Page 63 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 51

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The production of NH 3 and, hence NH 4 , is higher in the presence of enough water
and is accelerated by the presence of enzyme Urease, the activity of which was
reported to increase with the increase in applied Urea-N (Kumar and Wagenet,
1984; Moyo et al., 1989) (Fig. 4.10). This reaction also acidifies soils because of the
CO 2 resulted by the reaction.

30

25
Urease Activity (mg-N g -1 h -1 ) 20 Kilburn

15
Nibley
10
Dagor
5

0
250 500 750 1000
-1
N Level (mg N g )

Fig. 4.10. The effect of Urea-N on the activity of Urease in three soils.
(Redrawn from Kumar and Wagenet, 1984)

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The nitrification is beneficial to produce NO 3 that is available to plants.
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However, a fast conversion of NH 3 to NO 3 is not beneficial, since NO 3 is not
adsorbed by soil solids and is thus mobile in soils. Therefore, it is important to have
a method to inhibit the nitrification process. Roots of particular plants may satisfy
this need. For example, the of grass of Brachiara humidicola (Randle) Schweick
excrets nitrification inhibiting substances. This substances may inhibit the work of
nitrifying bacteria but may not affect other important soil microorganisms
(Gopalakrishnan et al., 2009).
The inhibition can also be conducted on the work of Urease by Urease
Inhibitors . Urease Inhibitors may inhibit the work of Urease so that the hydrolysis
of urea and, hence, the production of NH 3 is limited (Broadbent et al., 1985;
Hendrickson and O’Connor, 1987; Cai et al., 1989; Zhao and Zhou, 1991;

Abdul Kadir Salam - 2014