Climatic and edaphic controls over soil δ(15)N in temperate grassland of northern China: A PLS-PATH analysis

Identifying the impact path of climate and soil factors on soil δ(15)N is very crucial for better understanding the N turnover in soils and the integrated information about ecosystem N cycling. Many studies have showed that climate and soil variables influence the change of soil δ(15)N. However, most of the existing studies focused on the overall impact of factor on soil δ(15)N, without distinguishing between the direct and indirect effect. Although scholars have studied the relationships among temperature, precipitation, soil N, soil pH, and soil δ(15)N rather than estimating all the causal relationships simultaneously. To answer the above-mentioned questions, a regional-scale soil collection was conducted across a temperate grassland in northern China. Meanwhile, a PLS-PATH analysis was utilized to evaluate the direct and indirect effects of various factors on soil δ(15)N and to explore the causal relationships among variables. The results showed that along the transect, mean annual precipitation (MAP) and mean annual temperature (MAT) directly and significantly reduced soil δ(15)N, and indirectly affected soil δ(15)N through their effects on soil pH, soil clay, soil N and soil C/N. Soil C/N ratio has a significant direct impact on soil δ(15)N with a negative correlation. Soil clay, soil N content, and soil pH have a total positive effect on soil δ(15)N, but the total positive impact of soil pH is very weak because it has a negative indirect impact on soil δ(15)N by affecting soil clay, soil N and soil C/N ratio. The total influence is, in order, MAP > MAT > soil C/N > soil clay > soil N > soil pH (in absolute value). The above results will provide valuable information about ecosystem N cycle in temperate grassland of northern China.