Ecology, 91(11), 2010, pp. 3261–3273 Ó 2010 by the Ecological Society of America Nitrogen regulation of the climate–carbon feedback: evidence from a long-term global change experiment SHULI NIU, 1,2,4 REBECCA A. SHERRY, 1 XUHUI ZHOU, 1 SHIQIANG WAN, 3 AND YIQI LUO 1 1 Department of Botany and Microbiology, University of Oklahoma, Norman, Oklahoma 73019 USA 2 State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093 China 3 Key Laboratory of Plant Stress Biology, College of Life Sciences, Henan University, Kaifeng, Henan 475004 China Abstract. Modeling studies have shown that nitrogen (N) strongly regulates ecosystem responses and feedback to climate warming. However, it remains unclear what mechanisms underlie N regulation of ecosystem–climate interactions. To examine N regulation of ecosystem feedback to climate change, we have conducted a warming and clipping experiment since November 1999 in a tallgrass prairie of the Great Plains, USA. Infrared heaters were used to elevate soil temperature by an average of 1.968C at a depth of 2.5 cm from 2000 to 2008. Yearly biomass clipping mimicked hay or biofuel feedstock harvest. We measured carbon (C) and N concentrations, estimated their content and C:N ratio in plant, root, litter, and soil pools. Warming significantly stimulated C storage in aboveground plant, root, and litter pools by 17%, 38%, and 29%, respectively, averaged over the nine years (all P , 0.05) but did not change soil C content or N content in any pool. Plant C:N ratio and nitrogen use efficiency increased in the warmed plots compared to the control plots, resulting primarily from increased dominance of C 4 plants in the community. Clipping significantly decreased C and N storage in plant and litter pools (all P , 0.05) but did not have interactive effects with warming on either C or N pools over the nine years. Our results suggest that increased ecosystem nitrogen use efficiency via a shift in species composition toward C 4 dominance rather than plant N uptake is a key mechanism underlying warming stimulation of plant biomass growth. Key words: biofuel harvest; carbon storage; climate change; C:N ratio; land use change; nitrogen use efficiency; soil carbon; warming. INTRODUCTION In the past decade, scientists have made substantial progress in understanding the feedbacks between terrestrial ecosystems and climate warming (Jones and Donnelly 2004, Field et al. 2007, Luo 2007, Heimann and Reichstein 2008). Most modeling studies predict ecosystem C storage will decrease as respiration is stimulated more than photosynthesis by rising temper- ature, with a consequent positive feedback to climate warming (Cox et al. 2000, Friedlingstein et al. 2006, Heimann and Reichstein 2008). Nevertheless, experi- mental studies have shown diverse responses of C storage to climate warming, observing increases (Welker et al. 2004, Oberbauer et al. 2007, Day et al. 2008, Sardans et al. 2008), decreases (Oberbauer et al. 2007), and no changes (Marchand et al. 2004, Luo et al. 2009). These diverse experimental results may be partially due to variance in N regulation of carbon processes over time scales and across ecosystems (Shaver et al. 2000, Luo 2007). Indeed, N regulation can cause large uncertainties in projections of climate–C feedbacks (Hungate et al. 2003, Heimann and Reichstein 2008). Interactions between C and N are important in the response of ecosystem C cycling to climate change because of the close coupling between these two elements (McGuire et al. 1992, Hungate et al. 2003, Reich et al. 2006). Some recent ecological models of climate–carbon cycle feedbacks have emphasized C–N interactions (Thornton et al. 2007, 2009, Sokolov et al. 2008), and suggested that C sequestration will be underestimated, resulting in inaccurate conclusions about terrestrial feedbacks to climate warming if terrestrial C–N interactions under climate warming are ignored (Sokolov et al. 2008). In these models, increased N mineralization (availability) due to the kinetic sensitivity of microbe activity to climate warming has been considered to be a major mechanism for the proposed increase in plant C uptake. However, no consistent results regarding N mineralization have been reported in experimental warming studies, which see increases (Hobbie 1996, Rustad et al. 2001, Shaw and Harte 2001) or no change (Verburg et al. 2009) in N mineralization with warming. Some of the studies also reported that the mineralization response to warming changed with time (Verburg and van Breemen 2000, Manuscript received 10 September 2009; revised 26 February 2010; accepted 5 April 2010. Corresponding Editor: J. B. Yavitt. 4 E-mail: sniu@ou.edu 3261