PLANT NUTRITION 255 



Symbiotic nitrogen fixation. It has been known lor a long period 

 oC time that legumes possess the power of enriching the soil with 

 nitrogen, and that this nitrogen is obtained from the atmosphere 

 through the agency of bacteria living symbiotically upon the legumes. 

 Symbiosis, as applied to nitrogen fixation, is derived from the Greek 

 word for "living together." The bacteria, which belong to the genus 

 Rhizobhim, live in the root cells of the legume host, where their 

 metabolic activity causes a swelling or nodule to form on the root. For 

 this reason the organisms are commonly called root-nodule bacteria. 



One of the facts established concerning the manner in which 

 nitrogen is fixed is that the nodules excrete nitrogen chiefly as aspartic 

 acid, glutamic acid, and ^-alanine. Aspartic acid alone constitutes over 

 half of the total in young cultures. As the culture ages the percentage 

 of aspartic acid liberated decreases, while the amount of ^S-alanine 

 increases. From this fact one can infer that aspartic acid is a primary 

 fixation product, whereas /3-alanine is formed by the decarboxylation 

 of the aspartic acid. 



It has also been suggested that hydroxylamine is one of the products 

 formed, and that carbohydrate metabolism is closely related to nitro- 

 gen fixation. On the basis of these three facts a theory has been ad- 

 vanced which postulates that hydroxylamine units with oxaloacetic 

 acid to form an oxime which is then reduced to aspartic acid. 



According to this theory the mechanism of biological nitrogen fixa- 

 tion would be as follows: 



N2 — > unknown intermediates — > NHoQH 



hydroxylamine 

 + 



Carbohydrates -^ GOGH • GO • GH. • GOOH 



oxaloacetic acid 



. . . . (reduction) 



Oxmimosuccmic acid > 



GOOH 



I 

 C=NOH 



I 

 GH2 



I 

 GOOH 



oximino- 

 succinic acid 



GOOH 



GHNH., 



1 

 GH, 



1 

 GOOH 



L-aspartic 

 acid 



Since symbiotic nitrogen-fixing organisms are aerobic in character, 

 a lack of oxygen inhibits nodule formation. The fact that nodules are 

 formed in larger numbers near the surface of the soil is attributed to 



