| 03 Nov 2025
Destabilization of buried carbon under changing moisture regimes
Abstract. Paleosols formed by the burial of topsoil during landscape evolution can sequester substantial amounts of soil organic carbon (SOC) over millennia due to protection from surface disturbances. We investigated the moisture sensitivity of buried SOC storage in the Brady paleosol, a loess-derived soil in Nebraska, USA, where historical aeolian deposition during the Pleistocene–Holocene transition buried soils up to 6 m deep. Topsoils from erosional (up to 1.8 m depth) and depositional (up to 5.8 m depth) transects were incubated under two moisture regimes – continuous wetting (60 % water-holding capacity) and repeated drying–rewetting – to assess SOM vulnerability to changing hydrologic conditions.
SOC decomposition rates modeled from CO2 fluxes were consistently higher in erosional than depositional settings, with surface re-exposure of Brady soils enhancing microbial accessibility and destabilization. A two-pool model showed that >96 % of SOC was stored in a slow-cycling pool, particularly in deeply buried soils where stabilization was linked to mineral association, fine particles, and Ca-mediated flocculation. However, this pool decomposed more rapidly in shallower Brady soils (higher turnover rate relative to buried soil), reflecting increased microbial responsiveness to surface-driven processes.
Drying–rewetting cycles caused greater SOC losses from Brady soils than continuous wetting, despite the dominance of the slow pool and depletion of labile SOC. These cycles also accelerated fast pool decay in modern soils and erosional transects, whereas burial dampened variability in Brady soils. Although continuous wetting increased overall decay in burial transects during the incubation period, wet–dry cycles destabilized the slow pool, which may result in greater long-term SOC loss. Together, these results underscore the importance of burial depth, geomorphic context, and moisture regime in shaping the long-term vulnerability of ancient SOC under climate change.
While the research question is timely, relevant, and the experiment itself is well designed and executed, the study ultimately does not provide the mechanistic depth required for publication in SOIL. The analytical framework relies almost exclusively on routine soil characterization (pH, EC, exchangeable bases, particle size distribution, TOC/TIC) and radiocarbon, combined with respirometry assays. These analyses are adequate for descriptive comparisons but do not allow inference about the biogeochemical mechanisms governing the observed differences in SOC decomposition.
In addition, the study does not include a sufficiently large or diverse sampling design that might justify a more limited analytical framework. Based on the description provided, it appears that only a relatively small number of samples were analyzed (two settings × three transects × three depths ≈ 18 sampling points). This places the study in an intermediate position: the authors could either have expanded the sampling to include additional geomorphic settings—providing a broader understanding of buried soils across landscape contexts—or, alternatively, delved more deeply into the analytical characterization to generate mechanistic insight from the existing sample set.
Given the stated goal of evaluating the resistance of buried SOC to disturbance and re-exposure, one would expect the study to investigate processes such as mineral interactions, aggregation, chemical composition of SOC, microbial community shifts, or stabilization pathways. However, no analytical techniques that allow to assess these mechanisms (e.g., FTIR, NMR, selective extractions, thermal analyses, or microbial functional assays) were employed. As a result, the manuscript interprets differences in CO₂ production without any supporting evidence for the underlying processes, making the broader conclusions speculative.
Introduction
The Introduction is well written and provides an extensive overview of SOC stabilization and destabilization mechanisms, but it remains overly general and does not sufficiently contextualize why buried SOC specifically warrants investigation. The manuscript would be substantially strengthened by framing the problem around the real-world vulnerability of buried paleosols.
At present, the Introduction does not clearly articulate that buried SOC persists mainly because it is physically and hydrologically isolated from the surface environment, and that this isolation is increasingly threatened. There is ample evidence that buried soils in many landscapes (including loess systems similar to the Brady) are undergoing accelerated erosion, gully incision, agricultural disturbance, and enhanced exposure linked to climate-driven increases in extreme rainfall and land-use pressures. These processes directly control the likelihood that paleosols become re-exposed to surface conditions.
Linking this geomorphic vulnerability to the central research question—How resistant is this ancient carbon once exposed to moisture and oxygen relative to modern SOC?—would provide a much stronger motivation for the study and highlight its relevance for carbon–climate feedbacks. Bringing in quantitative context (e.g., rates of erosion, the spatial extent of paleosol exposure, or documented cases of rapid exhumation) would further emphasize why studying the oxidation potential of buried SOC is urgent.
The Introduction would benefit from being more oriented toward the mechanisms that are actually investigated in the experiment. I encourage the authors to revise the Introduction so that it explicitly connects (1) the increasing exposure risk of buried soils, (2) the large stock of “stabilized” carbon they hold, and (3) the uncertainty surrounding their decomposition dynamics once reintroduced to surface-like hydrological regimes. This would substantially sharpen the narrative and more effectively justify the experimental focus.
Material and Methods
2.1 Site and sampling
The study site is well described; however, the sampling design could be presented more explicitly. While I assume that three samples were collected at each point (two from the overlying soil and one from the buried Brady horizon), the manuscript states that samples were taken “at three depths” but only specifies two fixed intervals (0–30 and 30–60 cm). If the third depth corresponds to the Brady horizon at variable depth, this should be clearly stated to avoid ambiguity.
Results
The Results section is clearly described and the figures are of good quality. However, the presentation of the MLR results is somewhat odd. The equation could be placed in the supplementary material, and the main text could focus instead on a table summarizing the coefficients, RMSE, R², etc.
Discussion
The Discussion section lacks depth, largely due to the limited analytical framework mentioned above. The section is mostly speculative and relies heavily on previous studies—for example, attributing greater resistance to decomposition to flocculation with Ca²⁺ or to the presence of pyrogenic carbon. Similarly, attributing SOC protection merely to greater silt and clay content is also obvious and does not provide new insights.