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Yield, Dinitrogen Fixation, and Aboveground Nitrogen Balance of Irrigated White Lupin in a Mediterranean Climate

Dep. of Agronomy and Range Science, Univ of California, Davis, CA 95616

^* Corresponding author.

As an initial effort to evaluate irrigated white lupin (Lupinus albus L.) in California, we evaluated seed yield, N₂ fixation, and the N economy of this crop in a 2-yr field study. Treatments were arranged in a completely randomized block design and included two (Year 1) or three (Year 2) lupin cultivars each either (i) inoculated with Bradyrhizobium sp. (Lupinus) and no N fertilization, (ii) with neither inoculation nor N fertilization, or (iii) with N applied at 200 kg N ha'1 (Year 1) or 275 kg N ha'1 (Year 2). Mean seed yield from inoculated treatments was 5.73 Mg ha^–1 with 46.4 g N kg^–1 in Year 1, and 4.79 Mg ha^–1 with 48.9 g N kg^–1 in Year 2. Seed yield was similar with or without inoculation, but total dry matter and N accumulation were highest in inoculated treatments. Nitrogen fertilization did not increase yield or N accumulation above that of inoculated treatments. Estimates of N₂ fixation by the N difference method ranged from 193 to 247 kg ha–1. Symbolically fixed N represented 67 to 78% of total aboveground plant N, but the N harvest index ranged from 0.80 to 0.91, and thus, total seed N exceeded the quantity of N₂ fixed by 23 to 68 kg N ha^–1. Despite the lack of a net increase in soil N after seed removal, the yield potential and N² fixation exhibited by white lupin during the cool-season are favorable attributes for a winter annual protein crop in a mediterranean climate.

Received for publication June 15, 1988.