The Role of the Lithological Factor of Soil Formation in the Short-Term Transformation of Fallow Land in North-West Russia

The role of fallow lands in the regional carbon balance is critical; due to the transition of soils from an arable to a fallow state, soils can turn from a carbon sink into a source. The accumulation and long-term storage of soil organic carbon (SOC) directly affects the ability of terrestrial ecosystems to absorb climatically active gases and store these in soil, making the study of post-agrogenic transformation a priority. Using 13C NMR spectroscopy and analysis of mineral-associated organic matter (MAOM), this study investigated fallow soils in the North-West of Russia (Leningrad region) developing on parent materials of different lithological composition. The results show that the degree of hydromorphism and lithological composition are the main factors determining the trajectory of soil transformation at both the morphological and molecular levels. In the waterlogged Gleyic Anthrosol (V2) formed on the territory of the Vuoksa River, almost all organic carbon (96.8% of SOC) was found in the MAOM fraction, indicating strong mineral stabilization under anaerobic conditions. Here, humic acids (HAs) retained aliphatic structures and low maturity, inheriting traits of precursors of humification. In contrast, the Endocalcaric Luvisol (V5) forming on carbonate material was characterized by the most mature HAs composition, due to high aromaticity (AR/AL = 0.79), increased oxygen and carboxyl group concentration, and ensures high resistance to biodegradation. Soils of fallow lands on outwash sands under meadow vegetation (V4) were characterized by the largest soil organic matter (SOM) stock (21.4 kg/m2); however, HAs extracted from these soils had a high degree of oxidation and a relatively high concentration of aliphatic structural fragments. The soil under a hayfield (V3) was characterized by a relatively high concentration of aromatic structural fragments (AR/AL = 0.69), but with a low nitrogen concentration in its HAs composition, due to anthropogenic activity and the removal of plant biomass that makes a significant contribution to the structure of HA molecules. This study shows that in a young fallow state (up to 10 years), local factors, primarily the carbonate content in the soil or waterlogging, set the trajectory of SOM transformation, which is crucial for predicting soil evolution and the sequestration potential of territories.