**3. Results**

#### *3.1. Climatic, Edaphic and Biotic Factors along the Transect*

Along the transect, MAT ranged from 1.2 to 4.5 ◦C with a mean of 2.6 ◦C and MAP ranged from 155 to 518 mm with a mean of 308 mm (Table 1). SOC across 22 sites varied from 1.97 to 53.36 g kg−1, with the lowest value in temperate desert and the highest value in the temperate meadow steppe (Table 1). Nt ranged from 0.17 to 4.69 g kg−1, while Pt varied from 0.1 to 0.9 g kg−1. Soil C:N ratios were between 9.0 and 13.0. Soil pH varied from 7.1 to 8.3 (Table 1). MBC ranged from 71.8 to 660.5 mg kg−<sup>1</sup> and MBN varied from 46.5 to 392.4 mg kg−<sup>1</sup> (Table 1). Aboveground biomass was in the range of 11.6 to 146.6 g dry weight (d.w.) m<sup>−</sup>2, while belowground biomass varied from 50.9 to 785.6 g d.w. m<sup>−</sup><sup>2</sup> (Table 1). All observed parameters decreased from temperate meadow steppe to temperate steppe to temperate desert along the transect.

#### *3.2. Soil* δ*15N and* δ*13C*

Soil δ15N cross 22 sites along this transect varied from −3.53‰ to 5.88‰ (Table 1). All seven temperate meadow steppe sites had positive δ15N values while some temperate steppe and temperate desert sites had negative δ15N values. Soil δ15N in temperate meadow steppes (5.12‰) were higher than those in temperate steppes and temperate deserts (*P* < 0.05). By comparison, soil δ13C cross the grassland transect ranged from −27.1‰ to −21.2‰ (Table 1). Soil δ13C values in temperate meadow steppes (−25.7‰) were lower than those in temperate steppes and temperate deserts (*P* < 0.05).

#### *3.3. Correlation Climatic, Edaphic and Biotic Factors with Soil* δ*15N and* δ*13C*

Along the transect, soil δ15N were not correlated with MAP (Figure 1a) and MAT (Figure 1b). However, soil δ13C linearly decreased with MAP (*R*<sup>2</sup> = 0.43, *P* < 0.001, Figure 1c) and MAT (*R*<sup>2</sup> = 0.52, *P* < 0.001, Figure 1d).

**Figure 1.** Relationships between soil δ15N and δ13C in the upper 5 cm with mean annual precipitation (MAP, **a**, **c**) and mean annual temperature (MAT, **b**, **d**) at 22 sites along the vegetation transect across Inner Mongolian temperate grasslands.

Soil δ15N logarithmically increased with concentrations of SOC (*R*<sup>2</sup> = 0.71, *P* < 0.001, Figure 2a), Nt (*R*<sup>2</sup> = 0.71, *P* < 0.001, Figure 2b) and Pt (*R*<sup>2</sup> = 0.51, *P* < 0.001, Figure 2c). By comparison, soil δ13C linearly decreased with SOC (*R*<sup>2</sup> = 0.58, *P* < 0.001, Figure 2d), Nt (*R*<sup>2</sup> = 0.53, *P* < 0.001, Figure 2e) and Pt (*R*<sup>2</sup> = 0.37, *P* < 0.003, Figure 2f).

**Figure 2.** Relationships between soil δ15N and δ13C in the upper 5 cm with soil organic carbon (SOC, **a**, **d**), total nitrogen (N, **b**, **e**) and total phosphorus (P, **c**, **f**) at 22 sites along the vegetation transect across Inner Mongolian temperate grasslands.

Similarly to SOC and Nt, soil δ15N logarithmically increased with concentrations of MBC (*R*<sup>2</sup> = 0.63, *P* < 0.001, Figure 3a) and MBN (*R*<sup>2</sup> = 0.48, *P* < 0.001, Figure 3b). Soil δ13C linearly decreased with MBC (*R*<sup>2</sup> = 0.58, *P* < 0.001, Figure 3c) and MBN (*R*<sup>2</sup> = 0.58, *P* < 0.001, Figure 3d). Plant belowground biomass linearly increased with Soil δ15N (*R*<sup>2</sup> = 0.37, *P* = 0.004, Figure 3e), but decreased with soil δ13C (*R*<sup>2</sup> = 0.41, *P* = 0.002, Figure 3f). Soil δ15N decreased with soil δ13C along the transect (*R*<sup>2</sup> = 0.30, *P* = 0.008, Figure 4).

**Figure 3.** Relationships between soil δ15N and δ13C in the upper 5 cm with microbial biomass carbon (MBC, **a**, **c**) and microbial biomass nitrogen (MBN, **b**, **d**) as well as with plant belowground biomass (**<sup>e</sup>**, **f**) at 22 sites along the vegetation transect across Inner Mongolian temperate grasslands.

**Figure 4.** Relationships between soil δ15N and δ13C in the upper 5 cm at 22 sites along a vegetation transect across Inner Mongolian temperate grasslands.

The result of model averaging approach after model selection showed that MBC, total N, total P and SOC significantly positive related with soil δ15N. Microbial C, total P, and MAT were three most important factors which significantly negatively affected soil δ13C (Table 3).

**Table 3.** Relative importance and regression coefficients (given in parenthesis) of microbial biomass carbon (MBC), total nitrogen (N), mean annual precipitation (MAP), total phosphorous (P), soil organic carbon (SOC), and mean annual temperature (MAT) for determining soil 15N and 13C values. Values were derived through a model averaging approach. Factors with parameter values highlighted in bold significantly affected soil 15N and 13C (*P* < 0.05).

