Today, a significant amount of land has transitioned from agricultural use to fallow state. These lands are undergoing processes such as shrub encroachment, waterlogging, and afforestation, which lead to degradation and the loss of carbon from soils. To analyze the carbon state of soils on fallow lands in the North-West of Russia, a calculation of the content and stock of carbon in the soils was carried out, as well as elemental and molecular composition of humic acids extracted from the soils was performed. To investigate the dynamics of molecular composition, the 13C NMR spectroscopy method was used. The study revealed that soils transitioning to fallow state accumulate carbon due to the formation of coarse humus fractions, which contribute primarily to the labile carbon pool. Molecular composition of humic acids demonstrated that plant community succession in initial fallow stages promotes the formation of stable molecular complex. However, with an increase in the age of the lands in the fallow state, a decrease in the content of stable molecular complexes is observed, which can lead to an increase in the rate of biodegradation of organic matter.