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

Foliar stable nitrogen (N) isotopes (delta N-15) generally reflect N availability to plants and have been used to infer about changes thereof. However, previous studies of temporal trends in foliar delta N-15 have ignored the influence of confounding factors, leading to uncertainties on its indication to N availability. In this study, we measured foliar delta N-15 of 1811 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 delta N-15 and N concentrations ([N]) and identified whether N availability decreased in southern China. Across all species, foliar delta N-15 significantly decreased by 0.82 parts per thousand 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 delta N-15 were explained by mean annual temperature (MAT), atmospheric CO2 (PCO2$$ {\mathrm{P}}_{{\mathrm{CO}}_2} $$), atmospheric N deposition, and foliar [N]. The spatiotemporal trends of foliar [N] were explained by MAT, temperature seasonality, PCO2$$ {\mathrm{P}}_{{\mathrm{CO}}_2} $$, and N deposition. N deposition within the rates from 5.3 to 12.6 kg N ha(-1) year(-1) substantially contributed to the temporal decline in foliar delta N-15. The decline in foliar delta N-15 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.

Automated summary

The decrease in foliar delta N-15 does not necessarily indicate a decline in N availability, as foliar [N] remains constant in the studied plant species from southern China.