116 Decomposition of Plant and Animal Residues 



nose, galacturonic acid, acetic acid, and methyl alcohol. Some poly- 

 uronides are simple polymers of uronic acid, (CeHioOo)^, whereas 

 others are even more complex than pectins. 



Hemicelluloses are attacked by a great variety of bacteria and 

 fungi; they can also be digested by most animal forms. There is 

 greater variation in the digestibility and in the rate of decomposi- 

 tion of the hemicelluloses than of cellulose, because of greater differ- 

 ences in chemical nature between various hemicelluloses. Some, 

 like the mannans, are attacked readily, similarly to the starches, 

 whereas others, like the galactans, are more resistant to decomposi- 

 tion and can be attacked only by highly specific organisms. In the 

 rotting of fruits and vegetables, either in a growing state or in stor- 

 age, the breakdown of the pectins is particularly important. This 

 is carried out first by a group of enzymes, designated as pectase, 

 pectinase, and pectolase, as follows: 



C41H60O36 + 9H2O = CeHiaOe + CsHioOs + 



Pectin Galactose Arabinose 



2CH3COOH + 2CH3OH + 4C6H10O7 



Acetic acid Methyl Galacturonic 



alcohol acid 



Similar reactions are involved in the retting of flax and other 

 fibers by aerobic bacteria and fungi; anaerobic bacteria change the 

 sugars and sugar acids of the pectin to alcohols and lower acids. 



Since arabans and xylans make up 20-30 per cent of cereal straw, 

 of corn cobs, and of other plant residues, their breakdown in com- 

 posts and in soil is of great importance. They are usually attacked 

 by a variety of organisms somewhat more readily than is cellulose. 

 Hemicelluloses also form an important group of constituents of 

 microbial cell substance ( capsular material ) and may thus contribute 

 materially to the humus produced. 



The decomposition of cellulose and hemicelluloses in oat straw 

 harvested at different stages of growth is brought out in Table 21. 

 When the plant is young, and its cellulose and lignin contents are 

 low, it decomposes very rapidly: as much as 56.3 per cent of the 

 total material has been destroyed by the microorganisms in 59 days. 

 As the plant grows older and as its cellulose and lignin contents in- 

 crease, its rate of decomposition decreases; that this is due largely 

 to an unbalanced nitrogen condition is brought out by the fact that, 

 when a soluble nitrogen compoimd is added, the mature plant ma- 

 terial decomposes as rapidly and as extensively as the young plant. 



