Variation in the Stable Carbon and Nitrogen Isotope Composition of Plants and Soil along a Precipitation Gradient in Northern China

Water availability is the most influential factor affecting plant carbon (δ (13)C) and nitrogen (δ (15)N) isotope composition in arid and semi-arid environments. However, there are potential differences among locations and/or species in the sensitivity of plant δ (13)C and δ (15)N to variation in precipitation, which are important for using stable isotope signatures to extract paleo-vegetation and paleo-climate information. We measured δ (13)C and δ (15)N of plant and soil organic matter (SOM) samples collected from 64 locations across a precipitation gradient with an isotherm in northern China. δ (13)C and δ (15)N for both C(3) and C(4) plants decreased significantly with increasing mean annual precipitation (MAP). The sensitivity of δ (13)C to MAP in C(3) plants (-0.6±0.07‰/100 mm) was twice as high as that in C(4) plants (−0.3±0.08‰/100 mm). Species differences in the sensitivity of plant δ (13)C and δ (15)N to MAP were not observed among three main dominant plants. SOM became depleted in (13)C with increasing MAP, while no significant correlations existed between δ (15)N of SOM and MAP. We conclude that water availability is the primary environmental factor controlling the variability of plant δ (13)C and δ (15)N and soil δ (13)C in the studied arid and semi-arid regions. Carbon isotope composition is useful for tracing environmental precipitation changes. Plant nitrogen isotope composition can reflect relative openness of ecosystem nitrogen cycling.