Large inter-annual variation in carbon sink strength of a permanent grassland over 16 years: Impacts of management practices and climate

•Grassland CO2 fluxes were measured for 16 years, and C budgets were assessed.•In 9 out of 16 years, the long-term net biome production showed a carbon (C) sink.•Soil C stocks indicated a C neutral behavior of the intensively managed grassland.•Reduced organic C fertilization led to years as C source.•Maintaining the grassland C sink in the future will require organic C fertilization.