CARBON AND NITROGEN CYCLES IN THE SOIL 2og 



about by " frische Conferven, wie sie so haufig in stehendem 

 Wasser vorkommen " ; after lo to 15 minutes' boiling, how- 

 ever, the property was lost Meusel in 1875 (19S) showed 

 that it was bacterial, and could be stopped by antiseptics. 

 The property appears to be generally possessed by bacteria 

 and was shown by no fewer than 85 out of 109 kinds investi- 

 gated by Maassen (184). 



The formation of gaseous products is effected by a smaller 

 but still considerable number of organisms ; these were first 

 investigated by Gayon and Dupetit (103), and by Deherain 

 and Maquenne (8i<2). 



The physiological significance of the reduction appears to 

 be that nitrates can supply oxygen to the organisms when 

 free oxygen is no longer obtainable. It is not simply a re- 

 action between the organism and the nitrate : easily oxidisable 

 organic matter must be present at the same time. The 

 partially decomposed organic matter of the soil — the " humus " 

 — does not seem to be very serviceable (274). 



There is a very sharp contrast between the bacterial pro- 

 duction and the bacterial destruction of nitrates. Nitrate 

 production is the work of one organism only at each stage, 

 and the end result is a single product quantitatively equivalent 

 to the original ammonia ; no single chemical process oxidises 

 ammonia in this complete manner. The bacterial reduction 

 of nitrates, on the other hand, gives no single product, but a 

 number of products not in any simple ratio, whilst the chemical 

 reduction can readily be made to go quantitatively to ammonia. 



Whether denitrification goes on to any extent in properly 

 drained agricultural soils is doubtful, because the three essential 

 conditions, lack of air, presence of much easily decomposable 

 organic matter and of nitrate are rarely obtained. In 1895 

 Wagner and Maercker startled the agricultural world by an- 

 nouncing that unrotted dung destroys the nitrates in the soil 

 and reduces the crop yield (291). Their experiments were 

 criticised by Warington (297) who pointed out that their dress- 

 ings of dung were enormous and their results would not apply 

 to ordinary farm practice. But it may well occur in rich soils 



14 



