multi-layered profiles (such as in Fig. 2.4). Complexing
between allophane and organic matter means that organic
carbon is captured and stored in the soil, much in micro- or
nanoaggregates. Allophanic Soil, therefore, has a high
capacity to hold carbon in a form that is not readily broken
down and so provides a carbon sink. Thus relatively deep,
dark, topsoils are formed. Soil carbon contents are medium
to high (up to *8–12% organic carbon, Fig. 2.8) and
organic/mineral complexes are stable.
In subsoils and unfertilized topsoils the sum of bases are
low to very low and range from less than 1 to about 10
cmol
(+) kg
−1 . Allophanic Soils, although having excellent
physical properties, need careful management of nutrients
(especially phosphorus) to maintain high productivity. Sulphate reserves are held in B-horizons.
Allophane, and allophanic soil material, has pH-dependant
charge. This means that measured soil cation exchange
capacity, which is a measure of the negative charge on the
surfaces of clay and organic matter within the soil, increases as
the pH of the soil increases.
2.4.4 Biological Properties
The Allophanic Soils are possibly New Zealand’s most
favourable biological substrate, having low drought risk,
generally moderate acidity, high soil organic matter, good
rooting depth, high porosity, and moderate soil temperature.
The soils have the capability of hosting large and diverse
populations of soil organisms, and microbial biomass is generally high. They form in regions where rainfall is generally
>1000 mm year
−1 and soil-water deficits are uncommon.
It has been recognised by Yu-Tuan (Doreen) Huang,
David Lowe, and colleagues that DNA can be entrapped and
preserved by allophane. They determined that the formation
of stable allophane nanoaggregates and microaggregates
enabled large amounts of DNA to be adsorbed. In an
experiment, about 80% of added DNA was physically
adsorbed within tiny spaces (nanopores) between allophane
spherules and allophane nanoaggregates. The rest of the
added DNA (*20%) was adsorbed chemically on the surfaces of the allophane spherules. The stability of the
allophane-DNA aggregates prevents encapsulated DNA
from being exposed to oxidants, and DNA within small
pores between allophane spherules and nanoaggregates is
not accessible to enzymes or microbes, hence enabling its
protection and preservation. Thus, substantial organic carbon
is likely to be sequestered and protected, in this way, in
Allophanic Soils.
The extraction of miniscule amounts of DNA from the
soil is difficult as the DNA has strong bonds with soil organic matter and with the allophane spherules (bound tightly
to it through the DNA’s phosphate group). To release the
trapped DNA, Huang and colleagues took a novel approach
and dissolved all the allophane using acidified ammonium
oxalate (Tamm’s reagent), and then purified the resultant
DNA. They obtained amplifiable DNA, mainly of native
New Zealand plants, from the buried Ah-horizon on Rotoma
Tephra, about 9500 years old, from a site near Mt Tarawera
(Fig. 2.4). The extraction of non-modern DNA from allophanic materials, especially paleosols, has potential for
reconstruction and improved understanding of past environments including their biodiversity, the effects of volcanic
activity, and even for use in forensic science.
Table 2.2 Typical example of
chemical properties of an
Allophanic Soil: Horotiu series
(Typic Orthic Allophanic Soil)
SB09944 (SWAMP)
Horizon
Depth cm
pH
(in H 2 O)
Carbon
%
Nitrogen
%
CEC
a
cmol
(+) kg
−1
Sum bases
cmol
(+) kg
−1
P retention
%
Ap1
0–6
5.7
8.2
0.77
28.2
13.9
85
Ap2
6–17
5.3
5.5
0.55
21
5.5
91
AB
17–31
5.9
3.3
0.32
17.3
7.1
96
Bw1
31–55
6.6
1.7
0.15
12
7.5
96
Bw2
55–73
6.7
1.7
0.14
12.9
8.5
96
Bw3
73–91
6.7
1.2
0.12
11.6
7.5
94
2Cu(g)
91–107
6.7
0.7
0.07
6.5
4.5
76
2Cu
107–130
6.7
0.1
0.01
1.6
1.2
18
a CEC = Cation exchange capacity
2.4 Key Soil Properties
33
between allophane and organic matter means that organic
carbon is captured and stored in the soil, much in micro- or
nanoaggregates. Allophanic Soil, therefore, has a high
capacity to hold carbon in a form that is not readily broken
down and so provides a carbon sink. Thus relatively deep,
dark, topsoils are formed. Soil carbon contents are medium
to high (up to *8–12% organic carbon, Fig. 2.8) and
organic/mineral complexes are stable.
