58 NITROGEN METABOLISM 



have been seriously challenged [23]. The addition of haemo- 

 globin increases the respiration of i^feo^mw, but this appears 

 to be an indirect effect [13] and not connected with the 

 transport of O2 . 



It will be evident that there are still many aspects of 

 nitrogen fixation to be explored, and apart from their bio- 

 chemical interest, their economic importance should not be 

 underestimated. The growth of plants, the leaching effects of 

 rain-water and the activities of denitrifying organisms all 

 tend to remove from the soil the nitrogenous compounds 

 which are essential to the continued existence of most forms 

 of plant life. This loss is in part restored naturally by the 

 decomposition of plants and animals, and artificially by the 

 application of inorganic or organic fertilizers. Fixed nitrogen 

 compounds produced commercially probably account for no 

 more than 15% of the nitrogen returned annually to the soil. 

 By far the greatest proportion is due to Na-fixation by bio- 

 logical agents, and it has been estimated that symbiotic 

 systems and free-living organisms are responsible respectively 

 for returning to the soil 5-46x106 and 4-37x10^ tons of 

 nitrogen per year, yet, even allowing for this, there appears 

 to be an annual overall loss in soil nitrogen [42]. 



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