**5. Conclusions**

In summary, a large-scale soil collection was carried out in temperate grasslands along a vegetation transect in Inner Mongolia to examine how climatic, edaphic and biological factors affect the soil δ13C or δ15N signature. Along the transect, soil δ15N was not correlated with MAP and MAT, while soil δ13C linearly decreased with MAP and MAT. Soil δ15N logarithmically increased with concentrations of SOC, Nt and Pt, but soil δ13C linearly decreased with concentrations of SOC, Nt and Pt. Similarly, soil δ15N logarithmically increased with MBC and MBN, while soil δ13C linearly decreased with MBC and MBN. Soil δ15N linearly increased with plant belowground biomass, but soil δ13C decreased with plant belowground biomass. Multiple linear regressions showed that MBC especially, but also Nt to a lesser extent, affect soil δ15N, while MAT and Pt have more impact on soil δ13C. Thus, biotic factors controlled soil 15N signature most dominantly while climatic and edaphic factors exerted greater control on soil δ13C signature. These results indicate that soil C and N cycles are to some extent decoupled in these temperate grasslands. Further investigations should focus on those ecological processes leading to decoupling of C and N cycles in temperate grassland soils for a better understanding of SOC dynamics [12].

**Author Contributions:** X.C. and X.X. conceived and designed the experiments; X.Z. and F.W. performed the experiments; X.Z., M.L. and L.Z. analyzed the data; X.C. contributed reagents/materials/analysis tools; X.Z. wrote the paper; X.C., X.X., R.C. and I.G. contributed to interpret the results and improve the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Program of China (2016YFC0501802; 2017YFA0604802) and by the National Natural Science Foundation of China (41877089).

**Acknowledgments:** This study was supported by the National Key Research and Development Program of China (Grant No. 2016YFC0501802; 2017YFA0604802).

**Conflicts of Interest:** The authors declare no conflict of interest.
