Assessing the effect of arable management practices on carbon storage and fractions after 24 years in boreal conditions of Finland

Soil organic matter (SOM) plays an important role in soil functions that are crucial for sustainable agriculture. Understanding how agricultural management and soil properties affect SOM in different soil depths would aid in maintaining and increasing SOM throughout the soil profile. We sampled a 24 year-old cultivation field experiment with organic and conventional cropping systems, and an adjacent unmanaged meadow to 70 cm soil depth to assess the total organic carbon (OC) stocks and the distribution of OC into mineral-associated (MAOM), particulate (POM) and dissolved (DOM) organic matter. We found that >83% of the soil OC was in the MAOM fraction, and that the distribution of OC across the MAOM, POM and DOM within a soil depth was not strongly affected by soil management. Largest OC stocks (169 t ha⁻¹) together with the largest plant root biomass was found in the unmanaged meadow, which highlights the potential of deep-rooting plants in sequestering OC into the soil. The OC saturation state of the soil was assessed based on the clay to OC ratio and Hassink's Equation (Hassink 1997). Hassink's Equation seemed to underestimate the MAOM accrual capacity of these soils and thus overestimated soil OC saturation state, whereas the clay to OC ratio indicated potential for OC accrual in all soil depths except for the meadow topsoil. These varying results suggest that the applied metric should be soil type and -depth specific. We also determined the contribution of clay content and aluminum and iron oxides in explaining the amount of total OC, MAOM-C and POM-C. In contrast to the aluminum and iron oxides, clay was not well correlated to any of the OC fractions below the 20 cm depth, suggesting that estimating the OC accrual potential of the deeper soil should not be based on the soil texture alone. Our results indicate that aluminum and iron oxides can play an important role in transporting and stabilizing of OC in the soil profile.