| 21 Sep 2026
Cropping-system diversification drives long-term soil carbon gains more strongly than reduced tillage
Abstract. Long-term measurements across the soil profile are essential for understanding how agricultural management affects soil organic carbon (SOC) storage, accumulation, and vertical distribution. We analyzed a 40-year record (1984–2023) from the Farming Systems Trial in Pennsylvania, USA, comparing conventional (CNV), legume-based organic (LEG), and manure-based organic (MNR) cropping systems. Seven sampling events were used to evaluate long-term surface SOC trajectories, three campaigns included the complete 0–100 cm soil profile, and full- and reduced-tillage treatments were compared from 2008 onward. Surface SOC increasingly diverged among cropping systems over time (system × year: χ² = 35.0, P < 0.001). By 2023, SOC concentrations in the 0–20 cm layer were 2.46% in MNR, 2.30% in LEG, and 1.92% in CNV. Equivalent-soil-mass-corrected SOC stocks were 70.3, 65.7, and 54.7 Mg C ha⁻¹, respectively, corresponding to gains of 15.6 Mg C ha⁻¹ in MNR and 11.0 Mg C ha⁻¹ in LEG relative to CNV. Across the full 1 m profile, MNR and LEG stored 119.0 and 108.9 Mg C ha⁻¹, respectively, compared with 92.9 Mg C ha⁻¹ under CNV. The effect of cropping system varied strongly with soil depth (system × depth: χ² = 77.9, P < 0.001). In both 2021 and 2023, 91–94% of the additional SOC stored under the organic systems occurred within the upper 30 cm. Permanganate-oxidizable carbon followed the same vertical pattern, indicating that both total SOC and management-responsive carbon increased primarily within the surface and transition layers. Cropping-system diversification and organic inputs had a substantially greater influence on SOC than tillage intensity, which had no significant overall or system-dependent effect. These findings show that diversified organic cropping systems can produce substantial and persistent SOC gains over four decades. Full-profile sampling combined with equivalent-soil-mass accounting also clarifies where those gains occur, providing a stronger basis for evaluating agricultural soil-carbon storage.
Dear Editor and Authors,
Thank you for the opportunity to review this manuscript. The authors use a long-term field experiment to compare soil carbon dynamics among an organic-based system, an organic legume-based system and a coventional grain system.The topic is timely and relevant to the scope of SOIL, and has potencial to make a meaningful contribution
This manuscript uses a long-term field experiment, established in 1981 with data from 1984 to 2023, to compare soil carbon dynamics among three cropping systems: an organic manure-based system, an organic legume-based system, and a conventional grain system. All systems were managed under conventional tillage until 2008, when reduced tillage was adopted. The dataset is valuable, and the question of how diversification and tillage contribute to long-term soil carbon gains is relevant to the scope of the journal.
However, in its current form the manuscript reads more like a comprehensive thesis chapter than a focused research article. It addresses many topics at once, including management practice effects, carbon distribution with depth, comparisons among systems, and permanganate-oxidizable carbon (POXC). As a result, the main message is diluted and several of the secondary analyses are not sufficiently supported by the data. I recommend that the authors narrow the scope to the comparison among cropping systems, which is the core contribution suggested by the title, and restructure the text to be more concise. My specific comments are detailed below.
Major comments:
1. Scope and focus of the manuscript. The manuscript attempts to cover too many topics, which makes the discussion fragmented and the text difficult to follow. I suggest that the authors focus on the comparison among cropping systems and remove or substantially reduce the analysis of carbon distribution with depth. In my view, the depth data are not robust enough to support the conclusions drawn from them, particularly given that the systems differ in crop sequences and management. A more focused manuscript, with fewer discussion subsections and a more fluid narrative, would considerably strengthen the paper.
2. POXC analysis. I recommend removing the POXC results as a separate topic and conclusion. Interpreting POXC across depths is problematic when the compared systems involve different crops and management practices, which makes it difficult to attribute differences to the treatments.
3. Bulk density and carbon stock calculation. Bulk density was not measured in every sampling campaign. Instead, a single bulk density value appears to be applied to the whole period before the adoption of reduced tillage and another to the period after it. Given the substantial variation in crops and management practices over four decades, this approach may introduce considerable error into the carbon stock estimates, even with the equivalent soil mass correction. I ask the authors to discuss this limitation explicitly.
4. Presentation of statistical comparisons. Figures 1 and 2 show the treatment comparisons graphically, but the statistical results are hard to extract from the figures alone. I suggest adding supporting tables (in the main text or the supplement) reporting means, measures of variability, and the results of the multiple comparison tests (e.g., Tukey) for each treatment and period.
5. Structure of the Materials and Methods. Several parts of this section could be condensed or reorganized:
a) The content of Section 2.10 (quality control of analyses) related to carbon determination should be merged into the sections where the carbon methods are described (Sections 2.4 and 2.5), and Section 2.10 removed.
b) The bulk density correction is described in unnecessary detail. It would be sufficient to state that stocks were corrected by the equivalent soil mass approach and cite the reference, rather than reproducing the full calculation.
6. The crop sequences would be easier to follow if accompanied by a figure illustrating the crop rotations of each system over time. A visual timeline would help readers understand the differences among systems and the transition to reduced tillage in 2008.