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Environmental Implications of N Fixation by Soybeans¹

ABSTRACT

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.

Key Words: fertilizer N • soil N • symbiotic fixation • net N removal

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana, IL 61801. Financial support for this study was provided by the Rockefeller Foundation and the Illinois Agricultural Experiment Station. Cooperation from the Division of Agricultural Development, Tennessee Valley Authority is gratefully acknowledged.

² Associate Agronomist and Professors of Agronomy, University of Illinois. The present address of the senior author is Ohio State University, Columbus, Ohio.

Received for publication August 15, 1974.

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