Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 5 2008 10.5194/bg-5-1411-2008 http://www.biogeosciences.net/5/1411/2008/ http://www.biogeosciences.net/5/1411/2008/bg-5-1411-2008.html http://www.biogeosciences.net/5/1411/2008/bg-5-1411-2008.pdf 1411 1423 2008-10-21 The CO₂ exchange of biological soil crusts in a semiarid grass-shrubland at the northern transition zone of the Negev desert, Israel B. Wilske brkwils@yahoo.com J. Burgheimer A. Karnieli E. Zaady M. O. Andreae D. Yakir J. Kesselmeier Department Environmental Science and Energy Research, Weizmann Institute of Sciences, Rehovot 76100, Israel Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020, Mainz, Germany The Remote Sensing Laboratory, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, 84990, Israel The Desertification and Restoration Ecology Research Center, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, 84990, Israel now at: Department of Environmental Sciences, The University of Toledo, Toledo, Ohio 43606-3390, USA now at: Department of Natural Resources, Agriculture Research Organization, Gilat Research Center, Ministry of Agriculture, Mobil Post Negev 85280, Israel Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO₂ fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO₂ deposition between –11.31 and –17.56 mmol m^−2 per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO₂ deposition by BSC was calculated to compensate 120%, –26%, and less than 3% of the concurrent soil CO₂ efflux from November–January, February–May and November–May, respectively. Thus, BSC effectively compensated soil CO₂ effluxes when CO₂ uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO₂ deposition within the period November–January by 11–123% and on average by 27%. 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