Data from: Drivers of foliar 15N trends in southern China over the last century

Foliar stable nitrogen (N) isotopes (δ15N) generally reflect N
availability to plants and have been used to infer about changes thereof.
However, previous studies of temporal trends in foliar δ15N have ignored
the influence of confounding factors, leading to uncertainties on its
indication to N availability. In this study, we measured foliar δ15N of
1,811 herbarium specimens from 12 plant species collected in southern
China forests from 1920 to 2010. We explored how changes in atmospheric
CO2, N deposition and global warming have affected foliar δ15N and N
concentrations ([N]) and identified whether N availability decreased in
southern China. Across all species, foliar δ15N significantly decreased by
0.82‰ over the study period. However, foliar [N] did not decrease
significantly, implying N homeostasis in forest trees in the region. The
spatiotemporal patterns of foliar δ15N were explained by mean annual
temperature (MAT), atmospheric CO2 (PCO2), atmospheric N deposition, and
foliar [N]. The spatiotemporal trends of foliar [N] were explained by MAT,
temperature seasonality, PCO2, and N deposition. N deposition within the
rates from 5.3 – 12.6 kg N ha-1 yr-1 substantially contributed to the
temporal decline in foliar δ15N. The decline in foliar δ15N was not
accompanied by changes in foliar [N] and therefore does not necessarily
reflect a decline in N availability. This is important to understand
changes in N availability, which is essential to validate and parameterize
biogeochemical cycles of N.