Katharina Lenhart1, Claudia Kammann1,2, Pascal Boeckx3, Johan Six4, Christoph Müller1,5. 1. Department of Plant Ecology, Justus-Liebig-University Giessen, Heinrich-Buff Ring 26-32, D-35392, Giessen, Germany. 2. Department for Soil Science and Plant Nutrition, Hochschule Geisenheim University, Von-Lade-Str. 1, D-65366, Geisenheim, Germany. 3. Isotope Bioscience Laboratory - ISOFYS, Gent University, Coupure Links 653, Ghent, 9000, Belgium. 4. Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH Zurich, Tannenstrasse 1, 8044, Zurich, Switzerland. 5. School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, 4, Ireland.
Abstract
RATIONALE: 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.
RATIONALE: 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.