Nitrogen isotopic composition of plants and soil in an arid mountainous terrain: south slope versus north slope

Abstract. Nitrogen cycling is tightly associated with environment. The south slope of a given mountain could significantly differ from north slope in environment. Thus, N cycling should also be different between the two slopes. Since leaf δ¹⁵N, soil δ¹⁵N and Δδ¹⁵N_leaf-soil (Δδ¹⁵N_leaf-soil =  leaf δ¹⁵N − soil δ¹⁵N) could reflect the N cycling characteristics, we put forward a hypothesis that leaf δ¹⁵N, soil δ¹⁵N and Δδ¹⁵N_leaf-soil should differ between the two slopes. However, such a comparative study between two slopes has never been conducted. In addition, environmental effects on leaf and soil δ¹⁵N derived from studies at global scale were often found to be different from those on a regional scale. This led to our argument that environmental effects on leaf and soil δ¹⁵N could depend on local environment. To confirm our hypothesis and argument, we measured leaf and soil δ¹⁵N on the south and north slopes of Tian Shan. Remarkable environmental differences between the two slopes provided an ideal opportunity for our test. The study showed that leaf δ¹⁵N, soil δ¹⁵N and δ¹⁵N_leaf-soil on the south slope were greater than those on the north slope, although the difference in soil δ¹⁵N was not significant. The result confirmed our hypothesis and suggested that the south slope has higher soil N transformation rates and soil N availability than the north slope. In addition, in this study it was observed that the significant influential factors of leaf δ¹⁵N were temperature, precipitation, leaf N, leaf C ∕ N, soil moisture and silt ∕ clay ratio on the north slope, whereas on the south slope only leaf C ∕ N was related to leaf δ¹⁵N. The significant influential factors of soil δ¹⁵N were temperature, precipitation, soil moisture and silt ∕ clay ratio on the north slope, whereas on the south slope, mean annual precipitation and soil moisture exerted significant effects. Precipitation exerted contrary effects on soil δ¹⁵N between the two slopes. Thus, this study supported our argument that the relationships between leaf and soil δ¹⁵N and environmental factors are localized.