CARBON AND NITROGEN CYCLES IN THE SOIL 177 



gested an interesting distinction between high and low 

 potentials in oxidation which may prove of value in in- 

 vestigations. 



Stoklasa and Ernest {2710} found about 0*05 grpi. CO^ 

 evolved per kg. of soil in 24 hours at 20'' C. under aerobic 

 conditions. Under anaerobic conditions the quantities were 

 less. 



The Decomposition of the Carbon Compounds. 



Cellulose forms the chief constituent of the plant residues 

 added to the soil, and its decomposition is probably the largest 

 single process occurring in the soil, accounting for the chief 

 part of the " humus " found. The classical investigations of 

 Omelianski showed that both marsh gas and hydrogen arise 

 under anaerobic conditions, such as obtain in swamps 

 and ponds, but in normal soils the decomposition proceeds 

 aerobically and without the evolution of these gases. 



It is generally supposed that fungi, and particularly 

 actinomycetes, play an important part in cellulose decomposi- 

 tion : McBeth (i85<^, c,) has also described bacteria to which he 

 attributes this change. The only organism that has been 

 studied in detail is Spirochceta cytophaga (Hutchinson and 

 Clayton, 140/). This organism is distributed in soils; it is 

 aerobic and very selective in its action, cellulose being the 

 only compound with which growth was secured : it is indeed 

 inhibited by many carbohydrates, especially those containing 

 reducing groups. Its nitrogen requirements are met by simple 

 compounds such as ammonium salts, nitrates, amino-acids, 

 but not by higher compounds such as peptone (except in 

 weak solution), gelatine, etc. The products of decomposition 

 of cellulose include small quantities of volatile acids ; mucilage 

 soluble in ammonia but insoluble in acids, and which does not 

 give rise to optically active compounds on hydrolysis ; and a 

 pigment apparently related to the carotin group : there is no 

 obvious gas in the cultures. 



The organism possesses certain morphological character- 

 istics which explain some of the difficulties of previous investi- 



12 



