Wednesday, November 15, 2006

Nitrogen Fixation and N Fertilization in Soybeans: An Analysis of Published Results.

Fernando Salvagiotti and Achim Dobermann. Univ of Nebraska, 243 Keim Hall, Lincoln, NE 68503

Soybean growth and seed yield (SY) are closely related to nitrogen (N) which is provided by soil N sources, fertilizer, and N2 fixation. In order to achieve yield potential, soybeans must maintain high seed N concentrations while sustaining high photosynthesis rates. We summarized differences in N uptake and N2 fixation by soybean as affected by environments, genotypes or management, based on data published in scientific journals from 1966 to 2004 and experiments conducted at the University of Nebraska-Lincoln. Most frequent protein content ranged from 37 to 42% and N harvest index averaged 0.8. The slope of the linear regression between SY and N uptake was 11 kg of SY per kg of N uptake (r2 = 0.86, N = 412; P<0.01). Most frequent values of N2 fixation ranged from 61 to 151 kg N ha-1(36 to 61% of total N uptake), with an overall range of 0 to 337 kg ha-1. Nitrogen fixation increased as N uptake rose, reaching a maximum at 300-350 kg ha-1. The strength of the relationship between SY and fixed N decreased when N fertilizer was applied. An inverse non-linear relationship between N fertilizer rate and N fixation was observed if fertilizer was applied at planting in the top layer of soil. However, experiments where N was placed deeper or applied in reproductive stages did not follow this trend. Significative response to N fertilizer was recorded in lower-yielding environments (below 3500 kg ha-1) in which the crop had constraints for growth, as well as in higher-yielding environments, probably due to a higher N demand. Seed yield above the 75% percentile (3320 kg ha-1), had 74% and 81% higher N uptake and N fixation. The lack of data above this value, made difficult the prediction of crop behavior and the role of N2 fixation in high-yielding soybeans.