Simplified methods for assessing quantities of N2 fixed by Lupinus angustifolius L. and its benefits to soil nitrogen status

Abstract

Procedures for assessing the quantity of symbiotically fixed nitrogen (kg N/ha) in standing crops of lupin and for estimating variation of N₂ fixation by lupins in different years were determined empirically and described. In standing crops, N₂ fixation was estimated from crop height, plant population density, and a bioassay of soil mineral nitrogen (cereal crop N; kg N/ha). In addition it was also estimated from rainfall, sowing date, and cereal N, which consequently enabled prediction of seasonal variation in fixed N using historical rainfall data. Procedures for estimating the potential contribution of N₂ fixation to soil N, and the effects of lupin and cereal N budgets on soil N balance based on differences in fixed N and grain N (grain yield×estimated grain N concentration) are also given. The collective procedures are applied to a site in south-eastern Australia and the predicted crop effects on soil N balance compared with actual field data. Perceived limitations of the procedures are discussed.