Abstract <p>The properties of soils, C-biomass, and the structure of humic acids of arable chestnut soils of Western Transbaikalia were studied. The objects of this study were arable chestnut soils of the southern basins of Western Transbaikalia (Tugnuyskaya basin, Selenginskaya basin, Ivolginskaya basin). For comparison, arable soils were studied in pairs with virgin soil analogues. Arable chestnut soils are characterized by low fertility: low humus content of 0.98 ± 0.07%, low total nitrogen content of 0.08 ± 0.01%, arable horizon thickness of 21.33 ± 2.31 cm, and a light loamy granulometric composition. During the growing seasons, the average biomass carbon accumulation rates in chestnut soils were 35.77 ± 3.09 mg C/100 g of soil. The share of C-biomass in the total stock of soil organic carbon for the studied soils is up to 2.5%. The content of aliphatic carbon in arable chestnut soil is also lower than in virgin soil. The content of the amino group and methoxyl group carbon in virgin soil is 7.9 ± 0.14%; in arable soils this indicator is almost two times lower. The amount of carbon in carboxyl groups is 15.5 ± 0.42% in arable chestnut soils; in virgin soils the indicator was 12.55 ± 0.92%. Spectrum analysis of ¹³C-nuclear magnetic resonance (NMR) spectroscopy of humic acids (HAs) in soils can provide important information on the chemical composition and structural features of organic matter in soils. For arable and virgin chestnut soils of Transbaikalia, such an analysis can reveal differences associated with the use of land in agriculture. Aliphatic carbon in the HA of arable soil is expressed less strongly, which indicates a greater role of aromatic carbon in its macromolecular structure. A pattern is observed: in arable lands the content of aliphatic compounds is lower than in virgin lands. It has been found that agricultural tillage leads to a change in the molecular structure of humic acids, which is expressed in an increase in the proportion of aromatic carbon in arable soil. In the elemental composition of HA in cultivated chestnut soils of Transbaikalia, the carbon content is higher and the hydrogen content is lower compared to virgin analogues. The high hydrogen content in virgin soils leads to relatively developed lateral carbon chains in HAs. In cultivated chestnut soil, the HA macromolecule contains more nitrogen (2.9 at %) than in virgin soil (2.7&#xa0;at&#xa0;%). The atomic ratio H : C in cultivated chestnut soil was 1.0, while in virgin soil it was 1.1. In cultivated variants, the degree of benzenoid content (BC) is 22.5, while in virgin soil the indicator is lower at 16.4. It was established that the elemental composition of cultivated chestnut soils is characterized by a more condensed structure compared to virgin soils.</p>