OOS 34-1
Nitrogen isotopes of the biosphere: Excess N and the global N balance
Nitrogen (N) is simultaneously a key component of biomolecules (DNA, protein) and a carrier of electrons for energy – the quintessential biological resource. Ultimately, the dualistic role of N in organism growth and energetics is manifest in a highly dynamic global N balance. Here we examine controls on the N balance via understanding and application of natural stable isotopes. This approach centers on measurements of N in all phases – gaseous, dissolved and solid – and application of kinetic principles acting on enzymes that bear out on patterns that occur in ecosystems, biomes, and global N pools. Two global aggregate models (one natural, one cropland) are used along with a spatially-explicit isotope model to identify source areas of N loss pathways, with key uncertainties and future directions critically discussed.
Results/Conclusions
Globally, isotope mass-balance of residual N in the terrestrial biosphere reveals that approximately 30% of the N balance occurs via gaseous loss pathways, or that the majority of N is lost via leaching from the land. This points to constraints on soil denitrifying bacteria and their capacity to consume terrestrial N inputs; leaching of N is in excess of gaseous N production from the soil on average. Isotopic modeling points to the highest fractions of gaseous N losses in arid ecosystems, followed by tropical biomes and high latitude ecosystems. The N isotope balance of global croplands reveals a much lower gaseous N proportion (~10% of total N losses) than the natural biosphere; most of the N from croplands is lost via harvest (~50%). Further, in terms of denitrification/N leaching ratios, croplands export more N via leaching than gaseous losses compared to natural ecosystems. Nevertheless, whether natural or managed sites, N isotope composition reveals widespread constraint to denitrification globally.