Nitrogen Nutrition in Soybean – Fertilization, Fixation, and a Paradox.

A primary advantage soybean and other legumes have over cereal grains is their symbiotic relationships with rhizobia, which eliminates the reliance upon nitrogen fertilizers in most situations. Soybean, in particular, is valued for the high protein concentration (35 to 40%) of grain. The importance of N2 fixation for soybean production varies greatly among different locations and environments depending upon the amount of inorganic nitrogen in the soil (mineralized and/or carried over from the previous crop), environmental conditions that impact N2 fixation, and the efficiency of the symbiotic partnership. Depending upon these factors, the proportion of nitrogen in soybean derived from N2 fixation ranges from about 10% to 90%. Previous research indicated that N2 fixation declined at beginning seedfill, but more recent research documents that N2 fixation continues at high rates until seedfill is near completion, provided that there is adequate soil moisture season long. Drought stress during seedfill, however, results in declines in nodule activity from which the crop is unable to recover. Inorganic and mineralized nitrogen in the soil inhibit nodule formation. Likewise, the addition of nitrogen fertilizer to a well-nodulated crop decreases the N2 fixation rate within hours. If nitrogen fertilizer decreases both nodulation and N2 fixation rates, the value of adding small amounts of supplemental nitrogen fertilizer (e.g., 100 kg N ha-1) is questioned as it may be necessary to completely replace the nitrogen required by the crop (e.g., ~450 kg N ha-1 for 7000 kg grain ha-1) to realize yield gains. Nevertheless, as yield levels in soybean continue to increase, and as yield-contest levels exceed 7000 kg ha-1, there has been an increased interest in nitrogen fertilization of soybean because of uncertainty about the capacity of N2 fixation to provide sufficient nitrogen at these high yield levels.