the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Dec 2025
Two Decades of Conservation Agriculture Enhances Soil Structure, Carbon Sequestration, and Water Retention in Mediterranean Soils
Abstract. Conservation agriculture offers a pathway for enhancing soil health with climate co-benefits in Mediterranean agricultural systems. This study examined long-term impacts of combining no-till management with cover cropping over 20 years in California's Central Valley, providing rare insights into soil system equilibrium under sustained conservation management. We assessed soil physical, chemical, and structural properties comparing reduced tillage with cover crops (CTCC) to standard tillage without cover crops (STNC), employing density fractionation and spectroscopic analysis to understand carbon protection mechanisms. After two decades, conservation agriculture achieved dynamic equilibrium characterized by fundamental shifts in carbon stabilization pathways. Water-stable aggregate analysis revealed the most pronounced management effects, with CTCC exhibiting 136% greater stability than STNC, indicating substantial improvements in soil structural integrity. These structural enhancements corresponded with a reorganization of carbon protection mechanisms: CTCC disproportionately enriched the occluded light fraction (44.1% vs. 35.4% of total recovered carbon in STNC), demonstrating that physical protection within aggregates becomes a dominant carbon stabilization pathway under long-term conservation management. Mineral-associated organic carbon saturation analysis revealed that both management systems remained well below theoretical maximum capacity (11.5% vs. 7.4% saturation for CTCC and STNC, respectively), indicating substantial remaining potential for carbon sequestration even after reaching equilibrium. Physical property improvements under CTCC included 15% lower bulk density and 13% greater water retention at field capacity, though benefits were concentrated in the surface horizon. Our findings demonstrate that two decades of conservation agriculture fundamentally transforms soil functioning through aggregate-mediated physical protection, while creating substantial improvements in soil structural integrity and water retention capacity. This mechanism shift represents a new soil system equilibrium that maintains enhanced functionality and continued carbon sequestration potential in Mediterranean agricultural systems.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6047', Anonymous Referee #1, 29 Jan 2026
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RC2: 'Comment on egusphere-2025-6047', Anonymous Referee #2, 17 Feb 2026
This manuscript will be a valuable contribution to our understanding of long-term conservation agriculture impacts in Mediterranean agricultural systems. The multi-method analytical approach of combining density fractionation, FTIR spectroscopy, aggregate stability, and comprehensive physical characterization is well-suited to the research questions. The figures are visually consistent and well-constructed, and the paper is engaging to read. The core finding, that sustained conservation management reorganizes carbon protection mechanisms toward aggregate-mediated physical protection while improving soil structural and hydraulic properties, is well-motivated and contributes meaningfully to ongoing discussions about carbon permanence and sequestration potential in dryland agricultural systems.
My principal comments are related to the dynamic equilibrium framing, the reporting of statistics, and management. I have listed general comments followed by specific comments below.
General comments:
The framing of the results as evidence that the soil system has reached "dynamic equilibrium" after 20 years is not supported by the study design. Demonstrating equilibrium requires temporal data showing that carbon stocks have stabilized, whereas this study provides a single time-point comparison after 20 years of divergent management, which is valuable on its own terms but cannot establish whether stocks are still changing. I recommend the authors reframe this language throughout, using the equilibrium concept as theoretical context drawn from the literature rather than a demonstrated outcome of this study. Related to this, the discussion leans heavily on Tian et al. (2024) as a basis for the temporal phases of carbon protection mechanisms. Although this reference is useful, it pertains to a different climate, soil type, and crop rotations, and it is also just one study. More significantly, citing it as the basis for the 0-5, 5-15, and 15+ year phase framework is problematic, because Tian et al. (2024) was a 10-year study, and so did not observe the mature phase beyond 15 years that the authors use to interpret the results. I recommend either citing literature that documents this, or being more cautious in how the temporal progression is framed.Relatedly, the discussion would benefit from addressing potential disconnects between the equilibrium framing and MAOC saturation. The authors assert that the system has reached management equilibrium and that much unsaturated mineral capacity remains. These are not mutually exclusive, but it's unclear how additional carbon will be sequestered to fill this capacity if management-related carbon sequestration has plateaued.
The characterization of the conservation treatment as a 20-year no-till experiment warrants more careful handling. As described in section 2.2, the no-till system permitted shallow cultivation for tomato crops from 1999-2011, with true no-till only achieved in 2012, meaning that for roughly the first 13 of the 20 study years, the system was more accurately described as reduced tillage than no-till. The paper's framing implies a consistency of management that is not supported by the methods. This has bearing on the equilibrium claim; if true no-till conditions were established relatively recently within the study period, the system could still be responding to that transition rather than having stabilized under consistent management. The authors should address this trajectory more explicitly, both in how they describe the treatment throughout the manuscript and in how they interpret the maturity of the soil system response.
Specific comments:
Throughout the manuscript, most results are presented as boxplots, which display medians and interquartile ranges. However, parametric tests comparing means (ANOVA, Welch's t-test) are reported for several variables. These are incongruent: where parametric tests are used, means with standard error bars would be the more appropriate display, and would also be more consistent with the error bar descriptions in several figure captions. The authors should either reconcile the figure types with the tests applied, or justify the mixed approach explicitly.
The Games-Howell post-hoc test appears in Figures 7 and 11 but is not mentioned anywhere in section 2.11. Please add it to the statistical methods description.
Figure 4 caption refers to a Wilcox test while the methods section describes Welch's t-test for two-group comparisons. Please reconcile.
The hydraulic conductivity results show a reversal between depths, with CTCC being higher at 0-5 cm and lower at 5-10 cm relative to STNC, that receives little discussion despite potentially meaningful implications for subsurface drainage and water redistribution. A brief interpretation would be useful.
