Page 89 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 77

The work of soil enzymes on substrate, measured by p-Nitrophenol per soil
mass per hour, is found to be linearly correlated with the amount of substrate as
organic matter (Organic C and Total N) (Nannipieri et al., 1980; Baruah and Mishra
1984; Tate III, 1984; Salam et al., 1998d; 1999d); the higher the contents of organic
matter the higher the rates of the biochemical reactions. This phenomenon is easily
understood because the substrate is the energy source for microorganisms
producing enzymes.
Organic C or organic matter content is controlled by several factors, among
which are types of land-uses, types of vegetations, and soil tillage system. As
mentioned previously, the organic matter contents in soils of primary forest were
much higher than those in the secondary forest and agriculture lands (Table 5.10)
(Salam et al., 1998d). The organic matter in the primary forest had been preserved
by minimum disturbance and high litter inputs and organic matters of plant
residues, while those in agricultural lands had been degraded by soil tillages. Lands
with grasses (for example Paspalum conjugatum) also showed higher organic
matter content than did lands with natural vegetation (Salam et al., 2001). In
general, soil tillage systems also affect soil organic matter content (Table 5.8, Table
5.11). In general, minimum soil tillage preserves soil organic matter, while
intensive soil tillage drastically decreases the soil organic matter content (Klein and
Koths, 1980; Trasar-Cepeda and Gil-Sotres, 1987; Joner and Jakobsen, 1995; Deng
and Tabatabai, 1996;). Salam et al. (1998c) also showed that the activity of acid
phosphatase in minimum tillage or no tillage soils were higher that those in
intensively tillage soils. Previously, Klein and Koths (1980) reported that the
activities of urease, protease, and acid phosphatase were all higher in no-tillage
soils than those in plowed soils. Bergstrom et al. (1998a) also reported that the
activities of soil enzymes including dehydrogenase, urease, glutaminase,
phosphatase, arylsulfatase, and -glucosidase, increased in no tillage soils.
As mentioned previously, the positive relationship between the soil
enzymatic activities and soil organic C or organic matter contents is related the role
of organic matter as an energy source for soil microorganisms and
macroorganisms. The populations and activities of microorganisms and
macroorganisms increase with the increase of organic matter contents. As
previously shown, earthworms are found more frequently in topsoils, with higher
soil organic matter contents, than that in underlying subsoils, that have lower
organic matter contents. Earthworms also live better in soils of secondary forest,
shrubs, and alang-alang (I. cylindica) lands compared to that of coffee plantation,
that is intensively tillaged with low organic matter content (Yusnaini et al., 2002).
Higher population of earthworms then causes higher activities of soil enzymes. It

Abdul Kadir Salam - 2014