(death) 



/ 



AMMONIFICATION: 



proteins »- ammonia (NHg) compound 



Heterotrophic bacteria 



Actinomycetes, fungi 



Animal katabolism, excretions 



ANIMAL ANABOLISM:- 



plant proteins *- animal proteins *- protoplasm 



(death) 



PLANT ANABOLISM: 



nitrates — *- amino acids *- proteins — ^ protoplasm 



NITROGEN FIXATION: 

 N2 ^NHg (bacteria) 



(atmosphere) 



/ 

 DENITRIFICATION: 

 NO2 — *-N2 (bacteria) 



NITRIFICATION: 



NHg compounds — ^nitrites (NO2) »- nitrates (NO3) 



Autotrophic bacteria 



Steps and processes In the nitrogen cycle 



surrounding desert soil generally (Greene and Rey- 

 nard 1932). It is a reasonable estimate that the total 

 bird population in a deciduous forest would deposit 

 0.1 g dry weight of organic excrement per square 

 meter in a year's time ; the mammal population, per- 

 haps 0.5 g; and the total invertebrate fauna, possibly 

 2-3 g. The accumulation of excrement under the 

 roosts of birds is sometimes enough to kill the ground 

 vegetation and even the trees (Young 1936). The 

 guano deposits on the coast of and islands off Peru 

 and elsewhere in the world were originally several 

 meters thick, as the result of centuries of occupancy 

 by nesting colonies of marine birds, but have now 

 been largely depleted by man for use as crop fertilizer 

 (Hutchinson 1950). Bat excrement, deposited in 

 caves, was exploited in years past as a source of salt- 

 peter for gunpowder. 



The conversion of raw organic matter into ma- 

 terials suitable for re-absorption and utilization by 

 plants is a complicated process and depends almost 

 entirely on the reactions of plants and animals (Lutz 

 and Chandler 1946, Waksman 1952). The digestion 

 of animals produces both mechanical and chemical 

 changes in raw humus that can be measured quanti- 

 tatively (Franz and Leitenberger 1948). The non- 

 nitrogenous substances in fresh litter are sugars, 

 starches, pectins, pentosans, celluloses, cutins, tan- 

 nins, lignins, oils, fats, waxes, and resins. Most of 

 these substances are readily broken down in the soil 

 by fungi, actinomycetes, bacteria, and protozoans, but 

 tannins, lignins, waxes, and resins decompose very 

 slowly. The end products of complete decomposition 

 are HoO and COo, but sometimes decomposition is 

 incomplete and organic acids are formed instead. 



The most important soil organisms concerned in 



the decomposition of the litter are the bacteria, both 

 aerobic and anaerobic forms. They are commonly 

 divided into two types. Heterotrophic bacteria obtain 

 their energy from the oxidation of the carbohydrates 

 and fatty substances as above described. They use 

 this energy for the synthesis of cell substances and the 

 production of enzymes that break down complex com- 

 pounds in the litter into simpler compounds, including 

 proteins into ammonia compounds. They then use 

 part of the ammonia compounds in synthesizing the 

 amino acids they need in building their own proteins. 

 Autotrophic bacteria, in turn, are of two types : Chem- 

 osynthetic species that obtain their energy from the 

 oxidation of inorganic compounds (hydrogen, sulfur, 

 hydrogen sulfide, iron, ammonia) and photo synthetic 

 species, which include purple and green sulfur bac- 

 teria, possess a form of chlorophyll, and utilize the 

 energy of sunlight. Chemosynthetic bacteria convert 

 ammonia compounds into nitrites and nitrates, part of 

 which they use in their own anabolism, the rest becom- 

 ing available for plants to absorb. Photosynthetic 

 bacteria use the ammonia compounds in their own 

 anabolism but do not render them directly available 

 to plants. Chemosynthetic bacteria are more abundant 

 than photosynthetic bacteria in soil : photosynthetic 

 bacteria are the more abundant in water. 



Nitrogen cycle 



In the nitrogen cycle proteins are broken down 

 yielding ammonia (NH3) compounds in the course 

 of the metabolic processes of all animals and by the 

 activities of heterotrophic bacteria, filamentous fungi, 

 and actinomycetes. The process is called ammonifi- 

 cation. Some of the ammonia is oxidized to form 



166 Ecological processes and dynamics 



