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 (P_CO2), atmospheric N deposition, and foliar [N]. The spatiotemporal trends of foliar [N] were explained by MAT, temperature seasonality, P_CO2, 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.