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 in the three perennial cropping systems, while they decreased by 3.4 Mg C ha in the two annual cropping systems. The 0–100 cm soil C stocks increased by 6.8 Mg C ha in the three perennial cropping systems and increased by 2.3 Mg C ha in the triticale system, but decreased by 2.5 Mg C ha in the maize system. Topsoil N stocks increased by 0.18 Mg N ha in three perennial systems while they decreased by 0.08 Mg N ha 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 year, 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.