Table 1.
The general information of the sampled sites in the study area.
MAT and MAP in Table 1 are the abbreviations of mean annual temperature and mean annual precipitation, respectively (the same below). These climate data were the averages of observation data collected during a 30 year period (1987–2017). TMS, TTS and TD represent temperate meadow steppe, typical temperate steppe and temperate desert, respectively. The dominant soil types were from the “1:1,000,000 Soil Map of China in 2007”.
Table 2.
Statistical description and normality test of all the variables in the study transect.
The critical ratio is the skewness (or kurtosis) of each variable divided by its standard error.
Table 3.
Pearson’s correlation coefficients for climatic and edaphic variables in explaining soil δ15N.
Fig 1.
The PLS-PATH analysis was used to explore the direct and indirect effects of each variable on soil δ15N value, as well as the causal relationship between variables. The number adjacent to arrow is standardized path coefficients, showing the influence size of the relationship; Continuous and dotted black arrows denote positive and negative relationship, and green lines indicate no significant relationship. *, ** and *** represent significant differences at the levels of 0.05, 0.01 and 0.001, respectively.
Table 4.
The decomposition results of the path analysis.
Through PLS-PATH analysis, the total impact of each variable on soil δ15N is divided into direct and indirect effects. The negative sign indicates that the variable makes the soil δ15N decrease.
Fig 2.
Influence path of climate and soil factors on soil δ15N.
(A) The paths of MAP on soil δ15N. (B) The paths of MAT on soil δ15N. (C) The paths of soil clay on soil δ15N. (D) The paths of soil pH on soil δ15N. (E) The path of soil C: N on soil δ15N, yet no indirect effect of soil C: N ratio on soil δ15N was found. (F) The paths of soil N on soil δ15N.
Fig 3.
Changes of soil δ15N with various factors and the relationships between some factors.
The linear regression method was used to analyze how the soil δ15N changes with various environmental factors (MAP, MAT, soil N, soil C/N, pH, soil clay and longitude) and the relationships between some factors. (A) Change of soil δ15N with the MAP. (B) Change of soil δ15N with the soil N. (C) Change of soil pH with the MAP. (D) Change of soil δ15N with the MAT. (E) Change of the MAP with the longitude. (F) Change of the MAT with the longitude. (G) Change of soil δ15N with the MAT at the western part of the transect. (H) Change of soil δ15N with the soil clay. (I) Change of soil δ15N with the soil pH. (J) Change of soil δ15N with the soil C:N ratio.