title: Quantification of ecosystem C dynamics in a long-term FACE study on permanent grassland.
abstract: BACKGROUND
  Because of the wide-ranging appearance and high soil organic carbon (C) content of grasslands, their ecosystems play an important role in the global C cycle. Thus, even small changes in input or output rates lead to significant changes in the soil C content, thereby affecting atmospheric [CO2 ]. Our aim was to examine if a higher C supply provided under elevated CO2 will increase the soil C pool. Special attention was given to respirational processes, where CO2 emission rates and its sources (plant vs. soil) were considered.
  
  
  METHODS
  The Giessen-FACE experiment started in 1998 with a moderate CO2 enrichment of +20% and +30% above ambient on an extensively managed grassland. The experiment consists of three control plots where no CO2 is applied, three plots where [CO2 ] is enriched by +20% and one plot receiving [CO2 ] +30%. To exclude initial CO2 step increase effects, a detailed examination of respirational processes over 30 months was carried out after 6 years of CO2 enrichment starting in June 2004. At that time, the δ(13) C signature of the enrichment-CO2 was switched from -25 ‰ to -48 ‰ without a concomitant change in CO2 concentration.
  
  
  RESULTS
  After 9 years, the fraction of new C under [CO2 ] +20% was 37 ± 5.4% in the top 7.5 cm but this decreased with depth. No CO2 effect on soil carbon content was detected. Between June 2004 and December 2006, elevated [CO2 ] +20% increased the ecosystem respiration by 13%. The contribution of root respiration to soil respiration was 37 ± 13% (5 cm) and 43 ± 14% (10 cm) for [CO2 ] +20% and 35 ± 13% and 40 ± 13% for [CO2 ] +30%, respectively.
  
  
  CONCLUSIONS
  Our findings of an increased C turnover without a net soil C sequestration suggest that the sink strength of grassland ecosystems might decrease in the future, because the additional C may quickly be released as CO2 to the atmosphere. Copyright © 2016 John Wiley & Sons, Ltd.
authors.affiliation: Department of Plant Ecology, Justus-Liebig-University Giessen, Heinrich-Buff Ring 26-32, D-35392, Giessen, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Lenhart
authors.affiliation: Department of Plant Ecology, Justus-Liebig-University Giessen, Heinrich-Buff Ring 26-32, D-35392, Giessen, Germany.|Department for Soil Science and Plant Nutrition, Hochschule Geisenheim University, Von-Lade-Str. 1, D-65366, Geisenheim, Germany.
authors.forename: Claudia
authors.initials: C
authors.lastname: Kammann
authors.affiliation: Isotope Bioscience Laboratory - ISOFYS, Gent University, Coupure Links 653, Ghent, 9000, Belgium.
authors.forename: Pascal
authors.initials: P
authors.lastname: Boeckx
authors.affiliation: Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH Zurich, Tannenstrasse 1, 8044, Zurich, Switzerland.
authors.forename: Johan
authors.initials: J
authors.lastname: Six
authors.affiliation: Department of Plant Ecology, Justus-Liebig-University Giessen, Heinrich-Buff Ring 26-32, D-35392, Giessen, Germany.|School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, 4, Ireland.
authors.forename: Christoph
authors.initials: C
authors.lastname: Müller