Quantification of ecosystem C dynamics in a long‐term FACE study on permanent grassland

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 [CO 2 ]. Our aim was to examine if a higher C supply provided under elevated CO 2 will increase the soil C pool. Special attention was given to respirational processes, where CO 2 emission rates and its sources (plant vs. soil) were considered. Methods The Giessen‐FACE experiment started in 1998 with a moderate CO 2 enrichment of +20% and +30% above ambient on an extensively managed grassland. The experiment consists of three control plots where no CO 2 is applied, three plots where [CO 2 ] is enriched by +20% and one plot receiving [CO 2 ] +30%. To exclude initial CO 2 step increase effects, a detailed examination of respirational processes over 30 months was carried out after 6 years of CO 2 enrichment starting in June 2004. At that time, the δ 13 C signature of the enrichment‐CO 2 was switched from −25 ‰ to −48 ‰ without a concomitant change in CO 2 concentration. Results After 9 years, the fraction of new C under [CO 2 ] +20% was 37 ± 5.4% in the top 7.5 cm but this decreased with depth. No CO 2 effect on soil carbon content was detected. Between June 2004 and December 2006, elevated [CO 2 ] +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 [CO 2 ] +20% and 35 ± 13% and 40 ± 13% for [CO 2 ] +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 CO 2 to the atmosphere. Copyright © 2016 John Wiley & Sons, Ltd.