PREAMBLE TO CHAPTER 5
W h e r e a s t h e u l t i m a t e source of carbon for all biological processes is the relatively small concentration of carbon dioxide of the atmosphere, most plants fail to u s e for their needs even t h e a b u n d a n t supplies of e l e m e n t a r y nitrogen i n t h e atmosphere b u t m u s t needs d r a w this essen- tial element from t h e u s u a l l y scanty concentration of n i t r a t e i n t h e aqueous media w i t h w h i c h t h e y a r e i n contact. Nevertheless, m o d e r n technology does m a k e available to crop plants relatively large a m o u n t s of erstwhile atmospheric nitrogen i n t h e form of chemically fixed products. A l t h o u g h t h e absolute a m o u n t s of nitrogen so applied are large in tonnage, t h e y represent, nevertheless, b u t a small percentage of the total nitrogenous t u r n o v e r i n n a t u r e w h i c h in fact can only be m a i n t a i n e d t h r o u g h t h e activities of soil microorganisms. T h e role of microorganisms i n p l a n t nutrition, including those t h a t reconvert plant a n d a n i m a l r e m a i n s to nitrate, receives consideration i n Chapter 6, especially from t h e standpoint of t h e conditions w h i c h prevail i n the i m m e d i a t e vicinity of root surfaces, i.e. i n the rhizosphere. C h a p t e r 5, however, selects for separate discussion those biological situations in w h i c h e l e m e n t a r y nitrogen is converted to the organic form. T h i s separate t r e a t m e n t is w a r r a n t e d alike b y t h e i m p a c t of biological nitro- gen fixation u p o n t h e cyclical t u r n o v e r of nitrogen in n a t u r e a n d b y the intrinsic interest w h i c h n o w attaches to the m e c h a n i s m b y w h i c h t h e living cells, or even some cell-free extracts, fix t h e e l e m e n t a r y nitrogen. T h i s chapter, then, is not only p e r t i n e n t to t h e over-all study of inorganic p l a n t nutrition, b u t it contributes also to knowledge of t h e subsequent fate of nitrogen in plants i n general a n d is t h u s p a r t of this major topic w h i c h will be discussed in V o l u m e I V u n d e r nitrogen metabolism of plants.
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