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

摘要

The natural abundance of (15)N (delta(15)N) 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(15)N values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and delta(15)N 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(15)N patterns and climate factors (air temperature and precipitation) indirectly affected foliar delta(15)N patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO (3) (-) -N, whereas foliar delta(15)N was most closely associated with the delta(15)N of soil NH (4) (+) , the most abundant form of N in soil solution. (15)N enrichment in surface mineral soil in high elevation forests was mainly attributed to (15)N-enriched organic N accumulation. Furthermore, the foliar enrichment factor (epsilon(p/s) = I '(15)N(foliage)-I '(15)N(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(15)N 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(15)N patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil delta(15)N within and across forest ecosystems.