L−)) integrates both litter-derived CO₂ and priming-induced SOM mineralization. Because priming is strong, transient, and temperature-dependent (Fig. 4), the statement (line 388) that “the LIR optimum was consistent across both soils … suggesting that microorganisms decomposing the litter operate near their physiological optimum at this temperature”implicitly interprets LIR as litter decomposition.
Without explicitly presenting isotopically partitioned litter-derived CO₂ fluxes, the observed LIR maximum at 16.5 °C cannot be uniquely attributed to a physiological optimum of litter-decomposing microorganisms. It may instead reflect differences in priming dynamics, microbial biomass persistence, carbon use efficiency, stress-related community shifts at 20.5 °C, or the integration of short-lived respiration pulses in cumulative fluxes. Tempering this interpretation and, if possible, presenting cumulative priming and/or litter-derived CO₂ fluxes would help clarify the underlying mechanisms.
5-Site replication and ecosystem effects
Only two sites (one forest and one pasture) are included. As a result, site-specific effects are confounded with ecosystem type. It would therefore be helpful to acknowledge more explicitly that conclusions regarding vegetation effects are based on limited replication and should be interpreted cautiously.
6-PLFA interpretation
PLFA results are reported as absolute concentrations (µg g⁻¹), but presenting relative abundances (%) would help support statements regarding changes in community composition. Interpretations of cyclopropyl PLFAs as stress indicators could also be phrased more cautiously, as increases in cyclopropyl fatty acids can reflect stationary phase, nutrient limitation, or general stress, not exclusively heat stress.
Minor comments and clarifications
- Line 23: Gobiet et al. do not directly demonstrate SOM vulnerability to climate change; please revise or update the reference.
- Line 47: Please clarify what is meant by “classical theory” (cite explicitly) or rephrase.
- Line 115: Please clarify the sampling strategy (field replication vs pooling).
- Line 409: Forest stand age is given, but pasture age/history is not; please clarify.
- Lines 342 and 439: Avoid acronyms such as “L−” in the Discussion; spelling out terms would improve readability.
Citation: https://doi.org/10.5194/egusphere-2025-5429-RC2
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- 1
This manuscript presents a well-designed and carefully executed laboratory incubation study investigating the effects of temperature, litter input, and microbial community dynamics on soil organic carbon turnover in subalpine forest and pasture soils. The combination of respiration measurements, δ¹³C tracing, priming calculations, and PLFA-based microbial analyses provides a robust and comprehensive framework to address soil carbon–climate feedbacks. The results are clearly presented and well discussed in the context of existing literature. Overall, the study makes a valuable contribution to the understanding of SOM dynamics under warming in alpine and subalpine ecosystems. The manuscript is of high quality, and I only suggest minor revisions aimed at improving clarity, methodological transparency, and precision in interpretation.