Uptake of Soil Nitrogen by Soybean as Influenced by Symbiotic N₂-Fixation or Fertilizer Nitrogen Supply

Nodulated and N2-fixing soybean plants (Glycine max. L.) grown in a pot experiment took up significantly more soil N (labelled with 15N) than non-nodulated control plants. The organic matter in the experimental soil was labelled with 15N during a previous incubation, and the pool of labelled inorganic N originated mainly from mineralization of organic matter.

Addition of non-labelled ammonium or nitrate-N to non-nodulated plants did not increase their uptake of labelled soil N. Plants grown with the various N-sources exhausted the soil for KCl-extractable N to almost the same low concentration. Where non-nodulated plants were grown, 60–75% of the inorganic N initially present could be accounted for in plants and KCl-extracts at harvest.

An amount corresponding to 98% of the KCl-extractable N initially present was found in nodulated plants and the pool of KCl-extractable N.