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.