Page 82 - Enzymes in Tropical Soils

70 Enzymes In Tropical Soils

by soil microorganisms as energy sources (% Organic Matter Content = 1.5 – 2.0
times % C), while N is needed to build proteins required for the growth of soil
microorganisms. By this reason, organic matters with high C and high N or low C/N
ratios will decay more easily. Organic matter with low N or high C/N ratios will
decompose more difficult because the available N will be used to build proteins for
microorganisms. Therefore, low N organic matters must be enriched with Urea-N
to decay faster.
The decomposition of organic matters is one process completing the C-
Cycle. Carbon cycle is initiated by photosynthesis that uses atmospheric CO 2 and
H 2 O absorbed from soils to produce simple sugar and then build plant tissues.
Parts of plants will then be consumed by animals and humans and their residues
return to the soil system. These materials will then be decomposed by soil
microorganisms on the soil surface and/or in the soils. Parts of the residues will be
incoorporated in the soil system. Finally, all materials containing C will be
decomposed by soil microorganisms to produce CO 2 emitted into the atmosphere.
CO 2 will then enter a new C-Cycle when O 2 is reused by plants. The decayed
materials will finally be replaced by new organic matters.
In a closed system, organic matters will decompose and be replaced by new
organic matters. Conversely, in an open system the replacement will be minimum
or ceased because parts or all organic residues are not returned to the soil system
but are used for other purposes outside the soil system. This phenomenon will
decrease the C pools in the soil system. This may eventually decrease the
populations and activities of microrganism and may also the decrease in soil fetility.
As mentioned previously, organic matters and humus in soils are originated
from decaying animal and plant residues and are parts of soil solids. Structurally
and chemically, organic matters and humus are different from mineral matters.
Soil organic matters and humus are mostly composed of C chains completed with H
and O atoms and posses functional sites. Mineral matters are mostly composed of
Si, Al, and O. Organic matters are different from humus in the point of view that
organic matters are the accumulation of animal and plant residues which are
partialy decomposed, while humus is organic matters that have been transformed
by soil microorganisms to be more stable forms.
The contents of soil organic matters are different from one soil to other
soils. The contents of organic matters in Peat and Muck were in general > 50%.
The contents of organic matters in mineral soils range from 0% in desert soils to >
10% in forest soils. The gereral values ranges from 1 – 3%, decreases with soil
depth. Some reports showed that organic C of tropical soils ranges in values of
1.06 – 1.55% (Alfisols), 0.8 – 1.61% (Ultisols), 2.01 – 2.07% (Oxisols), and 2.44%

Abdul Kadir Salam - 2014