Soil Biology & Biochemistry 38 (2006) 2448–2460 Response of soil microbial biomass and enzyme activities to the transient elevation of carbon dioxide in a semi-arid grassland Ellen Kandeler a,Ã , Arvin R. Mosier b , Jack A. Morgan c , Daniel G. Milchunas d , Jennifer Y. King e , Sabine Rudolph a , Dagmar Tscherko a a Institute of Soil Science, University of Hohenheim, Emil Wolff Str. 27, D-70599 Stuttgart, Germany b Agricultural & Biological Engineering Department, University of Florida, Gainesville, FL 32611, USA c USDA-ARS Rangeland Resources Research Unit, Crops Research Laboratory, 1701 Centre Ave., Fort Collins, CO 80526, USA d Forest, Rangeland, and Watershed Stewardship Department, and Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA e Departments of Soil, Water, and Climate, and Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108, USA Received 2 December 2005; received in revised form 28 January 2006; accepted 28 February 2006 Available online 18 April 2006 Abstract Although elevation of CO 2 has been reported to impact soil microbial functions, little information is available on the spatial and temporal variation of this effect. The objective of this study was to determine the microbial response in a northern Colorado shortgrass steppe to a 5-year elevation of atmospheric CO 2 as well as the reversibility of the microbial response during a period of several months after shutting off the CO 2 amendment. The experiment was comprised of nine experimental plots: three chambered plots maintained at ambient CO 2 levels of 360 mmol mol 1 (ambient treatment), three chambered plots maintained at 720 mmol mol 1 CO 2 (elevated treatment) and three unchambered plots of equal ground area used as controls to monitor the chamber effect. Elevated CO 2 induced mainly an increase of enzyme activities (protease, xylanase, invertase, alkaline phosphatase, arylsulfatase) in the upper 5 cm of the soil and did not change microbial biomass in the soil profile. Since rhizodeposition and newly formed roots enlarged the pool of easily available substrates mainly in the upper soil layers, enzyme regulation (production and activity) rather than shifts in microbial abundance was the driving factor for higher enzyme activities in the upper soil. Repeated soil sampling during the third to fifth year of the experiment revealed an enhancement of enzyme activities which varied in the range of 20–80%. Discriminant analysis including all microbiological properties revealed that the enzyme pattern in 1999 and 2000 was dominated by the CO 2 and chamber effect, while in 2001 the influence of elevated CO 2 increased and the chamber effect decreased. Although microbial biomass did not show any response to elevated CO 2 during the main experiment, a significant increase of soil microbial N was detected as a post-treatment effect probably due to lower nutrient (nitrogen) competition between microorganisms and plants in this N-limited ecosystem. Whereas most enzyme activities showed a significant post-CO 2 effect in spring 2002 (following the conclusion of CO 2 enrichment the previous autumn, 2001), selective depletion of substrates is speculated to be the cause for non- significant treatment effects of most enzyme activities later in summer and autumn, 2002. Therefore, additional belowground carbon input mainly entered the fast cycling carbon pool and contributed little to long-term carbon storage in the semi-arid grassland. r 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon dioxide; Climate change; Soil enzymes; Microbial biomass; Shortgrass steppe; Carbon cycling; Below ground processes 1. Introduction Increased atmospheric CO 2 concentration often stimu- lates plant photosynthesis, enhances carbon allocation belowground, increases plant nutrient uptake, and im- proves the efficiency of plant water use (Hu et al., 1999). Results from 16 free-air CO 2 enrichment (FACE) sites representing four different vegetation types indicate that most herbaceous species had reduced leaf nitrogen con- tent and modest enhancement of leaf CO 2 assimilation under elevated CO 2 (see review by Nowak et al., 2004). ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2006.02.021 Ã Corresponding author. Tel.: +49 711 459 4220; fax: +49 711 459 3117. E-mail address: kandeler@uni-hohenheim.de (E. Kandeler).