Foliar and soil 15N natural abundances provide field evidence on nitrogen dynamics in temperate and boreal forest ecosystems

The natural abundance of 15N (δ15N) in plants and soils is an ideal tool for assessing ecosystem N dynamics. However, many of the mechanisms driving the variability of foliar and soil δ15N values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and δ15N values in leaves, bulk soils and soil mineral N as well as soil N turnover rates across four temperate and boreal forest ecosystems along a mountain transect. The results showed that plant species and soil properties directly controlled soil δ15N patterns and climate factors (air temperature and precipitation) indirectly affected foliar δ15N patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO3−-N, whereas foliar δ15N was most closely associated with the δ15N of soil NH4+, the most abundant form of N in soil solution. 15N enrichment in surface mineral soil in high elevation forests was mainly attributed to 15N-enriched organic N accumulation. Furthermore, the foliar enrichment factor (εp/s = δ15Nfoliage−δ15Nsoil) was significantly correlated with N transformation and loss rates, and was negatively correlated with the ratio of NH4+ to total inorganic N. These results suggest that foliar δ15N value and foliar N concentration together accurately reflect the N availability of forest ecosystems. Foliar εp/s can act as an integrated proxy to reflect the status of N cycling within or across forest ecosystems. Soil nitrification and species’ NH4+ to NO3− uptake ratios are key processes controlling foliar δ15N patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil δ15N within and across forest ecosystems.