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

摘要

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 iδ/isup15/supN, soil iδ/isup15/supN and Δiδ/isup15/supNsubleaf-soil/sub (Δiδ/isup15/supNsubleaf-soil/sub?=??leaf iδ/isup15/supN ? soil iδ/isup15/supN) could reflect the N cycling characteristics, we put forward a hypothesis that leaf iδ/isup15/supN, soil iδ/isup15/supN and Δiδ/isup15/supNsubleaf-soil/sub 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 iδ/isup15/supN 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 iδ/isup15/supN could depend on local environment. To confirm our hypothesis and argument, we measured leaf and soil iδ/isup15/supN 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 iδ/isup15/supN, soil iδ/isup15/supN and iδ/isup15/supNsubleaf-soil/sub on the south slope were greater than those on the north slope, although the difference in soil iδ/isup15/supN 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 iδ/isup15/supN 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 iδ/isup15/supN. The significant influential factors of soil iδ/isup15/supN 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 iδ/isup15/supN between the two slopes. Thus, this study supported our argument that the relationships between leaf and soil iδ/isup15/supN and environmental factors are localized.