In subsoils and unfertilized topsoils the sum of bases are
low to very low and range from less than 1 to about 10
cmol
(+) kg
−1 . Allophanic Soils, although having excellent
physical properties, need careful management of nutrients
(especially phosphorus) to maintain high productivity. Sulphate reserves are held in B-horizons.
Allophane, and allophanic soil material, has pH-dependant
charge. This means that measured soil cation exchange
capacity, which is a measure of the negative charge on the
surfaces of clay and organic matter within the soil, increases as
the pH of the soil increases.
2.4.4 Biological Properties
The Allophanic Soils are possibly New Zealand’s most
favourable biological substrate, having low drought risk,
generally moderate acidity, high soil organic matter, good
rooting depth, high porosity, and moderate soil temperature.
The soils have the capability of hosting large and diverse
populations of soil organisms, and microbial biomass is generally high. They form in regions where rainfall is generally
>1000 mm year
−1 and soil-water deficits are uncommon.
It has been recognised by Yu-Tuan (Doreen) Huang,
David Lowe, and colleagues that DNA can be entrapped and
preserved by allophane. They determined that the formation
of stable allophane nanoaggregates and microaggregates
enabled large amounts of DNA to be adsorbed. In an
experiment, about 80% of added DNA was physically
adsorbed within tiny spaces (nanopores) between allophane
spherules and allophane nanoaggregates. The rest of the
added DNA (*20%) was adsorbed chemically on the surfaces of the allophane spherules. The stability of the
allophane-DNA aggregates prevents encapsulated DNA
from being exposed to oxidants, and DNA within small
pores between allophane spherules and nanoaggregates is
not accessible to enzymes or microbes, hence enabling its
protection and preservation. Thus, substantial organic carbon
is likely to be sequestered and protected, in this way, in
Allophanic Soils.
The extraction of miniscule amounts of DNA from the
soil is difficult as the DNA has strong bonds with soil organic matter and with the allophane spherules (bound tightly
to it through the DNA’s phosphate group). To release the
trapped DNA, Huang and colleagues took a novel approach
and dissolved all the allophane using acidified ammonium
oxalate (Tamm’s reagent), and then purified the resultant
DNA. They obtained amplifiable DNA, mainly of native
New Zealand plants, from the buried Ah-horizon on Rotoma
Tephra, about 9500 years old, from a site near Mt Tarawera
(Fig. 2.4). The extraction of non-modern DNA from allophanic materials, especially paleosols, has potential for
reconstruction and improved understanding of past environments including their biodiversity, the effects of volcanic
activity, and even for use in forensic science.
Table 2.2 Typical example of
chemical properties of an
Allophanic Soil: Horotiu series
(Typic Orthic Allophanic Soil)
SB09944 (SWAMP)
Horizon
Depth cm
pH
(in H 2 O)
Carbon
%
Nitrogen
%
CEC
a
cmol
(+) kg
−1
Sum bases
cmol
(+) kg
−1
P retention
%
Ap1
0–6
5.7
8.2
0.77
28.2
13.9
85
Ap2
6–17
5.3
5.5
0.55
21
5.5
91
AB
17–31
5.9
3.3
0.32
17.3
7.1
96
Bw1
31–55
6.6
1.7
0.15
12
7.5
96
Bw2
55–73
6.7
1.7
0.14
12.9
8.5
96
Bw3
73–91
6.7
1.2
0.12
11.6
7.5
94
2Cu(g)
91–107
6.7
0.7
0.07
6.5
4.5
76
2Cu
107–130
6.7
0.1
0.01
1.6
1.2
18
a CEC = Cation exchange capacity
2.4 Key Soil Properties
33