Clover N‐Fixation Measurement by Total‐N Difference and ¹⁵N A‐Values in Lysimeters¹

Abstract

Abstract Assessment and improvement of N fixation by legumes on annual‐type range is becoming increasingly important in light of the increasing need for energy conservation. A comparison was made of two methods of assessing N fixation: a simple but crude method by measuring the total‐N difference in tops of an N‐fixing legume and a non N‐fixing grass, and a more expensive method using A‐values determined from 15 N applications to the same legume and grass species. Subterranean clover ( Trifolium subterraneum L.) and soft chess grass ( Bromus molis L.) were grown in field lysimeters (69 cm deep by 36 cm diam.) filled with Josephine loam (Typic Haplozerults, fine loamy, mixed mesic) for 3 years. Single (100 and 500 kg/ha) and repeated (100 kg/ha) applications of 15 N were made and determinations were made of soil and fertilizer N in the plants, soil, and leachate periodically. The relation between the two methods was strongly linear; fixation (kg/ha) by the A‐value method = 50.1 + 0.852 N fixation by difference (r = 0.98, n = 15). Consequently, previous values cited for N fixation measured by the difference method for winter annual legumes in California may have been underestimated by about 40%.