Page 59 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 47

Panama, Turner (2010) found that the optimum pH values of
phosphomonoesterase were consistent for all soils, i.e. 4 to 5 for acid
phosphomonoesterase and 10 to 11 for alkaline phosphomonoesterase. The
optimum pH for phosphodiesterase varied with soil pH, with the most acidic
optimum pH (3.0) for most acidic soils and the most alkaline optimum pH (pH 10) in
near-neutral soils. The activity of arylsulfatase showed a very acidic optimum pH in
all soils (pH ≤3.0) irrespective of soil pH. Based on a report by Salam et al. (1998d),
differences in the values of pH used in phosphatase determination will cause
differences in the degrees of soil phosphatase activity.

4.5 Enzymes in the Cycling of Sulfur

The cycle of sulfur or S-Cycle in the soil – plant system is just similar to the C-
2-
Cycle and P-Cycle. Sulfur in the form of SO 4 in soil water is absorbed by plant
roots and is transported to particular plant parts to form proteins. Since, S is
needed to form wax, in plants S is reduced to form – S – S – bonds or – S – H
functional group of proteins. Protein-S will then enter the food chain through
consumptions by animals and humans. The residues of plants, animals, and
humans will enter the soil system and will encounter decomposition process to
2-
form inorganic form of S available to plant root absorption such as SO 4 . The S-
Cycle is depicted in Fig. 4.8.
The decomposition of organic S, that is not available to plants, to inorganic
2-
SO 4 , that is readily available to plant root absorption, involves enzyme
arylsulfatase produced by soil microorganisms. The enzymatic reaction in the
decomposition of organic S is shown in Eq. 4.5 as follows (Tabatabai, 1982).

+
2-
-
R—OSO 3 + H 2 O  R—OH + H + SO 4 ....... Eq. 4.5

It is shown in Eq. 4.5 that the decomposition of organic S also needs the presence
of water molecules. Therefore, moistuning the organic S is important for the
release of S from the organic materials. Unlike the decomposition of organic P, the
decomposition of organic S will acidify the soil system because this reaction
+
produces H ions. The soil containing organic S may become more acidic with time
of decomposition.

Abdul Kadir Salam - 2014