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

Abstract

Abstract As an initial effort to evaluate irrigated white lupin ( Lupinus albus L.) in California, we evaluated seed yield, N 2 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 N 2 fixation by the N difference method ranged from 193 to 247 kg ha −1 . Symbiotically 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 N 2 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 N 2 fixation exhibited by white lupin during the cool‐season are favorable attributes for a winter annual protein crop in a mediterranean climate.