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

The natural abundance of N-15 (delta N-15) 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 delta N-15 values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and delta N-15 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 delta N-15 patterns and climate factors (air temperature and precipitation) indirectly affected foliar delta N-15 patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO (3) (-) -N, whereas foliar delta N-15 was most closely associated with the delta N-15 of soil NH (4) (+) , the most abundant form of N in soil solution. N-15 enrichment in surface mineral soil in high elevation forests was mainly attributed to N-15-enriched organic N accumulation. Furthermore, the foliar enrichment factor (epsilon(p/s) = I ' N-15(foliage)-I ' N-15(soil)) was significantly correlated with N transformation and loss rates, and was negatively correlated with the ratio of NH (4) (+) to total inorganic N. These results suggest that foliar delta N-15 value and foliar N concentration together accurately reflect the N availability of forest ecosystems. Foliar epsilon(p/s) can act as an integrated proxy to reflect the status of N cycling within or across forest ecosystems. Soil nitrification and species' NH (4) (+) to NO (3) (-) uptake ratios are key processes controlling foliar delta N-15 patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil delta N-15 within and across forest ecosystems.