Page 106 - Enzymes in Tropical Soils

94 Enzymes In Tropical Soils

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higher than 30 C may be degradative and may lower their populations and
activities in soils. Curtin et al. (2012) noticed from their experiment in some New
Zealand soils that the microbial biomass carbon was particularly sentitive to
o
temperature; the post-incubation biomass C decreased 18-35% between 5 – 25 C.
As the soil temperature is related to soil cover, the forest conversion to
other land-uses may change the soil temperature (Dash et al., 1981; Moyo et al.,
1989; Neal, 1990) and also the types of vegetation, plant roots, and soil
microorganisms (Duxbury and Tate III,1981; Frankenberger and Dick, 1983; Jha et
al., 1992). For example, in the regions of Northeast India, it was found that the
activities of some soil enzymes such as dehydrogenase, urease, and phosphatase
were higher in the regions that had not been degraded than those in the degraded
regions. This phenomenon was related to the lower populations of fungi and
bacteria in the degraded regions.

6.4 Soil Water and Temperature versus Soil Biochemical
Reactions

The rates of soil biochemical reactions increase with the increase in soil
water content and soil temperature below their respective optimum values. This
facts enable the use of these soil properties to manage the rates of organic matter
decomposition. The rates of the soil biochemical reactions can be accelerated or
lowered by manipulating these soil properties.
As mentioned previously, water molecule is one of the most important
reactants in the decomposition of organic matters. For example, the
decomposition of organic P (Eq. 4.1) and organic S (Eq. 4.5) in soils may not occur
in the absence of water. Therefore, the decomposition of organic matters in dry
soils does not occur. The decomposition may start after the air-dry soil is
moistened and may increase significantly as water is added until an optimum value.
The general optimum water content for the biochemical reactions is the soil field
water capacity. The soil field water capacity is the water hold by soils at water
potential of 10 – 35 kPa, which is different between soils (Singer and Munns, 1987).
The rate of a biochemical reaction is expressed by the activity of enzymes,
which indicates the amount of products of an enzymatic reaction per unit soil mass
per unit time. For example, the activities of phosphatases are usually expressed in
-1 -1
mg p-Nitrophenol per gram soil per hour (mg p-Nitrophenol g h ). Related to the
role of water, Balligar et al. (1988) reported that the activities of phosphatases of

Abdul Kadir Salam - 2014