Page 62 - Enzymes in Tropical Soils

50 Enzymes In Tropical Soils

there are also free nitrogen fixing bacteria in soils living outside of plant root
nodules. Among these bacteria are Azotobacter and Azospillum. These bacteria
may fix atmospheric N. This N is eventually available to plants after the bacteria
are dead and decayed.
The protein-N is not available to plants and, therefore, plants can not utilyze
it before this protein-N is converted into inorganic forms that are available to
plants. Various soil microorganisms decompose and detach the protein-N in the
form of NH 3 , which will encounter an ammonification process upon reaction with
+
+
water to form NH 4 . Ammonium is easily absorbed by plant roots. However, NH 4
usually can not suffice the plant need of N except for paddy plants.
Ammonium ions are then oxydized by particular bacteria through a two step
+ -
reactions called nitrification. This reactions change of NH 4 to NO 2 with
- -
Nitrosomonas as the oxydizing agent and of NO 2 to NO 3 with Nitrobacter as the
oxydizing agent (Bartlett, 1981). These reactions follow Eq. 4.6 and Eq. 4.7.

-
+
+
2 NH 4 + 3 O 2  2 NO 2 + 4 H + 2 H 2 O ....... Eq. 4.6

-
-
2 NO 2 + O 2  2 NO 3 ....... Eq. 4.7

The above reactions can be summarized as shown by Eq. 4.8 as follows.

+ - +
2 NH 4 + 2 O 2  NO 3 + 2 H + H 2 O ....... Eq. 4.8
This reaction shows that the nitrification process needs the presence of enough O 2 .
The nitrification do not occur in waterlogged soils because the absence of O 2 . This
reaction also suggests that the soil porosity controls the nitrification process.
Eq.4.8 also shows that the nitrification acidify soils as indicated by the production
+
of H ions.
Wu et al. (2011) reported that the Ammonia Oxydizing Bacteria sharply
increased about 50 times in soils fertilized with long-term N. This indicates that
nitrification process occurs faster in the presence of N fertilizers. For example, the
+
presence of Urea increases NH 3 which very quickly becomes NH 4 upon hydrolysis.
The decomposition of urea that produces NH 3 progresses as shown in Eq. 4.9 as
follows (Tabatabai, 1982).

NH 2 CONH 2 + H 2 O  CO 2 + 2 NH 3 ....... Eq. 4.9

Abdul Kadir Salam - 2014