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Nitrogen (N) oligotrophication is increasing globally across terrestrial ecosystems and manifested in decreasing nitrogen concentration ([N]) and changes in the stable nitrogen isotope composition (δ15N) of foliage. Heterogeneity in plant nitrogen sources makes it challenging to detect the effects of N oligotrophication even at a small catchment scale with complex topography. Understanding the spatial and temporal variation of foliar δ15N and [N] at such a scale is required to develop useful ecological indicators and monitoring methods to support catchment management with a potential N oligotrophication problem. This study examined spatial and high-resolution temporal variation of foliar δ15N and [N] and their influencing factors in ten trees grouped by Eucalyptus and Acacia in a native forest vegetation catchment. Over 16 sampling campaigns within a 12-month period, foliar δ15N and [N] increased in Eucalyptus but were constant in the N2-fixing Acacia. The higher foliar [N] and δ15N in Acacia reflected its N2-fixation ability. Topographic flow accumulation area (NDVI) explained 46% (77%) of spatial variation in dry-season Eucalyptus foliar δ15N ([N]). For Eucalyptus, foliar δ15N was higher at the downslope than the upslope locations, but no hillslope location differences were observed for foliar [N]. These results suggest that in the non-N2-fixing Eucalyptus, seasonal water stress related nitrogen availability may be reflected in foliar δ15N rather than foliar [N]. As such, foliar δ15N of non-N2-fixing plants potentially is a more sensitive indicator of seasonal or topographical N availability than foliar [N].
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