Changes in soil C‐isotopic composition in an agroecosystem under Free Air Carbon dioxide Enrichment (FACE) treatment during a crop rotation period

FACE (Free Air Carbon dioxide Enrichment) has been used since 1999 to evaluate the effects of future atmospheric CO 2 concentrations on an arable crop agroecosystem. The experiment conducted at the Institute of Agroecology at the Federal Research Centre in Braunschweig consists of a typical local crop rotation of winter barley, a cover crop, sugar beet and winter wheat. The atmospheric CO 2 concentration of ambient air is about 375 ppm with a δ 13 C value of −7 to −9‰, and 550 ppm ( δ 13 C value = −20.2‰) during daylight hours in the rings fumigated with additional CO 2 . Thus, the surplus C can be traced in the agricultural system. Over the course of the first experimental period (3‐year crop rotation period), the C‐isotopic composition and the C concentration in soil were monitored monthly. Plant samples were analysed according to the relevant developmental stages of the crop under cultivation. A 13 C depletion was observed in plant parts, as well as in soil samples from the FACE rings under CO 2 enrichment, indicating that labelled C has reached both respective ecosystem compartments. Albeit farming management practice (especially ploughing) leads to a mixing of ‘old’ and ‘new’ C compounds throughout all soil horizons down to the end of the ploughing layer and resulted in a heterogeneous distribution of newly formed C compounds in the soil, isotope analysis of soil C reflected where the surplus C went. Copyright © 2005 John Wiley & Sons, Ltd.