Nitrogen Management in Soybean: Biological N Fixation, N Fertilization or Both?.

Although relationships among soybean (Glycine max [L.] Merr) seed yield, nitrogen (N) uptake, biological N2 fixation (BNF), and response to N fertilization have received considerable coverage in the scientific literature, a comprehensive summary and interpretation of these interactions with specific emphasis on high yield environments is lacking. Six hundred and thirty-seven data sets (site–year–treatment combinations) were analyzed from field studies that had examined these variables and had been published in refereed journals from 1966 to 2006. A mean linear increase of 0.013 Mg soybean seed yield per kg increase in N accumulation in aboveground biomass was evident in these data. The lower (maximumN accumulation) and upper (maximumN dilution) boundaries for this relationship had slopes of 0.0064 and 0.0188 Mg grain kg_1 N, respectively. On an average, 50–60% of soybean N demand was met by biological N2 fixation.  The gap between crop N uptake and N supplied by BNF tended to increase at higher seed yields for which the associated crop N demand is higher. Soybean yield was more likely to respond to N fertilization in high-yield (>4.5 Mg ha_1) environments. A negative exponential relationship was observed between N fertilizer rate and N2 fixation when N was applied on the surface or incorporated in the topmost soil layers. Deep placement of slow-release fertilizer below the nodulation zone, or late N applications during reproductive stages, may be promising alternatives for achieving a yield response to N fertilization in high-yielding environments. The results from many N fertilization studies are often confounded by insufficiently optimized BNF or other management factors that may have precluded achieving BNF-mediated yields near the yield potential ceiling. More studies will be needed to fully understand the extent to which the N requirements of soybean grown at potential yields levels can be met by optimizing BNF alone as opposed to supplementing BNF with applied N.