Fallow Chernozem soils store the most stable carbon in Russian forest-steppe study

Soils left to recover naturally for six decades after cultivation have built up the largest and most chemically stable pools of organic carbon among four contrasting land-use sites in Russia’s Chernozem belt, according to a new molecular study of humic acids published in One Ecosystem. The research, conducted on the territory of a proposed Carbon Polygon in the Tambov Region, found that fallow soils had accumulated up to 50.81 ± 0.96 kg of soil organic carbon per square metre in the top metre, significantly more than any other land use examined, and that their humic acids showed the highest proportion of aromatic structural fragments, a marker of resistance to biodegradation.

The study set out to characterise how different types of land use shape both the quantity and the molecular quality of soil organic matter in the northern forest-steppe. Researchers from the Carbon Polygon study area selected four sites within a single landscape: long-term arable land planted with winter and spring wheat, fallow land undergoing roughly 60 years of self-restoration, a shelterbelt site representing the zonal reference soil, and an eroded site where a firebreak channel excavated about 70 years ago buried part of the old arable surface under overburden material. Soil sampling took place in mid-April 2025 across five soil profiles, yielding 14 samples from all diagnosed horizons, of which seven were used for molecular analysis of humic acids.

The soils themselves belong to some of the most fertile in the world. The territory lies in the zone of northern and typical forest-steppe on the Oka-Don Plain, where Haplic and Luvic Chernozems and Luvic Greyzemic Phaeozems develop on cover loess-like loams. The climate is temperate continental, with an average annual temperature of +6°C and about 550 mm of precipitation. Because the four profiles co-exist within the same landscape under comparable lithological and geomorphological conditions, differences among them can be attributed primarily to land-use history rather than to differences in parent material or climate.

Carbon and nitrogen stocks were calculated for an active zone of 0–30 cm, which responds most directly to management, and for a passive zone of 0–100 cm. The fallow soil held 16.54 ± 0.15 kg/m² of carbon in the top 30 cm and 50.81 ± 0.96 kg/m² in the top metre, a result that differed significantly from all other profiles. Arable soils held 36.24 ± 1.02 and 34.03 ± 0.61 kg/m² respectively in the metre profile, values that did not differ significantly from each other but were clearly below the fallow figure. The eroded site showed only 8.97 ± 0.12 kg/m² in the 0–30 cm layer, reflecting the slow formation of its modern surface horizon, yet 37.86 ± 0.56 kg/m² over a metre because of the buried old-arable horizon beneath. The lowest stocks of all belonged to the zonal shelterbelt soil, at 3.49 ± 0.08 kg/m² for 30 cm and 6.34 ± 0.46 kg/m² for 100 cm, which the authors attribute to its contrasting genesis and lack of regular organic inputs.

Nitrogen stocks broadly followed the carbon pattern. The fallow soil held the highest nitrogen stock in the metre profile at 3.19 ± 0.04 kg/m², while the highest nitrogen stock in the active 0–30 cm layer belonged to one of the arable profiles, a difference the authors link to the use of organic fertilisers and reclamation techniques on cultivated land. The shelterbelt soil showed the lowest nitrogen stocks in the metre layer, indicating depletion relative to the other sites. Carbon-to-nitrogen ratios also separated the land uses: arable horizons showed ratios of 14–15, fallow soils the widest and, in the authors’ assessment, optimal range of 15–16, the shelterbelt soil a very narrow ratio of about 7 suggesting mobile organic material rapidly engaged in the carbon cycle, and the eroded site intermediate values of 12 and 13 in its surface and buried horizons.

The molecular work relied on elemental analysis and solid-state 13C NMR spectroscopy. Humic acids were extracted with sodium hydroxide following a standard international protocol, decalcified, and analysed for carbon, hydrogen and nitrogen on a CHN analyser, with oxygen calculated by difference. Spectra were recorded on a Bruker Avance 500 spectrometer and divided into eight regions of structural fragments, from alkyl carbon through aromatic and carboxyl groups, from which hydrophobicity and humification indices were derived. Statistical comparisons used ANOVA with post-hoc pairwise tests, and results are reported as mean ± standard deviation with significance at p < 0.05.

