Varying soil organic matter composition differences between pools in acidic and calcareous soils

Intensive inversion tillage operations triggered a considerable organic carbon release from cultivated topsoils. Decreasing tillage intensity or land use change are reported to increase the soil organic matter (SOM) content of the degraded topsoil even within a decade. However, the chemical composition, storage mechanisms, and fate of the SOM surplus are not fully understood. Among the environmental and soil conditions, the role of carbonates is considered essential. The present study compares the SOM surpluses due to tillage intensity drop under acidic forest soils and Chernozems. The topsoils (0-10cm) of three long-term (~20 years) field trials were investigated in Hungary. SOM composition was estimated using Fourier transform infrared spectroscopy. Tillage intensity drop and pasture construction significantly increased the SOM content in all sites’ topsoils. At two locations, both organic matter pools increased, whereas only the mobile SOM pool has grown due to pasture construction. This suggests that a considerable amount of additional carbon can be stored in the soil, even in crop fields, using conservation technics. The SOM surplus did not change the original SOM composition trends of the labile and stable OM pools; consequently, the OM composition of the pools is rather the functions of soil and environmental conditions than sequestration mechanisms. In the acidic soil and the Chernozem under pasture, the mineral phase associated OM (stable pool) was ruled by aliphatic components, whereas the mobile, aggregation occluded OM was more aromatic. In agreement with the mainstream literature, we found the opposite trend on the other Chernozem site. Therefore the role of carbonates solely does not explain the differences. For more general trends, additional case studies must be involved. This research was funded by the National Research, Development, and Innovation Office (NKFIH), grant number: K-123953, and supported by the Eötvös Loránd Research Network SA41/2021, and EJP Sic-Soc-Dyn.