Carbon dioxide fluxes of a mountain grassland: Drivers, anomalies and annual budgets
Julian Rogger, Lukas Hörtnagl, Nina Buchmann, Werner Eugster
Abstract
The potential of grasslands to mitigate climate change by carbon (C) sequestration in soils depends on agricultural management and the response of the grassland C cycle to a warming and more variable climate. A 15-year time series of eddy covariance carbon dioxide (CO2) flux measurements on a medium-intensively managed grassland at 1000 m above sea level in Switzerland was analysed to assess how CO2 fluxes respond to environmental drivers as well as grazing and cutting events. Further, the effects of management, recent warming trends and anomalous weather conditions on the annual C sequestration potential were assessed. To this end, C budgets including atmospheric and management-related C fluxes (fertilization, cutting/grazing) were estimated. Generally, the main environmental drivers of CO2 fluxes were light and temperature. The removal of the aboveground canopy by management significantly reduced photosynthesis and induced a saturation in the photosynthetic response to light at lower intensities compared to periods without management. Ecosystem respiration remained largely unaffected by management. With a net ecosystem exchange of −357 (± 76) g C m−2 year−1 and a net biome production of −154 (± 80) g C m−2 year−1 (including management C fluxes), the grassland on average acted as a net C sink. Recent climate warming favors spring CO2 assimilation due to earlier starting vegetation periods. However, the warming also resulted in an increased ecosystem respiration. Further, anomalous and especially unfavorable weather conditions for photosynthesis and regrowth following harvest events induced several periods of an anomalously increased net CO2 release. In the year 2015, such release anomalies contributed to an annual net C loss. Under future climatic conditions, the C sequestration potential at the mountain site will depend on the extent to which spring photosynthesis is offset by such climate-management interactions and a warming-induced increase in ecosystem respiration.