Response of aboveground grassland biomass and soil moisture to moderate long-term CO2 enrichment

Rising atmospheric CO? concentrations may alter C cycling and community composition, however, long-term studies in (semi-)natural ecosystems are still rare. In May 1998, the Giessen FACE (Free Air Carbon dioxide Enrichment) experiment started in a grassland ecosystem near Giessen, Germany, consisting of three enrichment (E plots) and three ambient control rings (A plots). Carbon dioxide concentrations were raised to +20% above ambient all-year-round during daylight hours. The wet grassland (Arrhenatheretum elatioris Br.-Bl; not ploughed for > 100 years) has been fertilized with 40 kg ha(-1) yr(-1) N, and mown two times each year for decades. Since 1993, the biomass has been monitored and since 1997 it was divided into grasses, legumes and non-leguminous forbs. During the 5 years prior to CO2 enrichment, the annual biomass yield from the A plots was non-significantly higher (3%) than the later E plots yield. Under CO2 enrichment, the biomass increased significantly from the third enrichment year on by 9.8%, 7.7% and 11.2% in the years 2000-2002, respectively. The increase was surprisingly high considering the moderate CO2 enrichment regime of only +20% and sub-optimal N supply, possibly suggesting a non-linear response of temperate grassland ecosystems to rising atmospheric CO2 levels. The leaf area index did not change significantly under elevated CO2, nor did the soil moisture in the top 15 cm increase. No correlation existed between the magnitude of the yield stimulation under elevated CO2 and the precipitation sums preceding the respective harvests. The grass biomass increased significantly under FACE, while the forb biomass declined strongly in the fourth and fifth year. The legume fraction was mostly below 1% of the total yield, and did not respond to CO2 enrichment. These findings are in contrast to other grassland results and possible reasons are discussed. (c) 2005 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.