This study investigates woody encroachment on agricultural land and evaluates its biomass accumulation and carbon sequestration potential. By integrating spatial data analysis with field-based measurements, areas of naturally afforested agricultural land were identified and assessed for the period 2016–2025. The results show that over ten years, 59,797 ha of agricultural land became naturally overgrown with woody vegetation, primarily on light-textured soils (42.6%) and in undulating terrain (65.5%). Field surveys were conducted in 80 naturally afforested plots, where tree height, diameter at breast height, stand density, and species composition were recorded. Based on these measurements, biomass accumulation and carbon content were estimated. The findings indicate that after approximately a decade of natural succession, the studied areas accumulated on average 36.0 t ha⁻¹ of biomass, containing 18.0 t ha⁻¹ of carbon, equivalent to 66.1 t CO₂ ha⁻¹. When extrapolated to the total area of 59,797 ha, this corresponds to approximately 2.15 million tonnes of biomass, 1.08 million tonnes of carbon, and 3.95 million tonnes of CO₂ stored in naturally afforested land. These results demonstrate that woody encroachment on abandoned agricultural land serves as a significant natural carbon sink, accumulating on average 66 t CO₂ ha⁻¹ over a decade—several times higher than in unmanaged or extensively managed grasslands.