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The Chemistry and Fertility of Soils under Tropical Weeds 61

The phenomena of lower-activities of soil enzymes in degraded land-uses are
related to some soil environmental factors like soil water and temperature, soil C
and N, and soil pH. The activities of soil enzymes are generally higher in soils with
high temperature and higher water contents or with the increase in soil C or
organic matter and N contents, and the increase in soil pH. The changes in the soil
environment that cause the changes in these soil properties may also drive the
changes in soil enzymatic activities.
Soil water and soil temperature are important soil physical properties and are
both closely related and significantly affect the soil enzymatic activities. The
activities of soil enzymes increase with the increase in soil water contents and/or
temperature until particular values, after which the soil enzymatic activities
decline. In case of soil water, the declining pattern is due to the limitation of O 2
diffusion into the soil system which then lowers the respiration by soil
microorganisms. The resulted decreases in the populations and activities of soil
microorganisms then decrease the production of soil enzymes. In the case of soil
temperature, the declining pattern is suggested to be influenced by the enzyme
denaturation or the decrease in the activity of soil organisms at temperature higher
than the maximum value. Forest degradation might have caused the changes in
these two important soil physical properties. Forest degradation may decrease the
soil water content and increase the soil temperature.
In a laboratory experiment, Salam (1998a) treated an air-dry Oxisol sample
with distilled water at 0 to 20 – 120%. The activity of phosphatase was measured
after a one-week incubation at room temperature and produced data shown in Fig.
4.1. The activity of phosphatase increased in the presence of 20 – 40% water and
reached a maximum value at 40%. The activity of phosphatase then decreased
after the maximum activity and the lowest was found at the highest water addition
of 120%.
Salam (2014) explains that the experiment was conducted in the absence of
plant roots and soil macroorganisms (earthworms). Therefore, the increase in soil
phosphatase activity was attributed to the presence of soil microorganisms. The
presence of water until 40% (about the soil field water capacity) may have
increased the populations and activities of soil microorganisms and, hence,
increased the production of soil enzymes. To some extent the presence of more
water may have also dissolved part of the adsorbed soil enzymes previously
inactive. Above 40%, water filled more soil pores and, hence, limited the O 2
diffusion needed for respiration by microorganisms. The energy obtained by soil

Abdul Kadir Salam and Nanik Sriyani – 2019