Potentially Mineralizable Carbon Dynamics During Post-Agrogenic Succession in Soils of the Leningrad and Novgorod Regions Under Different Land-Use Types

Agricultural lands in Russia are increasingly being converted into fallow areas, making the study of potentially mineralizable carbon (PMC) in fallow soils essential for understanding post-agrogenic ecosystem functioning. This study provides a comparative assessment of the spatial and temporal dynamics of PMC in soils of the Leningrad and Novgorod regions (Bankovo, Belogorka, and Borovichi sites) under different land-use types, including fallows of various ages, arable, garden, pasture, hayfield soils, and secondary forests. Total carbon content was determined by high-temperature dry combustion, while basal respiration was measured using a standard incubation method. PMC parameters were estimated using biokinetic fractionation of soil organic matter (SOM), and cumulative carbon release was calculated as the sum of emissions over the incubation period. The highest basal respiration values among the studied fallow soils were observed in fallow soils at the Bankovo site (1.44–1.66 µg CO2–C g−1 h−1). Carbon stocks in most fallow soils were lower than those of the corresponding background soils, although the magnitude of differences varied among sites. Both the size of the PMC pool and its mineralization rate varied among fallow soils depending on post-agrogenic succession, vegetation type, and site-specific environmental conditions. The highest cumulative C-CO2 production was recorded in degraded pasture soils and secondary forests at Borovichi. Restorative ecosystems generally showed higher carbon-mineralizing activity than arable and garden soils, although turnover characteristics varied among sites. Region-specific patterns related to parent material and environmental conditions were identified. Even after long-term fallowing (up to 120 years), several soil properties remained different from background conditions, indicating prolonged and site-dependent recovery of organic matter dynamics.