| 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.
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.