Shifts in Respiratory Quotient and δ¹³C Isotopic Signature of CO₂ Produced by Soils under Long-Term Agricultural Use in Arid Steppe Ecosystems

Abstract

Geochemical shifts in carbon cycling induced by agriculture in arid and semiarid ecosystems are closely linked to global climate. Here, we evaluated (i) how different agricultural practices in arid steppe ecosystems of north Kazakhstan influenced CO₂/O₂ exchange rate, respiratory quotient (RQ), and C-transformation efficiency and (ii) shifts in δ¹³C-CO₂ signature induced by long-term and uniform agricultural practices when a natural C3-plant community was substituted with a C3-crop. Conservation tillage and intensive mold-board ploughing were compared with soil that had never been tilled. The content of microbial C was significantly lower in soil subjected to tillage. For all tilled treatments RQ was close to 1.0 and higher than in never tilled soil suggesting complete oxidation of soil organic matter. δ¹³C-CO₂ value for never tilled soil was significantly lower (−26) than in all tilled treatments (around −20), suggesting that agricultural soils produce CO₂ with heavier isotope composition compared with undisturbed soil.

Keywords:

• carbon transformation efficiency
• respiratory quotient
• ¹³C-CO₂ signature
• microbial physiology

ACKNOWLEDGMENTS

Authors are very grateful to Prof. Eva-Maria Pfeifer and Dr. Christian Knoblauch for offering perfect analytical facilities and support; the German Research Foundation for financial support to participate at the 19th International Symposium for Environmental Biogeochemistry (ISEB); Dr. A. Bektemirov and Dr. K. Akshalov for maintaining field studies; Mrs. Jennifer Kane-Zabolotskaya from U.S.-diplomatic mission in Kazakhstan for kind support; Dr. Andreas Ulrich from Universität Konstanz for sharing his experience; Dr. Catherine Alexander from University of London for comments. We also appreciate contribution of Prof. Shamil Mamilov.