Page 27 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 15

those that involve in the cycles of N, P, and S, which are recently most extensively
investigated, i.e. urease, phosphatases, and arylsulfatase. The presence of
phosphatases and arylsulphatases are expected because these soil enzymes
accelerate the decomposition of organic P and organic S, respectively, which are
not available for plant root absorption, to release inorganic P and S such as H 2 PO 4 -
2-
2-
or HPO 4 and SO 4 , that are available for plant root absorption (Tabatabai, 1982;
Rojo et al., 1990). Unlike phosphatases and arylsulfatases, the presence of urease
in soils is not expected. Urease may waste the use of Urea-N because its presence
+
increases the hydrolysis of Urea to produce ammonium (NH 4 ). Ammonium may
-
encounter nitrification to form NO 3 in an oxydative condition or denitrification to
form N 2 in a reductive condition. Nitrate-N molecules are mobile in soils or may
volatilize to the atmosphere. Therefore, the presence of urease is usually inhibited
by Urease Inhibitors (Broadbent et al., 1985; Hendrickson and O’Connor, 1987; Cai
et al., 1989; Zhao and Zhou, 1991; Hendrickson and Doughlass, 1993; Sanz-Cobena
et al., 2008).
The presence of enzymes like phosphatases and arylsulphatase may then
increase the availabilities of some nutrient elements from non-mineral sources.
The accumulated organic matters in the environment, which contain some
important nutrient elements, may be used as nutrient element sources. This may,
in turn, lower the need of nutrients that are usually supplied by commercial
fertilizers. The optimation of the environmental properties may be regulated to
manage the production of soil enzymes so that parts of the nutrient elements
needed by plants can be supplied from organic sources.

2.1 Definition and Properties of Enzymes

Enzymes are catalysts. A catalyst accelerates the rate of a reaction to a
magnitude of several orders. Enzymes are biocatalysts that are able to enhance
biochemical reactions. Soil enzymes are the biocatalysts working on biochemical
reactions in the soil system. For example, phosphatase is a biocatalyst working on
the biochemical reaction that decomposes soil organic P to produce
orthophosphates. Without the presence of phosphatase, the decomposition of soil
organic P occurs relatively slowly. This may result in a slow supply of phosphorus
available for plant root absorption and plants are more dependent on the supplies
of P from other sources.
Tabatabai (1982) stated that the most remarkable properties of enzymes are
their: (a) catalytic efficiency and (b) specificity. An enzyme may efficiently enhance

Abdul Kadir Salam - 2014