Page 67 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 55

The activities of phosphatases are among the soil enzymes greatly pH
dependent. As pointed out previously (Salam et al., 1998d), choosing the pH value
to measure the activities of phosphatase is based on a curve indicating the
relationship between the activities of phosphatases and baffered pH.
Frankenberger and Johanson (1982) found that acid phosphatase showed an
optimum pH at pH 6.5 for acid phosphatase and pH 11.0 for alkaline phosphatase.
The optimum pH in fact varies with soil types and is determined by various factors,
among which is the dominant vegetation affecting the soils. Salam (1998d) found
that the optimum pH for acid phosphatase in West Lampung, Sumatra, Indonesia,
shifted to higher pH due to land-use conversion from the primary forest to the
secondary forest and to the cultivated soils.
Arylsulfatase activity can be determined by the method previously reported
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Salam et al. (1999e). A 1 g of soil sample (0.2 mm and 105 C oven-dry equivalent) is
used. The microbial activity is stopped by the addition of 0.25 ml toluene, followed
by a 4 ml acetate buffer 0.5 M (pH 5.8) and a 1 ml of p-nitrophenol sulfate solution
of 0.025M (ca. 3.5 mg of p-nitrophenol equivalent). After gentle swirling, the
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mixture is incubated for 1 h at 30 C. A 1 ml of 0.5 M CaCl 2 and a 4 ml of 0.5 M
NaOH solution are the added. The concentration of p-nitrophenol in the solution
phase is determined with a spectrophotometer at 400 nm wavelength after
filtering through a Whatman No. 42 paper (Tabatabai, 1982).
Urease activity is determined by the amount of urea substrate remaining in
the soil system after incubation (Tabatabai, 1982). A 5 g (oven-dry equivelent) soil
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sample is incubated at 30 C for 5 h after an addition of 5 ml of a urea substrate
solution (2 mg urea per ml). The microbial and enzymatic reactions are stopped
after the incubation with the addition of phenylmercuric acetate solution (PMA).
Potassium chloride solution is employed for urea extraction. The amount of the
remaing urea in the extract is determined after development of a red color using
sulfuric acid, phosphoric acid, thiosemicarbazide, and diacetylmonoxime. The
concentration of urease is determined by measuring the absorbance of a red color
at 527nm with a spectrophotometer.
Bacause soil temperature affects greatly the activities of soil enzymes, the
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selection of temperature is very important. The temperature of 37 C will result in
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higher activities of soil enzymatic activities than that of 30 C or lower. However,
researchers use various velues of temperature for enzymatic determination. Some
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researchers use 30 C, not 37 C (Sahrawat, 1980; Salam et al., 1998d). Tempetarure
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of 37 C was used probably to simulate the optimum temperature of animal bodies.
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However, in general soils possess temperature lower than 37 C, therefore, other
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researchers use temperature of 30 C and not 37 C
Abdul Kadir Salam - 2014