Nitrogen—Isotope‐Ratio Analysis

This chapter focuses on certain problems associated with the nontracer methods, on methods for analysis of samples low in N content, on the analysis of 15N-depleted vs. 15N-enriched samples, and on optical spectroscopy vs. mass spectroscopy for N isotope-ratio analysis. Nitrogen isotope-ratio analysis of gases that are evolved from soils as a result of denitrification, N02 - reactions, or other processes is accomplished after purifying the gas of interest from other gases. Isotope-ratio analysis of sample N often involves: conversion of sample N to NH4 +-N by means of acid digestion, oxidation of NH4 +-N to N2 and determination of the isotopic composition of the N2. In N tracer experiments, the N forms have N isotopic compositions substantially different from the isotopic composition of the total soil N. Conversion of NH4 +-N to N2 for mass spectrometer analysis usually is done by reacting NH4 +-N with alkaline sodium hypobromite in the absence of air.