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

Permanent grasslands cover one third of the European agricultural area and are known to store large amounts of carbon (C) in their soils. However, long-term assessments of their C sink strength are still scarce. Thus, we investigated the C budget of an intensively managed, permanent grassland in Switzerland over 16 years, compared the results to changes in soil C stocks, and determined the most important drivers of the net ecosystem CO2 exchange (NEE). Combining NEE fluxes with C imports and C exports, we quantified the grassland C budget, i.e., net biome production (NBP). We observed a large inter-annual variation in NBP, with 9 of the 16 years indicating a C sink, and 7 years indicating a C source. On average, the grassland was a small C sink to C neutral, with a NBP of -70±106 g C m$^{−2}$ yr$^{−1}$ (mean±95% confidence interval). Mean NEE fluxes were -284±115 g C m$^{−2}$ yr$^{−1}$, C exports via harvest 335±73 g C m$^{−2}$ yr$^{−1}$, and organic C imports via slurry -121±43 g C m$^{−2}$ yr$^{−1}$. Soil C stocks from 0 to 0.7 m did not change significantly (decrease of 27.5 g C m$^{−2}$ yr$^{−1}$ over 13 years). Inter-annual variation in NBP was affected by management practices and environmental conditions. ... mehrIn the last five years, NBP was positive (C source), most likely due to decreasing C imports in combination with extreme weather conditions. Our study demonstrated the importance of covering multiple years with different management events when assessing the C sink strength of a site. Maintaining even a small grassland C sink in the future will be challenging and will require continuous organic C imports.