Drivers of long-term grassland CO 2 fluxes: effects of management and meteorological conditions during regrowth periods

Abstract. Grasslands serve a unique role in the global carbon (C) cycle and cover about 30 % of the European and about 70 % of the Swiss agricultural area. Carbon dioxide (CO2) fluxes of managed grasslands are substantially influenced by land management practices and meteorological conditions, but the temporal development of drivers and their effects are still uncertain. We used 20 years (2005–2024) of eddy-covariance (EC) fluxes, meteorological data, and detailed management information collected from an intensively managed grassland site (Chamau) in Switzerland, and employed machine learning approaches, i.e., eXtreme Gradient Boosting (XGBoost) models in combination with SHapley Additive exPlanations (SHAP) analyses, to identify drivers and their temporal contributions over two decades. Our study aimed to (1) identify intra- and inter-annual variations in grassland CO2 fluxes, (2) assess magnitude and drivers of gross primary production (GPP) and ecosystem respiration (Reco) during regrowth periods (i.e., the period after mowing, grazing, or reseeding events until the next event), and (3) quantify driver contributions to GPP and Reco over time, with focus on management and extreme events. Our results showed pronounced intra- and inter-annual variations in CO2 fluxes, driven by both management activities as well as meteorological conditions. Despite significant increases in temperature and decreases in soil water content (SWC) during the two decades, GPP and Reco during regrowth periods remained stable, and no significant trend over time was detected, suggesting adapted, climate-smart decision making of the farmer. The most important drivers of GPP in the long-term were light, management, and temperature, while Reco was mainly driven by temperature, GPP, and management. However, during extreme drought periods in the peak growing seasons (June, July, August), SWC increased in importance and limited GPP. In contrast, the impact of nitrogen (N) fertilization was more differentiated, either acting in parallel with SWC, suggesting low N availability during drought periods, or increasing GPP in years after sward renewal despite low SWC. Overall, our study provided novel insights into relevant drivers of grassland CO2 fluxes and their complex temporal contributions in the short- and long-term. Our results suggest that even small climate-smart management adaptations could be promising solutions for stabilizing important grassland processes, such as grassland regrowth, under current and future climate.