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

Identifying the impact path of climate and soil factors on soil delta N-15 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 delta N-15. However, most of the existing studies focused on the overall impact of factor on soil delta N-15, without distinguishing between the direct and indirect effect. Although scholars have studied the relationships among temperature, precipitation, soil N, soil pH, and soil delta N-15 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 delta N-15 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 delta N-15, and indirectly affected soil delta N-15 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 delta N-15 with a negative correlation. Soil clay, soil N content, and soil pH have a total positive effect on soil delta N-15, but the total positive impact of soil pH is very weak because it has a negative indirect impact on soil delta N-15 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.