Page 107 - Ilmu Tanah Book

94 The Chemistry and Fertility of Soils under Tropical Weeds

CO 2 (oxidized) and that of O decreases from 0 in O 2 (oxidized) to -2 in H 2 O
(reduced). This biochemical cycle is mathematically summarized in Table 6.1.
Based on Fig. 6.3 and Table 6.1, it is obvious that CH 2 O or hydrocarbon is an
energy source for organisms. Hydrocarbon produced during photosynthesis is used
by organisms involving respiration, its reverse reaction. Hydrocarbon is
decomposed into H 2 O, CO 2 , and energy during respiration. In the soil environment
hydrocarbons may be exist in various forms. Bohn et al. (1985) listed some forms
of organic matters as shown in Table 6.2.

PHOTOSYNTHESIS RESPIRATION
CO ---> CH O CH O ---> CO
2 2 2 2
(PLANTS) (ORGANISMS)

Fig. 6.3. The cycle of Carbon through photosynthesis and respiration
(After Salam, 2014).

Microorganisms, macroorganisms, and plant roots use O 2 as the main electron
acceptor. In the absence of O 2 , both plant roots and macroorganisms cannot
oxidize hydrocarbon bacause no chemical species will accept electrons released
during the oxidation of hydrocarbon. However, soil microorganisms may continue
the oxidation of hydrocarbon by employing some chemical species in soils as
electron acceptors. Using any of these acceptors, soil microorganisms will obtain
less energy and, consequently, conduct less activities. Bohn et al. (1985) showed
some of these electron acceptors as listed in Table 6.3.
The above explanation suggests that the highest energy obtained by soil
microorganisms is obtained when O 2 is used as the electron acceptor, as also used

Abdul Kadir Salam and Nanik Sriyani – 2019