Geoscience Reference
In-Depth Information
for around 50% and 30% of the total functional groups, respectively (Yu, 1976). Fur-
thermore, at high alkaline conditions, amino groups can also produce negative charge
on the soil surface (Horowitz, 1991; Stevenson, 1994; Yu, 1997). Humic substances
are the main body of humus (Dai, 2004). There are three kinds of humus, including
loosely, stably and tightly combined humus. The main components of organic matter
in soil are humic acid, fulvic acid and humin. Humin is the main composition of tightly
combined humus. Humic acid and fulvic acid are stably combined humus (Lu, 2000;
Chen and Wang, 2006).
The high charge ranges (200 to 500 cmol kg
−
1
) carried by humus may play an
important role in the surface charge of organic soil colloids (Schnitzer and Khan,
1989; Yu, 1997; Stevenson, 1994; Tan, 2008).
In an organo-mineral complex, the amount of surface charge is smaller than the
sum of charges carried by the two components separately because of various bonding
forces. The mechanisms of this phenomenon are not known. Charged organic colloids
can join with positively charged iron and aluminium oxides or with positive sites at
the edges of clay minerals. Organic matter as an important charge component can also
affect the zero point of charge of the mineral soil (Yu, 1997; Stevenson, 1994; Majzik
and Tombacz, 2007).
It is noteworthy that some humic nitrogen compounds can carry positive charges,
which can change the behaviour of humus colloids (Fuchsman, 1986a).
The concentrations of electrolytes, types of electrolyte, valences of ions and pH
are important factors that can influence surface charge. Among them, pH is the most
important one (Alkan
et al.
, 2005; Li and Xu, 2008; Yukselen and Erzin, 2008). The
dissociation of H
+
ions from the hydroxylated surface and the adsorption of H
+
ions
by the hydroxylated surface are pH dependent. As a result of this condition, variable
charge minerals, such as iron and aluminium oxides, carry a negative charge when the
pH is higher than their zero point of charge.
3.6 HUMIFICATION OF PEAT
As described above, decomposition or humification involves the loss of organic matter
either as gas or in solution, with the disappearance of the physical structure and changes
in chemical state. The breakdown of plant remains is brought about by soil microflora,
bacteria and fungi, which are responsible for aerobic decay. Therefore the end products
of humification are carbon dioxide and water, the process being essentially biochemical
oxidation. Immersion in water reduces the oxygen supply enormously, which in turn
reduces aerobic microbial activity and encourages anaerobic decay, which is much
less rapid. This results in the accumulation of partially decayed plant material as peat
(Fuchsman, 1986b)
The influences on metabolic activity, apart from the supply of oxygen, are tem-
perature, acidity and the availability of nitrogen. Normally, higher temperatures and
pH values enhance the decomposition activity. Decomposition tends to be most active
in neutral to slightly alkaline conditions. The more acid the peat, the better the plant
remains are preserved. The acidity of the peat depends on the rock types in the area
draining into the peat land, the types of plant growing there, the supply of oxygen and
the concentration of humic acid. In temperate regions, bog peat (blanket and raised
