Abstract <p>The aim of this work is to study the bioproductivity, C-biomass, and elemental composition of humic acids (HAs) as a unified ecosystem of dry-steppe soils. The study objects are the virgin chestnut soils of southern steppes of western Transbaikalia. It has been revealed that chestnut soils are formed under dry-steppe plant associations with sparse and short grass cover with a poor species composition. The net primary production (NPP) in chestnut soils of the study region is 4.80 ± 3.48 kg/m² yr, and the underground mass exceeds the aboveground one by nine times. Plants of more continental steppes with a greater moisture deficit allocate a larger proportion of photosynthetates (to 90%) to the root mass as compared to plants of western steppes. The BNP-to-ANP ratio is 1 : 9 and may reach 1 : 25. The participation of C-biomass of microorganisms in the total reserve of organic carbon of chestnut soils reaches 2.5% of C_org. The accumulation of C-biomass is limited by soil moisture and the reserve of energy resources. The elemental composition of HA of cold chestnut soils is characterized by a low carbon content and relatively high content of nitrogen and hydrogen. The molecule of HA of chestnut soils has a weakly condensed aromatic nature, and the aliphatic part predominates in it. The consequence of the weakened condensation reaction consists of insignificant benzenoidity. ¹³C-NMR spectroscopy shows that humic substances (HSs) in chestnut soils are characterized by a reduced content of aromatic components.</p>