the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Mar 2026
Long-term storage of air-dried samples compromises water-extractable organic carbon as a soil health indicator
Abstract. The assessment of soil health relies on sensitive indicators to detect management-induced changes, yet the analytical reliability of these indicators following long-term storage is rarely assessed. We investigated how multi-year storage of air-dried samples influenced the concentrations of several common soil health indicators, including water-extractable organic carbon and nitrogen (WEOC, WEN), mineralizable carbon (Cmin), and permanganate-oxidizable carbon (POX-C), using archived samples from a cover crop experiment. Concentrations of WEOC nearly doubled after three years of storage, while WEN decreased by 19%. A small but significant 6% increase in Cmin concentration was also observed. In contrast, POX-C concentrations remained stable, indicating robustness to storage effects. These storage effects were consistent among three treatments with different cover crop species. In addition, WEOC concentrations consistently declined over time in this experiment and four long-term agricultural sites in the USA, but bulk soil organic carbon (SOC) or soil organic matter (SOM) did not. These results suggest that multi-year storage of air-dried samples inflates the WEOC pool. Therefore, we caution the use of WEOC as a soil health indicator in archived samples, as the observed variations might reflect storage artifacts rather than genuine management impacts.
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Status: open (until 17 May 2026)
- RC1: 'Comment on egusphere-2026-980', Anonymous Referee #1, 26 Mar 2026 reply
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- 1
The manuscript “Long-term storage of air-dried samples compromises water-extractable organic carbon as a soil health indicator” authored by S. Mahmood and co-workers focused on the evolution with time of soil indicators with time of storage. The data refer to a common practice of “air drying” and storing at “room temperature”. Indeed, room temperatures are varying a lot across different locations and seasons, as for room moisture. In my opinion, air drying is rarely reaching moisture content below 5-10% in soils (at least in my region). This means that microbial communities will still have some residual activities at these moisture levels and room temperature, which is the main reason behind the modification of labile soil health indicators such as water-soluble C and N. For “labile” soil health indicators, other methods of drying (at controlled temperature and moisture) and storage (at colder temperature) are preferably, and this could be mentioned in the manuscript. Therefore, results from this study are relevant, but their novelty is relatively low.
Another aspect is that permanganate oxidizable carbon (POX-C) is proposed as a more stable indicator, as suggested by USDA, but there are many concerns about the methods used for measuring POX-C, and this is not reported in the document. I suggest to include some of the studies questioning POX-C used methods to give a better overview of this indicator, as those by Margenot’s group Margenot et al., 2024 (https://doi.org/10.1002/ael2.20124).
Lastly, the presentation of the experimental design, which comprises sample sets from 5 experiment, should be improved to clarify when samples were collected, when they were analysed the first time, when they where reanalysed and the variation occurred during the years of storage.
Detailed comments:
Abstract: line 19 “four long term agricultural sites” is not clear if it is referred to comparison of long-term storage or real field experiments in which WEOC is decreasing. Please revise.
Introduction
Lines 43-49: see my comment above about POX-C methodological problems
Lines 51-54: Why Florida and Midwest experiments are listed separately instead of simply saying “five long term experiments”? This will make things easier to understand, in my opinion.
Materials and Methods
Sections 2.2 and 2.3 and elsewhere: It is not clear the set-up of the re-analysis of stored samples. Which were the reference values for the samples stored from the other 4 long-term experiments? The comparison was done between the re-analysis values of stored samples against the freshly analysed soils?
Line 101: sampling time or storage duration?
Results
Line 116: archived sample sets would better explain the setup of the experiment
In my opinion, section 3.2 and 2.2 should clearly state when archived samples where reanalysed, mentioning the storage time elapsed after sampling, in a scheme. I found that the experimental design is not fully explaining if the samples from long term experiments were collected in year X and reanalysed for this study. In this case, can the authors compare the WEOC data from the first time they were analysed and the reanalysis here? The increasing or decreasing trends can be driven by the land use over years, not only storage.
line 139: why post-rewetting studies are cited to justify WEN decrease during storage?
Discussion
Lines 147-150: as I said above, a quick mention to doubts about POX-C methods and “lability” should be mentioned here.
Line 163: Again, the use of “temporal trends” could refer to trends driven by the agronomic practices inducing shifts in soil indicators or by the storage time at room temperature that is causing re-arrangements of C pools. I suggest to use different wording to avoid this confusion.
Line 181: “controlled conditions” does not equal to optimized condition to minimize soil microbial activity. WEOC can be analysed in stored samples if the moisture is kept very low and the storage temperature is low (but not on frozen samples). But I agree that measuring WEOC as soon as possible is the best option.
Conclusion
Line 190: see my comment about POX-C. I think it has potential to became a soil health indicator, but since the last 15 years the method has marginally improved and actually is a worst indicator than SOC, for example.