Speaker
Description
With the accelerating transition toward a low-carbon bioeconomy, diverse biomass crops are increasingly introduced into traditional agricultural systems to support biorefinery or provide renewable bioenergy. However, how different crops and different cropping systems impact soil organic carbon (SOC) across the soil profile remains poorly understood, partly because few long-term field experiments exist and SOC stock changes are often quantified using inconsistent approaches. Using a decade-long field experiment (Biobase) comprising annual monocultures, annual rotations, and perennial cropping systems, we quantified changes in SOC stocks using both the equivalent soil mass (ESM) and fixed-depth (FD) approaches, along with total soil nitrogen (TN) and nitrogen balance. Based on the ESM method, the topsoil SOC stock change after conversion from traditional rotation to perennial cropping systems (+2.6 Mg ha-1, 7%) was significantly higher than the conversions to annual crop monoculture (-0.9 Mg ha-1) and optimized annual crop rotation (-0.9 Mg ha-1). None of the cropping systems significantly affected SOC stocks below 20 cm. In contrast, the FD approach indicated that optimized rotation, together with most perennial cropping systems, increased topsoil SOC stocks by 9–17% (3.2–6.0 Mg ha-1). The changes in SOC stocks were highly associated with dynamics in soil TN, suggesting coupled carbon and nitrogen accumulation under perennial systems. Except for the two annual crop monocultures, all perennial cropping systems and crop rotations increased topsoil TN content by 8–30%, which could be linked to the mitigated nitrate leaching. These findings indicate that the carbon sequestration benefits of perennialization are largely confined to topsoil. The difference between the ESM and FD approaches highlights how calculation methods influence SOC stock assessment, and quantifying SOC stocks should account for variations in soil bulk density and consider the entire soil profile. Overall, our findings improve the understanding of long-term SOC dynamics following the conversion from annual to perennial cropping systems and highlight the importance of robust SOC stock estimation for climate impact evaluation.