Using nitrogen isotope ratios to assess terrestrial ecosystems at regional and global scales

Abstract

Advances in technology have made extensive surveys of 15N natural abundances in terrestrial ecosystems feasible at the regional and even global scale within the last decade. To date, such surveys have included measurements of plant (typically foliage) or soil δ15N. Correlations between plant δ15N and measures of N saturation have been reported in regional surveys of both Europe and North America, with plant tissue δ15N values increasing under N saturating conditions. Global analyses have shown positive relationships between both soil and foliar δ15N values and mean annual temperature and negative relationships between δ15N values and precipitation. Several factors can drive variations in plant and soil 15N natural abundances, thereby presenting challenges to the use of 15N for inferring nitrogen cycling patterns across large scales. These include: (1) prior land-use, which may leave long-lasting (decades to centuries) imprints on soil and hence plant δ15N; (2) variability in δ15N among species within a given site, which can mask patterns across sites; (3) mycorrhizal associations, which may fractionate strongly under certain conditions; (4) effects of climate, especially precipitation regime, which may influence the value of and temporal variability in plant δ15N. Despite these complicating factors, δ15N surveys at large scales may provide insight into N cycling patterns and processes controlling these patterns in terrestrial ecosystems; maps of δ15N in precipitation may also provide a useful tool for interpreting some patterns in terrestrial ecosystem N dynamics. © 2010 Springer Science+Business Media B.V.