The elemental composition revealed distinct signatures of anthropogenic transformation. Humic acids from the arable soils had reduced carbon concentrations of 51.62–54.36% and a predominance of oxygen at 39.84–43.00%, which the authors interpret as active intramolecular oxidation and the initial stages of soil degradation. Their oxidation index ranged from 0.302 to 0.468, indicating carboxylic aromatic fragments. By contrast, the highest carbon concentrations appeared in humic acids from the fallow land and the eroded site, and fallow samples showed relatively low hydrogen concentrations of 3.48–3.61%, signalling a high degree of carbonisation and the formation of biodegradation-resistant molecular complexes. The forest shelterbelt soil and the modern surface horizon of the eroded site were assessed as the least mature, with a predominance of aliphatic fragments and, in the eroded case, a hydrogen predominance suggesting low maturity and availability to microorganisms.

The NMR results sharpened this picture. The highest concentration of aromatic structural fragments belonged to the fallow soils, with an aromatic-to-aliphatic ratio of 2.23, consistent with advanced humification and the selective preservation of recalcitrant lignin-derived and condensed aromatic compounds. Arable soils reached an aromatic-to-aliphatic ratio of up to 2.13, but also carried a relatively high proportion of hydrophilic carboxyl fragments, up to 10% COO-R, which the authors link to tillage and organic fertiliser inputs and which indicates a slightly degraded, oxidised state of the organic matter. The modern humus horizon of the eroded soil had the lowest ratio, 1.00, with only 41% aromatic fragments, the weakest degree of transformation among the samples, while the buried old-arable horizon in the same profile showed a ratio of 2.13, demonstrating that organic matter buried by erosion decades ago has remained in a stable, condensed form even as the newly formed surface organic matter has not yet matured.

Taken together, the authors argue that the transition from arable cultivation to fallow increases both the stocks and the resistance of soil organic matter to biodegradation, while long-term agriculture, although associated with a relatively stable and mature humic complex, produces an oxidised organic pool marked by carboxyl groups. They also note that agricultural soils may hold considerable sequestration potential relative to the zonal reference soil, whose low stocks reflect its contrasting genesis rather than a superior conservation status. The finding that buried organic matter under the eroded site has retained molecular stability while newly formed organic matter requires decades to reach a comparable level offers what the authors describe as a direct view into the timescales of humification.

The team is careful to frame the results as preliminary. Bulk density was measured in a single replicate, which the authors note can introduce variation of up to 17% in carbon and nitrogen stock estimates for the top 30 cm, and the study covers only five soil profiles within one proposed carbon polygon, so the figures should not be extrapolated across the entire forest-steppe zone. The purpose, they stress, was to identify molecular mechanisms of humic acid transformation under different land uses rather than to produce regional inventories. Future work will examine spatial and temporal variability in soil properties across the polygon and assess its suitability for carbon monitoring and long-term carbon storage, filling a gap in the Russian Carbon Polygon network, which currently has almost no coverage of the Chernozem zone in southern Russia.

Subject of Research: Molecular and elemental composition of humic acids in soils under arable, fallow, shelterbelt and eroded land use in the northern forest-steppe of Russia

Article Title: Molecular and elemental compositions of humic acids of arable soils in north forest-steppe zone of Russia

Article References: Polyakov, V., Kulikovskiy, M., Bukovsky, M., Artukhov, E., Tian, G., Chebotaryova, S., Muraveva, A., & Abakumov, E. (2026). Molecular and elemental compositions of humic acids of arable soils in north forest-steppe zone of Russia. One Ecosystem, 11, Article e203814. https://doi.org/10.3897/oneeco.11.e203814

Image Credits: AI Generated

DOI: 10.3897/oneeco.11.e203814

Keywords: soil organic matter, humic acids, Chernozem, carbon sequestration, 13C NMR spectroscopy, forest-steppe, land use, fallow land, soil erosion, Tambov Region, nitrogen stocks, carbon polygon