232 



METABOLISM 



of denitrification has received considerable attention. In spite of a voluminous 

 literature on the subject (compare Lemmermann, 1901, [Jensen, 1904]) the 

 phenomena of denitrification are in several important aspects still obscure. 

 We speak of denitrification when nitrate is transformed into nitrite, nitrite 

 into ammonia, or finally nitrate or nitrite into free nitrogen. Since we have 

 seen that nitrification, that is, the transformation of ammonia into nitric 

 acid, is a source of energy to the organism, the reverse process can take place 

 only by the expenditure of energy, as when free nitrogen is formed from nitrate 

 or nitrite. Therefore denitrification obviously cannot be made to rank along- 

 side of fermentation where gain of energy is the significant point. A certain 

 likeness between fermentation, or, more accurately, respiration, and nitrification, 

 is perhaps, however, observable. According to Jensen (1898, 1899) denitrification 

 occurs only in the absence of oxygen, and the denitrifying Bacteria are anaerobic 

 in the presence of nitrates; when no nitrates, on the other hand, are present they 

 are strongly aerobic. The significance of denitrification rests solely in the fact 

 that there is gain of respiratory oxygen from the nitrates, and this view is 

 supported by an observation of Maassen (1901), that substances rich in 

 oxygen, such as chlorates, are able to inhibit the decomposition of nitric 

 acid. Probably these chlorates act in the place of the nitrate, giving up their 

 oxygen, and thus protecting, so to speak, the nitrates. Still various investigators 

 have shown that denitrification is also possible in the presence of oxygen, and this 

 fact does not at first sight appear to support the conception of the process just 

 advanced. When we remember, however, that yeast is also incapable of 

 developing alcohol when oxygen is abundantly present, we must admit the 

 possibility of the existence of Bacteria which habitually split off oxygen from 

 nitrogenous compounds, even if it be at their disposal in a free state. It may 

 be concluded from Maassen's researches that certain Bacteria are always speci- 

 fically denitrifiers, while others develop such powers only under definite external 

 conditions. The number of the former type is apparently limited, while the 

 power of occasionally inducing denitrification appears to be widely distributed. 



Denitrification is naturally a process of fundamental interest and impor- 

 tance to the agriculturist. In agricultural operations, as we have seen already, 

 large quantities of nitrogen are removed from the land in harvesting, and 

 manuring with nitrogen becomes one of the most necessary conditions of suc- 

 cessful agriculture. If this nitrogen be presented in the form of nitrate of potash 

 denitrification must be most carefully guarded against. It is not necessary 

 for us, however, to go into this question more fully ; we need only remark that 

 the land would indeed be in a parlous state, were there no process in nature 

 by which denitrification could be compensated for. The reconstruction of 

 nitric acid out of ammonia and nitrites we have already studied in the previous 

 lecture, but the free nitrogen of the air, which, owing to its chemical inertness, 

 is of no service to the organism (compare Bunge, 1889) is by no means ex- 

 cluded from taking part in the circulation of material in living nature. The 

 combination of free nitrogen has been conclusively proved, and it is both 

 practically and theoretically a phenomenon of the very greatest importance. 



Recently J. Kuhn (1901) has provided us with a very interesting proof 

 of nitrogen combination in arable land. He was able to obtain out of a certain 

 field good and even increasing harvests after being sown for twenty successive 

 years with winter rye, without any nitrogenous manuring whatever. This 

 showed that more nitrogen was annually combined in the soil than was removed 

 in the process of harvesting ; and since the rye is itself incapable of bringing 

 about such a combination, and since further the amount of combined nitrogen 

 precipitated on the soil nothing like makes up for the loss by harvesting (com- 

 pare p. 136), obviously atmospheric nitrogen must have been combined to 

 a very large extent in the soil. We have again to thank Winogradsky for a 



