Aims

Arid and semi-arid grassland ecosystems cover 41% of the global land surface but are currently being degraded by increases in aridity and changes in land use. As a consequence, ecosystem services, including carbon (C) and nitrogen (N) sequestration, are being reduced. In this study, we determined whether stable isotopic C (δ¹³C) and N (δ¹⁵N) values can be used to explain patterns of multiple ecosystem functions related to C and N cycling in arid and semi-arid grasslands at the regional scale.

Methods

Ecosystem (plant and soil) δ¹³C and δ¹⁵N values and ecosystem functions related to C and N cycling were investigated along an aridity gradient on the Mongolian Plateau. The gradient covered four vegetation grassland types with distinct characteristics of aridity and soil.

Results

We found that ecosystem δ¹³C and δ¹⁵N values increased while ecosystem functions decreased as aridity increased. Structural equation modelling revealed that aridity directly and indirectly affected plant δ¹³C values by affecting plant C/N and indirectly affected soil δ¹³C values by affecting plant and soil C/N and plant δ¹³C. Aridity directly and indirectly affected soil δ¹⁵N values by affecting soil C/N and soil pH, and indirectly affected plant δ¹⁵N values by affecting plant and soil C/N and soil δ¹⁵N. As indicated by ecosystem δ¹³C and δ¹⁵N values, the negative effects of aridity on ecosystem functions were mainly derived from the direct and indirect effects of aridity on C cycling and N cycling, respectively.

Conclusions

Our findings suggest that ecosystem δ¹³C and δ¹⁵N values can be used to explain patterns of multiple ecosystem functions related to C and N cycling in arid and semi-arid grasslands at the regional scale. Consideration of ecosystem δ¹³C and δ¹⁵N values in predictive models will greatly increase the ability to accurately simulate how ecosystem functions change in response to the rapid degradation of semi-arid grasslands.