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

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 ¹⁵N 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 ¹⁵N 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%.

Key Words: Symbiotic N-fixation • Winter annual legumes • Trifolium subterraneum L. • Bromus mollis L.

² Professor of agronomy and range science, Davis; agronomist, Univ. of Calif., Hopland Field Station, Hopland; and professor of geobiology, Davis.

Received for publication February 7, 1977.

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