Development of On-Farm Methods for Measuring Nitrogen Fixation

Legumes can utilise nitrogen (N) derived from symbiotic N2 fixation or from soil. The N derived from these 2 sources is transported in xylem sap from root to shoot in different forms. The products of N2 fixation were found to be exported from nodules as the ureides, allantoin and allantoic acid in many tropical forage species, and as amino acids in temperate pasture legumes. Assimilates from soil were transported from the roots predominantly as nitrate. Each of these different N compounds can be measured separately by simple colorimeti:ic test-tube reactions in a laboratory, or with chemical-impregnated paper strips in the field. It is possible to estimate a legume's dependence upon N2 fixation by analysing the xylem sap for its N compounds and relaling the relative solute composition to experimentally determined calibration curves. Sap analyses and 15N methods have given similar estimates of N fixation in several field trials,