Biomass responses in a temperate European grassland through 17years of elevated CO2

Future increase in atmospheric CO2 concentrations will potentially enhance grassland biomass production and shift the functional group composition with consequences for ecosystem functioning. In the GiFACE experiment (Giessen Free Air Carbon dioxide Enrichment), fertilized grassland plots were fumigated with elevated CO2 (eCO(2)) year-round during daylight hours since 1998, at a level of +20% relative to ambient concentrations (in 1998, aCO(2) was 364ppm and eCO(2) 399ppm; in 2014, aCO(2) was 397ppm and eCO(2) 518ppm). Harvests were conducted twice annually through 23years including 17years with eCO(2) (1998 to 2014). Biomass consisted of C3 grasses and forbs, with a small proportion of legumes. The total aboveground biomass (TAB) was significantly increased under eCO(2) (p=.045 and .025, at first and second harvest). The dominant plant functional group grasses responded positively at the start, but for forbs, the effect of eCO(2) started out as a negative response. The increase in TAB in response to eCO(2) was approximately 15% during the period from 2006 to 2014, suggesting that there was no attenuation of eCO(2) effects over time, tentatively a consequence of the fertilization management. Biomass and soil moisture responses were closely linked. The soil moisture surplus (c. 3%) in eCO(2) manifested in the latter years was associated with a positive biomass response of both functional groups. The direction of the biomass response of the functional group forbs changed over the experimental duration, intensified by extreme weather conditions, pointing to the need of long-term field studies for obtaining reliable responses of perennial ecosystems to eCO(2) and as a basis for model development.