ABSTRACT
Biological nitrogen fixation (BNF) of pulse crops can be quantified by methods such as acetylene reduction assay, nodule observations, xylem sap analysis, total nitrogen (N) difference, and N stable isotopic methods. This chapter aims to highlight the role of nitrogen stable isotopic techniques to quantify BNF by pulse crops. N isotopic dilution and N natural abundance are widely used to estimate BNF. The principle of these techniques is that N2-fixing plants such as pulses have different amounts of N in their tissues than non-fixing plants. Increasing emphasis on the use of pulse crops to provide food protein, enhance the adaptation of agricultural systems to drought and reduce the reliance of fertilizer N for cereal crops in pulse-based cropping rotations highlights the need to quantify BNF by pulse crops using N isotopic techniques. The xylem sap technique can be used to measure BNF for plant species that produce significant quantities of ureide in plant sap as a product of N2 fixation.
