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The carbon isotope composition (δ13C) of C3/C4 mixed grassland is reflected in the δ13C of diet, hair or faeces of grazers, if 13C discrimination (13Δ) between grassland vegetation and these tissues is known and constant. However, these relationships could be modified by selective grazing or differential digestibility of the C3 and C4 components, potentially creating a bias between grassland and grazer tissue δ13C. Importantly, these factors have never been studied in detail. We investigated the relation between δ13C of C3/C4 grassland vegetation and that of faeces and hair of sheep in a 3-year (2005–2007) experiment in the Inner Mongolian semi-arid steppe. The experiment employed six stocking rates (0.375–2.25 sheep ha−1 year−1; four replications), which allowed for a large variation in species composition, digestibility, and diet selection. Faecal-nitrogen content, a proxy for digestibility, decreased from 1.9% to 1.5% during the grazing period due to aging of the herbage. At the same time, the C3/C4 ratio decreased due to the later growth initiation of C4 species. 13Δ between diet and faeces (13Δ(DF); 0.6 ppt) and between diet and hair (13Δ(DH); −3.9 ppt) were not influenced by stocking rate, period in the season or C3/C4 ratio. Moreover, faeces–hair discrimination (13Δ(FH); −4.3 ppt), which reflects differences between digestibility of the C3 and C4 components, did not vary along the different gradients. The δ13C of grassland vegetation can be estimated from the δ13C of sheep faeces and hair, provided that 13Δ was accounted for. This is useful for landscape- or regional-scale investigations or reconstruction of C3/C4 vegetation distribution from faeces and hair, which provide different temporal and spatial integration of grassland isotope signals.
