Influence of Organic Nitrogen on Soil Nitrogen, Nodulation, Nitrogen Fixation, and Yield of Soybeans

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Abstract
Nitrogen was applied on a previous rye (Secale Cereale L.) cover crop for later determination of the effect of organic N on soybean (Glycine max L.) N2‐fixation rate, plant dry matter, N uptake, and yield. Nitrogen was applied at 0, 56, 112, and 224 kg N/ha as NH4NO3 or urea either on rye in mid‐March or at soybean planting time in a 4 × 2 × 2 factorial design. In both years of study, N2‐fixation rate as estimated from acetylene reduction was reduced throughout the season by increased fertilization rate. For 1 year, interactions showed that N2‐fixation rate and nodule mass were stimulated late in the season from increased N fertilizer applied to the rye but not at soybean planting. Interactions between NH4NO3 and urea and time of application showed that NH4NO3 as compared with urea applied on rye significantly increased soybean dry matter, N uptake, and yield. Urea applied at planting increased these parameters in comparison with NH4NO3. The observed increases in dry matter, N uptake and yield from rye‐applied NH4NO3 were not accompanied by a corresponding increase in N2‐fixation rate or nodule mass. This would suggest that the N2 fixing system was not responsible for the increase of dry matter, N uptake, and yield. Soil organic N was not measurably influenced by the treatments imposed.

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