Changes in the abundance of C3/C4 species of Inner Mongolia grassland: evidence from isotopic composition of soil and vegetation MAXIMILIAN H. O. M. WITTMER *, KARL AUERSWALD *, YONGFEI BAI w , RUDI SCHA ¨ UFELE * and H A N S S C H N Y D E R * *Lehrstuhl fu ¨r Gru ¨nlandlehre, Technische Universita ¨t Mu ¨nchen, Am Hochanger 1, D-85350 Freising-Weihenstephan, Germany, wState Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Xiangshan, Beijing 100093, China Abstract Global warming, increasing CO 2 concentration, and environmental disturbances affect grass- land communities throughout the world. Here, we report on variations in the C3/C4 pattern of Inner Mongolian grassland derived from soil and vegetation. Soil samples from 149 sites covering an area of approximately 250 000 km 2 within Inner Mongolia, People’s Republic of China were analyzed for the isotopic composition (d 13 C) of soil organic carbon (SOC). The contrast in d 13 C between C3 and C4 plants allowed for calculation of the C3/C4 ratio from d 13 C of SOC with a two-member mixing model, which accounted for influences of aridity and altitude on d 13 C of the C3 end-member and for changes in d 13 C of atmospheric CO 2 . Maps were created geostatistically, and showed a substantially lower C4 abundance in soil than in recent vegetation (10%). The difference between soil and vegetation varied regionally and was most pronounced within an E–W belt along 441N and in a mountainous area, suggesting a spread of C4 plants toward northern latitudes (about 11) and higher altitudes. The areas of high C4 abundance for present vegetation and SOC were well delineated by the isotherms of crossover temperature based on the climatic conditions of the respective time periods. Our study indicates that change in the patterns of C3/C4 composition in the Inner Mongolia grassland was mainly triggered by increasing temperature, which overrode the antagonistic effect of rising CO 2 concentrations. Keywords: d 13 C, carbon isotope discrimination, crossover temperature, geostatistics, precipitation, semivariogram, soil organic carbon, Suess effect, wool Received 29 January 2009; revised version received 18 May 2009 and accepted 9 July 2009 Introduction The carbon isotope composition (d 13 C) of plants and soil organic carbon (SOC) yields important information regarding carbon fluxes and linked biogeochemical cycles (Schimel, 1995; Ehleringer et al., 2000). In grass- land, the 13 C signal can vary considerably. This is related primarily to the presence of variable proportions of C3 and C4 photosynthetic types (Bird & Pousai, 1997; Tieszen et al., 1997; Collatz et al., 1998) and the large difference in carbon isotope discrimination ( 13 D) be- tween them (Farquhar et al., 1989). Variation in the C3/C4 ratio has wide biogeochemical and land use implications: it affects the magnitude and seasonal distribution of biomass production, soil carbon storage, water use, and nutrient cycling (Bird & Pousai, 1997; Tieszen et al., 1997; Epstein et al., 1998; Sage & Kubien, 2003; Semmartin et al., 2004). Hence, because it indicates the C3/C4 ratio, d 13 C is a useful proxy of vital functions of grassland. However, only a few regional-scale inves- tigations of d 13 C of C3/C4 mixed grassland have been undertaken (von Fischer et al., 2008). Soil is one of the most important terrestrial carbon reservoirs, storing more than twice as much carbon as the atmosphere (Trumbore, 2000). The SOC input to this pool is mainly composed of carbon from vegetation, and the output of carbon is by soil respiration (Schimel, 1995). With increasing soil depth, the soil carbon age increases greatly (e.g. as derived from radiocarbon dating) (Rumpel et al., 2002; Du ¨ mig et al., 2008; Lopez-Capel et al., 2008). Hence, SOC is affected by past vegetation, and therefore allows for the reconstruction of changes in the isotopically distinct C3/C4 ratio (Boutton et al., 1998; Ehleringer et al., Correspondence: Karl Auerswald, tel. 1 49 8161 713965, fax 1 49 8161 713243, e-mail: auerswald@wzw.tum.de Global Change Biology (2010) 16, 605–616, doi: 10.1111/j.1365-2486.2009.02033.x r 2009 Blackwell Publishing Ltd 605