Environmental Implications of N Fixation by Soybeans

The objective was to measure inorganic N removal from soil by soybeans (Glycine max L.) grown at different N levels. Nodulating and non‐nodulating isolines of ‘Clark’ soybeans were planted on Flanagan silt loam in 1972 and 1973. Fertilizer rates of 0, 112, 224, and 448 kg N/ha as Ca(NO3)2 enriched with 1% 15N were applied. Symbiotically fixed N decreased from 48 to 10% of the total N in the above‐ground plant as applied fertilizer N increased from 0 to 448 kg/ha. Net removals of N by soybean grain were 61, 109, 135, and 149 kg/ha at 0, 112, 224, and 448 kg fertilizer N/ha rates, respectively. Corn (Zea mays L.) grain grown on adjacent plots removed 62, 123, 129, and 133 kg N/ha at fertilizer N rates of 0, 112, 224, and 336 kg/ha, respectively. The study indicates that soybeans are good scavengers for inorganic N in soils. Inclusion of net N removal by soybeans in the calculated N removed by nonleguminous crops in Illinois from 1945 to 1973 increases net crop N removal by approximately 37%. Added fertilizer N has exceeded the N removed by the harvested portion of Illinois crops in only 5 years since 1945, all 5 of these occurring after 1965.