Perennial cropping systems increased topsoil carbon and nitrogen stocks over annual systems—a nine-year field study

Enhancing biomass yield simultaneously with soil carbon (C) sequestration is a key aim of climate-smart cropping systems. Perennialization is believed to be a suitable mitigation strategy for climate change with the potential for enhancing soil C stocks. Based on a nine-year field experiment in Denmark, we measured the changes in soil C and nitrogen (N) stocks, biomass yield, and yield stability of three perennial (low-fertilized miscanthus, high-fertilized festulolium, and no N-fertilized grass-legume mixture) and two annual (continuous triticale and maize) cropping systems. We found that the changes in topsoil (0–20 cm) and 0–100 cm soil C stocks and topsoil N stocks varied significantly between cropping systems. Over nine years, topsoil C stocks increased by an average of 1.4 Mg C ha −1 in the three perennial cropping systems, while they decreased by 3.4 Mg C ha −1 in the two annual cropping systems. The 0–100 cm soil C stocks increased by 6.8 Mg C ha −1 in the three perennial cropping systems and increased by 2.3 Mg C ha −1 in the triticale system, but decreased by 2.5 Mg C ha −1 in the maize system. Topsoil N stocks increased by 0.18 Mg N ha −1 in three perennial systems while they decreased by 0.08 Mg N ha −1 in the two annual cropping systems. Changes in 0–100 cm soil N stocks did not differ significantly between cropping systems. Miscanthus, festulolium, and maize showed the highest biomass yield (17.1, 16.7, and 16.4 Mg ha −1 year −1, respectively). There were no significant differences among cropping systems in yield stability. This study demonstrated the potential of perennial cropping systems in obtaining higher soil C stocks compared with annual cropping systems while maintaining high biomass yield, supporting perennialization as a promising option for climate-smart agriculture.