Nitrogen-15 signals of leaf-litter-soil continuum as a possible indicator of ecosystem nitrogen saturation by forest succession and N loads

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of ¹⁵N (δ¹⁵N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N₂O emission and inorganic N leaching as well as N concentration and δ¹⁵N of leaves, litters and soils. We found that foliar N concentration and δ¹⁵N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar δ¹⁵N (p < 0.05). Also, N addition decreased the δ¹⁵N of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests (p < 0.05). In addition, the soil N₂O emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the ¹⁵N enrichment factor (ε_p/s) of forest ecosystems. Our study indicates that δ¹⁵N of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future.