Nitrogen isotopic composition of plants and soil in an arid mountainous terrain: sunny slope versus shady slope

Abstract

<p><strong>Abstract.</strong> Nitrogen cycling is tightly associated with environment. Sunny slope of a given mountain could significantly differ from shady slope in environment. Thus, N cycling should also be different between the two slopes. Since leaf δ<sup>15</sup>N, soil δ<sup>15</sup>N and △δ<sup>15</sup>N<sub>leaf-soil</sub> (△δ<sup>15</sup>N<sub>leaf-soil</sub> = leaf δ<sup>15</sup>N − soil δ<sup>15</sup>N) could reflect the N cycling characteristics, we put forward a hypothesis that leaf δ<sup>15</sup>N, soil δ<sup>15</sup>N and △δ<sup>15</sup>N<sub>leaf-soil</sub> should differ across the two slopes. However, such a comparative study between two slopes has never been conducted yet. In addition, environmental effects on leaf and soil δ<sup>15</sup>N derived from studies at global scale were often found to be different from that at regional scale. This led to our argument that environmental effects on leaf and soil δ<sup>15</sup>N could depend on local environment. To confirm our hypothesis and argument, we measured leaf and soil δ<sup>15</sup>N on the sunny and shady slopes of Mount Tianshan. Remarkable environment differences between the two slopes provided an ideal opportunity for our test. The study showed that leaf δ<sup>15</sup>N, soil δ<sup>15</sup>N and △δ<sup>15</sup>N<sub>leaf-soil</sub> on the sunny slope were greater than that on the shady slope although the difference in soil δ<sup>15</sup>N was not significant. The result confirmed our hypothesis and suggested that the sunny slope has higher soil N transformation rates and soil N availability than the shady slope. Besides, this study observed that the significant influential factors of leaf δ<sup>15</sup>N were temperature, precipitation, leaf N, leaf C / N and silt / clay ratio on the shady slope, whereas on the sunny slope only leaf C / N was related to leaf δ<sup>15</sup>N. The significant influential factors of soil δ<sup>15</sup>N were temperature, precipitation and silt / clay ratio on the shady slope, whereas on the sunny slope MAP and soil moisture exerted significant effects. Precipitation exerted contrary effects on soil δ<sup>15</sup>N between the two slopes. Thus, this study supported our argument that the relationships between leaf and soil δ<sup>15</sup>N and environmental factors are local-dependent.</p>