sup 15^N fractionation between vegetation, soil, faeces and wool is not influenced by stocking rate

Understanding stable isotope fractionation in trophic networks is important for the interpretation of stable isotope composition of ecosystem components. This work explores the influence of grazing pressure on the nitrogen isotope composition (δ ^sup 15^N) of vegetation (standing biomass), soil, and sheep's faeces and wool in a three-years (2005-2007) experiment with different stocking rates (0.375-2.25 sheep ha^sup -1^ year^sup -1^) in semi-arid Inner Mongolia grassland. The ^sup 15^N of wool (from a yearly shearing) reflects vegetation at the whole-year grazing grounds-scale while faeces reflect that of the area grazed within a few days. Stocking rate had no effect on δ ^sup 15^N of vegetation and soil, and sheep's faeces and wool, although nitrogen content of bulk vegetation increased with stocking rate. Furthermore, δ ^sup 15^N of vegetation and diet did not differ between stocking rates. Hence, ^sup 15^N fractionations between vegetation and faeces ([straight epsilon]^sub veg-faeces^), vegetation and wool ([straight epsilon] ^sub veg-wool^), faeces and soil ([straight epsilon] ^sub faeces-soil^) and soil and vegetation ([straight epsilon] ^sub soil-veg^) were constants, with [straight epsilon] ^sub veg-faeces^=3.0[per thousand] (±0.1[per thousand], 95% confidence interval), [straight epsilon] ^sub veg-wool^=5.3[per thousand] (±0.1[per thousand]), [straight epsilon] ^sub faeces-soil^=1.1[per thousand] (±0.4[per thousand]) and [straight epsilon] ^sub soil-veg^=-4.1[per thousand] (±0.3[per thousand]). This finding is useful as it means that δ ^sup 15^N of wool or faeces can be used to estimate the ^sup 15^N of grazed vegetation, even if grazing pressure is unknown.[PUBLICATION ABSTRACT]