The response of soil CO 2 ¯ux to changes in atmospheric CO 2 , nitrogen supply and plant diversity JOSEPH M. CRAINE*, DAVID A. WEDIN ² and PETER B. REICH ³ *Department of Integrative Biology, University of California, Berkeley, CA 94720, USA, ²School of Natural Resource Sciences, University of Nebraska, Lincoln, NE 68583, USA, ³Department of Forest Resources, University of Minnesota, St. Paul, MN 55108 USA Abstract We measured soil CO 2 ¯ux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmos- pheric carbon dioxide (CO 2 ) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO 2 and N fertilization increased soil CO 2 ¯ux by 0.57 mmol m ±2 s ±1 (13% increase) and 0.37 mmol m ±2 s ±1 (8% increase) above average control soil CO 2 ¯ux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO 2 ¯ux by 0.23, 0.41 and 1.09 mmol m ±2 s ±1 (5%, 8% and 21% decreases), respectively. There were no statistically signi®cant pairwise interactions among the three treat- ments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO 2 increased soil CO 2 ¯ux most when soil moisture was low and soils were warm. Effects on soil CO 2 ¯ux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no signi®cant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correl- ated over time; neither were correlated with the CO 2 effect. Models of soil CO 2 ¯ux will need to incorporate ecosystem CO 2 and N availability, as well as eco- system plant diversity, and incorporate different environmental factors when determining the magnitude of the CO 2 , N and diversity effects on soil CO 2 ¯ux. Keywords: Nitrogen fertilization, CO 2 , biodiversity, soil CO 2 ¯ux, carbon cycle Received 28 February; revised version received and accepted 3 July 2001 Introduction Increasing atmospheric carbon dioxide (CO 2 ) concentra- tions, increasing rates of nitrogen (N) deposition, and declining plant diversity are three major global changes caused by human activities (Vitousek 1994; Sala et al. 2000). Soil CO 2 ¯ux represents an integrated measure of root and microbial activity (Raich & Nadelhoffer 1989; Schlesinger & Andrews 2000) and is an important source of atmospheric CO 2 with annual global ¯ux equivalent to approximately 10% of the atmospheric CO 2 pool (Schlesinger & Andrews 2000). Soil CO 2 ¯ux is an important component of below-ground ecosystem car- bon (C) cycling and storage and is likely to be altered by increases in resource availability and decreases in diver- sity (Knapp et al. 1998). Soil CO 2 ¯ux generally increases with elevated CO 2 in forest and grassland systems (Allen et al. 2000), with a wide variety of responses of soil CO 2 ¯ux to elevated CO 2 (±10% to +162%) (Zak et al. 2000). Nitrogen fertilization often does not alter or decreases soil CO 2 ¯ux (Raich & Tufekcioglu 2000). Little is known about the relationship between diversity and soil CO 2 ¯ux, or about potential interactions between elevated CO 2 ,N supply and plant diversity in their effects on soil CO 2 ¯ux. Temporal patterns of soil CO 2 ¯ux are important to models of ecosystem carbon balance (Raich & Potter Correspondence: Joseph M. Craine, Landcare Research, Private Bag 1930, Dunedin, NZ, tel +64 3477 4050, fax +64 3477 5232, jcraine@socrates.berkeley.edu Global Change Biology (2001) 7, 947±953 ã 2001 Blackwell Science Ltd 947