COS 111-8
Global foliar and soil nitrogen isotope patterns and the global N cycle convergence hypothesis
Describing patterns of terrestrial nitrogen cycling across global environmental gradients has relied heavily on interpretations of the natural abundance isotopic composition of N in plants and soils. Initial interpretations of patterns of N isotopes supported the broad conclusion of greater N availability and proportion of N lost from ecosystems in gaseous forms in hot, dry ecosystems. Yet, when the basic patterns of N cycling at the global scale were laid out, N isotope data were relatively sparse and there was little accounting for important covariates. In order to better understand global patterns of N cycling, I present the results of separate global syntheses of foliar and soil N isotopic composition (δ15N).
Results/Conclusions
Patterns of foliar δ15N suggested that at local scales, plants grown in soils with greater N availability were enriched in 15N. At the global scale, hot, dry sites had the highest δ15N, which supported the idea that these soils had the highest N availability. Analyses of soil δ15N suggests something different. Soil organic matter from hot, dry ecosystems did have high δ15N. But, the soil organic matter in these ecosystems had been processed more by microbes (lower C and N concentrations), which allows for more enrichment. In addition, hot, dry ecosystems also tend to have higher clay concentrations, which preferentially retains 15N-enriched organic matter. When the degree of microbial processing and clay concentrations are accounted for as covariates, there SOM δ15N is invariate across global climate gradients. Put another way, if the soil organic matter in a cold, wet site was allowed to decompose to the same degree as in a hot, dry site with the same clay content, they would have the same 15N signatures. The most parsimonious explanation for these patterns is that the proportion of N lost to fractionating pathways, such as gaseous pathways, is invariant across global climate gradients.