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

Abstract

Foliar stable nitrogen (N) isotopes (δ¹⁵ N) generally reflect N availability to plants and have been used to infer about changes thereof. However, previous studies of temporal trends in foliar δ¹⁵ N have ignored the influence of confounding factors, leading to uncertainties on its indication to N availability. In this study, we measured foliar δ¹⁵ N 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 δ¹⁵ N and N concentrations ([N]) and identified whether N availability decreased in southern China. Across all species, foliar δ¹⁵ N 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 δ¹⁵ N were explained by mean annual temperature (MAT), atmospheric CO₂ ( $P_{{\mathrm{CO}}_2}$ ), atmospheric N deposition, and foliar [N]. The spatiotemporal trends of foliar [N] were explained by MAT, temperature seasonality, $P_{{\mathrm{CO}}_2}$ , 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 δ¹⁵ N. The decline in foliar δ¹⁵ N 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.

Keywords: atmospheric CO2global change; foliar nitrogen concentrations; foliar nitrogen isotopes; nitrogen availability; nitrogen deposition.