Paired 0–20 cm Topsoil Organic Carbon Concentration and Stock Changes and an IPCC Tier 1 Accounting Benchmark in Commercial Cropland

Soil organic carbon (SOC) monitoring is important for climate change mitigation and sustainable agricultural management, but site-specific observations and Intergovernmental Panel on Climate Change (IPCC) inventory estimates need not define the same quantity. This study quantified paired changes in laboratory-measured organic carbon concentrations in the 0–20 cm layer between spring 2023 and late autumn 2025, alongside an IPCC Tier 1 accounting benchmark. The study covered a 528.06 ha mixed-tillage commercial farm in Southeastern Lithuania, comprising 19 agricultural fields. In each campaign, 119 georeferenced composite samples were collected within fixed management-zone footprints established using the EM38-MK2 apparent electrical conductivity, Sentinel-2 Normalized Difference Vegetation Index data, and geographic information system data. Laboratory-determined total organic carbon was used operationally as SOC. The whole-farm area-weighted mean SOC concentration decreased from 3.003% to 2.866%, and area-weighted field-level paired changes showed a downward shift (Wilcoxon signed-rank test, p = 0.040), with carbon stock-determined values of 71.82 and 66.76 Mg C ha−1 (difference, −5.05 Mg C ha−1; field-level p = 0.049). In comparison, the IPCC Tier 1 scenario forecast yielded +0.0382 to +0.0440 Mg C ha−1 yr−1 for 0–30 cm under the default 20-year transition. However, the measured stocks represented the 0–20 cm layer over three growing seasons, whereas the IPCC estimates represented the 0–30 cm layer and a default 20-year transition period. Parallel presentation therefore illustrates an estimated mismatch and associated monitoring requirements compared to Tier 1 predictive performance. Continued, seasonally aligned sampling to 30 cm, campaign-specific bulk density measurement, and explicit uncertainty analysis are required to evaluate persistent SOC stock trends.