The carbon retention efficiency estimate (19% of cover crop inputs retained) is a useful calculation but the denominator relies on estimated rather than directly measured cover crop biomass inputs over 20 years. This assumption should be stated explicitly and its uncertainty acknowledged.
Figure 4 caption cites a Wilcox test while the methods section describes Welch's t-test for two-group comparisons. Please reconcile.
The authors report MAOC saturation as point estimates but do not propagate the uncertainty inherent in the Georgiou et al. (2022) framework (±9 mg C g⁻¹ mineral) through their calculations. Given that this uncertainty represents roughly 10% of the central estimate, the saturation percentages carry uncertainty that could be reported. The authors' conclusion that both treatments remain well below theoretical maximum is likely robust to this, but reporting the propagated uncertainty would strengthen their conclusions.
Figure 8 presents proportional carbon and nitrogen distribution as stacked bars with no measure of variability, yet statistical comparisons between treatments are reported in the text for these proportions (section 3.5.3, p = 0.01 and p = 0.003). A reader has no way to visually assess the variability underlying those comparisons from the figure as presented. Error bars or an alternative figure type that conveys distributional information should be added.
Throughout the manuscript, significant figures are applied inconsistently. For example, aggregate stability is reported as 46.85 ± 15.22% alongside p-values carried to many decimal places, while other values are rounded to whole numbers. I recommend standardizing to reflect the actual precision of the measurements, following the conventions used elsewhere in this journal.
Citation: https://doi.org/10.5194/egusphere-2025-6047-RC2
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Alvarez-Sagrero et al. examine the combined effects of reduced tillage and cover cropping on soil structure, carbon storage, and organic matter composition. Overall, I do not have major concerns with the study. The narrative is generally coherent, the writing is easy to follow, and the data are clearly presented. Some sections of the Introduction and Discussion are somewhat wordy and could be streamlined, and a few inconsistencies in statistical treatment should be addressed. I provide general comments on the framing and analysis below, followed by additional specific comments.
My first general comment concerns the repeated use of the term “dynamic equilibrium” in the Abstract and Discussion (e.g., lines 6, 18, and 448). The authors argue that 20 years of conservation practices have led to the establishment of a new dynamic equilibrium. In my view, demonstrating a dynamic equilibrium typically requires time-series data showing stabilization of key variables, as was shown in Caruso et al. 2018 and Tian et al. 2024, both were cited by the authors. In the present manuscript, this inference is largely based on comparisons to other systems and on the assumption that 15 to 20 years is sufficient to reach equilibrium (line 448). While this is a reasonable hypothesis, it remains unclear whether the system has reached equilibrium or is still in the process of approaching it. This is not a critical flaw, but I would recommend softening the language. For example, the authors could emphasize that conservation agriculture can lead to such an equilibrium and that the observed differences are meaningful, while noting that additional temporal data, including future measurements or archived soil analyses, would be needed to confirm equilibrium status.
My second general comment relates to consistency in the statistical approach. In the Methods, the authors state that their primary comparison focuses on combined reduced tillage and cover cropping versus conventional tillage without cover crops, with intermediate treatments excluded due to limited effects (lines 176-180). However, Figure 1 presents the results on aggregate stability for all four treatments. For consistency, the authors should either include the intermediate treatments throughout the paper or remove them from Figure 1.
Relatedly, the statistical tests used to assess treatment effects deserve reconsideration. The manuscript reports the use of Welch’s t tests and Mann-Whitney U tests (Section 2.11). These tests do not account for block effects or the paired structure inherent in a randomized complete block design (line 106). A paired t-test, or a linear mixed model including block as a random factor would be more appropriate and would strengthen the statistical rigor of the study.
Specific comments:
Introduction: Parts of the Introduction are somewhat wordy and could be streamlined. For example, the paragraph describing advances in soil analytical techniques (lines 75-86) appears redundant, as the methods applied in this study are largely standard and well established.
Line 93: It is unclear why the authors hypothesize that the system is approaching a saturation limit for mineral-associated organic carbon, given the fine soil texture, relatively low carbon inputs, and dry climate. Later results indicate that observed carbon stocks are far below theoretical saturation levels. It is debatable whether these gaps represent realistic or achievable carbon sequestration potential in this system, and this point should be discussed more cautiously.
Lines 267-268: The assignment of the C=O functional group as microbially derived organic carbon appears overly simplistic. There is evidence that other functional groups, such as amides and aromatics, may also contribute to signals in this range (Parikh et al. 2014), and the terminology should be revised or qualified.
Section 2.8: Please report mass and carbon recoveries for the fractionation procedure. This information is important for evaluating data quality and consistency.
Figure 2 caption: There appears to be a typo indicated by two question marks.
Results, including lines 348 and 352: In several places, the manuscript uses comparative language such as “higher” or “lower” even when differences are not statistically significant. I recommend avoiding such terminology unless supported by statistical tests.
Sample sizes across figures: Sample sizes vary among analyses, for example eight replicates in Figure 1, seven for carbon stocks in Figure 5, and six for carbon fractions in Figure 7. Please explain the reasons for these differences, whether any samples were excluded, and the criteria used for exclusion.
Line 399: The figure citation here should refer to Figure 7.
Line 423: Claims of additive effects require explicit comparison with tillage-only and cover-crop-only treatments. Without these comparisons, the interpretation remains speculative.
Lines 443-454: As noted earlier, confirmation of a mature or equilibrium phase requires temporal data. Without time-series evidence, conclusions about reaching a stable phase should be softened.
Line 470: The large carbon saturation gap discussed here does not appear to represent a realistic or actionable target for conservation efforts in this system and should be framed more cautiously.
Section 4.6: The methodological insights are a bit wordy, given that these protocols areall standardized (line 556).
Section 5 can be streamlined, in my opinion.