Chemical Structure of Organic Matter of Water‐Stable Macro‐ and Microaggregates of Haplic Chernozem in Different Land‐Use Variants

ABSTRACT Background Soil organic matter (SOM) plays a significant role in the soil aggregation process, and aggregate stability, largely determined by OM quality, is the cornerstone of sustainable agroecosystem management. Aim This study investigates the effect of different land uses (steppe—NS, conventional tillage—CT, and long‐term bare fallow—LTBF) on the chemistry of OM pools within water‐stable structural units of Haplic Chernozem. Methods The quality of OM pools within water‐stable macro‐ (2–1 mm) and free microaggregates (<0.25 mm) isolated from the 1–2 mm air‐dry aggregates was investigated using 13 C cross‐polarization (CP)‐MAS NMR spectroscopy. Results Integral indices of the chemical structure of OM pools indicate negative changes in their qualitative composition when converting native soil to agricultural practice: an increase in their decomposition (DI), aromaticity (ARI), and hydrophobicity (HI) indexes compared to those in undisturbed native soil (NS), reaching the maximum observed values under extreme environmental conditions (LTBF). This is accompanied by a disruption of the natural differentiation of OM quality at different levels of structural organization. In arable soils (CT and LTBF), due to physical destruction caused by soil cultivation and the passage of heavy agricultural machinery, accompanied by the release of formerly physically protected OM, more degraded OM is localized not within macroaggregates (as under native conditions NS), but within free microaggregates. Conclusion A sharp deterioration in the OM quality in arable soils, in particular, an increase in its hydrophobicity and aromaticity, largely contributes to a decrease in the proportion of the 2–1 mm water‐stable macroaggregates within air‐dry aggregates of the same size in arable soils compared to that in native soil: by 35‐fold (CT) and by almost 300‐fold (LTBF).