Hydrogen (H, D with mass ratio 2:1) and nitrogen (14N, 15N with mass ratio 15:14) isotopes are powerful tracers of water sources and biological processes respectively. Hydrogen isotopes (delta-D, measured relative to VSMOW) co-vary with delta-18O in precipitation along the Global Meteoric Water Line (delta-D = 8 * delta-18O + 10), making them complementary hydrological tracers. The large mass difference (100%) produces the largest fractionation effects of any stable isotope system. Nitrogen isotopes (delta-15N, relative to atmospheric N2) trace biological nitrogen cycling: nitrogen fixation, nitrification, denitrification, and trophic-level enrichment each produce diagnostic fractionation patterns. Together, delta-D and delta-15N address questions from paleoclimate reconstruction to pollution source tracking.
Hydrogen and nitrogen isotope systems each occupy distinct niches in geochemistry. Hydrogen isotopes are primarily hydrological tracers, while nitrogen isotopes are primarily biological tracers. Together with oxygen and carbon isotopes, they form the core light stable isotope toolkit.
The hydrogen isotope system has the largest relative mass difference of any stable pair (D/H, mass ratio 2.0). This produces enormous fractionation effects: delta-D of precipitation ranges from ~0 per mil in the tropics to -400 per mil in central Antarctica. The Global Meteoric Water Line (GMWL), defined as delta-D = 8 * delta-18O + 10, reflects the equilibrium fractionation of both isotopes during condensation. The slope of 8 arises from the ratio of equilibrium fractionation factors for D/H and 18O/16O during condensation. Deviations from this line diagnose secondary processes: evaporation (slope <8), kinetic effects during snow formation (deuterium excess > 10), and water-rock interaction at high temperature (exchange shifts delta-18O but not delta-D in rocks with little hydrogen).
Nitrogen isotopes trace the biogeochemical nitrogen cycle -- one of the most complex and environmentally important element cycles. Atmospheric N2 is the reference standard (delta-15N = 0 per mil by definition). Biological nitrogen fixation produces organic N with delta-15N near 0. Nitrification (NH4+ to NO3-) and denitrification (NO3- to N2) both strongly fractionate nitrogen, with the residual substrate becoming enriched in 15N. In marine systems, water column denitrification in oxygen minimum zones produces NO3- enriched in 15N, which is propagated through the food web and recorded in sedimentary organic nitrogen -- providing a proxy for past ocean oxygenation.
Applied uses span environmental forensics (pollution source identification), paleoecology (trophic structure reconstruction from bone collagen delta-15N), paleoclimate (delta-D in ice cores and leaf waxes as temperature and moisture source proxies), and hydrology (using delta-D and delta-18O to identify groundwater recharge sources and mixing). The increasing precision of compound-specific isotope analysis -- measuring delta-D and delta-15N on individual organic molecules -- is extending these tools to ever more specific process tracers.