Maintaining favourable carbon balance in boreal clay soil is challenging even under no-till and crop diversification

We explored the carbon balance, including both gaseous and waterborne carbon, of a long-term experimental site in 2019 and 2020. Additionally, we assessed earthworm abundance and soil aggregation, aiming to uncover potential factors influencing the decomposition and stabilization capacity of organic carbon in the soil. The heavy clay soil site in southern Finland was under long-term cereal monocropping with conventional tillage (CT) and no-till (NT) treatments. Short-term diversification with cover crop and winter rapeseed as a break crop were applied on parts of the site, and the effects of climate, carbon input and soil conditions on the carbon balance were assessed. Net ecosystem exchange (NEE) ranged from -0.32 to 1.91 Mg CO2-C ha- 1 yr- 1 in CT and from 0.22 to 2.40 Mg CO2-C ha- 1 yr- 1 in NT. Net ecosystem carbon balance ranged from 1.49 to 3.27 Mg C ha- 1 yr- 1 and 1.51 to 3.67 Mg C ha- 1 yr- 1 in CT and NT, respectively, indicating carbon loss from soil. The differences in NEE or carbon balance between CT and NT were not statistically discernible. Earthworm abundance was higher in NT than in CT and increased vastly in the NT management after diversification of the rotation with winter rapeseed. Soil erosion measurements showed remarkably lower carbon loss (44%) for NT compared to CT, however, the role of erosion in the carbon balance was minor, ranging from 50 to 120 kg C ha- 1 yr- 1. The mean size of soil aggregates decreased during the study period, and soil aggregates tended to enlarge in the summer and diminish in the winter. The results highlight the difficulty of maintaining a positive carbon balance in boreal agricultural soils with limited productivity. Furthermore, future climatic conditions may worsen the situation by promoting decomposition and restricting carbon protection in soil aggregates.