, Volume 340, Issue 1-2, pp 25-33
Date: 06 May 2010

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

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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 15N) 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-1 year-1) in semi-arid Inner Mongolia grassland. The 15N 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 15N of vegetation and soil, and sheep’s faeces and wool, although nitrogen content of bulk vegetation increased with stocking rate. Furthermore, δ 15N of vegetation and diet did not differ between stocking rates. Hence, 15N fractionations between vegetation and faeces (εveg-faeces), vegetation and wool (ε veg-wool), faeces and soil (ε faeces-soil) and soil and vegetation (ε soil-veg) were constants, with ε veg-faeces = 3.0‰ (±0.1‰, 95% confidence interval), ε veg-wool = 5.3‰ (±0.1‰), ε faeces-soil = 1.1‰ (±0.4‰) and ε soil-veg = -4.1‰ (±0.3‰). This finding is useful as it means that δ 15N of wool or faeces can be used to estimate the 15N of grazed vegetation, even if grazing pressure is unknown.

Responsible Editor: Klaus Butterbach-Bahl.