Abstract
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 Δδ15Nleaf-soil (Δδ15Nleaf-soil = leaf δ15N − soil δ15N) could reflect the N cycling characteristics, we put forward a hypothesis that leaf δ15N, soil δ15N and Δδ15Nleaf-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 δ15Nleaf-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.
Highlights
In a natural terrestrial ecosystem, nitrogen (N) is the most required element but is usually a key limiting resource for plants (Vitousek et al, 1997); studying N cycling is of vital importance
The variations in nitrogen isotope ratio (δ15N) in plants and soil are tightly associated with many biogeochemical processes, including N mineralization, ammonia volatilization, nitrification and denitrification (Högberg, 1997; Houlton et al, 2006)
3.1 Comparisons of δ15N in leaf and soil between the north and the south slopes In Tian Shan, for all species pooled together, the arithmetic mean of leaf δ15N was 0.5 ± 0.2 and 2.0 ± 0.2 ‰ for the plants grown on the north and south slopes, respectively
Summary
In a natural terrestrial ecosystem, nitrogen (N) is the most required element but is usually a key limiting resource for plants (Vitousek et al, 1997); studying N cycling is of vital importance. The variations in nitrogen isotope ratio (δ15N) in plants and soil are tightly associated with many biogeochemical processes, including N mineralization, ammonia volatilization, nitrification and denitrification (Högberg, 1997; Houlton et al, 2006). Mineralization produces available N, including ammonium and nitrate, which are the substrates for ammonia volatilization, nitrification and denitrification. During these processes, gaseous N loss is more likely to be depleted in 15N, which will cause the remaining N pool and subsequent plants to enrich 15N (Högberg, 1997). Nitrogen isotopes have been widely applied in studies of terrestrial ecosystem N cycling (Handley et al, 1999; Evans, 2001; Robinson, 2001; Hobbie and Colpaert, 2003; Houlton et al, 2007)
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