CARBON AND NITROGEN CYCLES IN THE SOIL 193 



nitrogen during sewage purification though the published re- 

 sults show that it occurred. Later sewage workers recognised 

 the loss, and Letts, indeed, made measurements of the evolved 

 nitrogen, special gasimetric methods being devised for the 

 purpose.^ 



A serious attempt to grapple with the problem was made 

 in 1896 and 1897 at some of the German Experiment Stations, 

 notably Jena, in consequence of the request made by the 

 German Agricultural Society for an investigation into the 

 losses of nitrogen from farmyard manure. The first hypoth- 

 esis, set up by Wagner, was a reduction hypothesis to the effect 

 that nitrates are present in the manure, and decompose in ab- 

 sence of air, giving rise to nitrogen. This view, however, soon 

 proved to be erroneous, for it supposed that the loss only 

 occurred in absence of air, whereas, in point of fact, it only 

 occurs in presence of air. 



Immendorf (141a) recognised this fact and favoured the 

 oxidation hypothesis, considering that evolution of nitrogen 

 was the result of direct oxidation or combustion of the nitro- 

 gen compounds to gaseous nitrogen. No reduction of nitrate 

 was assumed. Pfeiffer and his assistants at Jena (224^) began 

 on the reduction hypothesis, and supposed that nitrates were 

 formed on the outside of the heap and then diffused inside, 

 where they were denitrified. But during the course of their 

 experiments they changed their view, and ended by accepting 

 the oxidation hypothesis. 



Two lots of cow manure (made with peat) were put up : 

 air was blown through one, and over the other. The losses 

 of nitrogen were : — 



Grams. Per Cent. 

 Air blown through .... 4*26 42*6 



„ „ over .... 276 27'6 



They argue that blowing air over the dung would be 

 favourable to their supposed nitrification and denitrification 

 process, while blowing air through would be unfavourable 



^ Letts, Fifth Report, Sewage Commission, Appendix 6, 171-194 ; also Report 

 to the Corporation of Belfast on the Purification of Belfast Sewage. 



13 



