Measuring Symbiotic Nitrogen Fixation by Legumes

This chapter provides a brief overview of the methodologies that have been used to quantify N2 fixation of legumes and then focuses in more detail on the particular ones that people believe are likely to be of most value for quantifying N2 fixation by field-grown legumes. It is possible to quantify legume N2 fixation by simply measuring the total amount of N accumulated during a given period of time, provided the plants are grown in N-free rooting medium such as coarse sand under controlled conditions and are supplied with all essential nutrients except combined N. The chapter describes many of the methodologies to separate legume total N into two components: that fraction of plant N taken up from the soil and that derived from the fixation of atmospheric N2. Other approaches attempt to quantify N2 fixation by measuring the rate of activity of nitrogenase, the N2-fixing enzyme in root nodules.