exceptionally high organic carbon contents of up to c. 30%).
Only in the Pumice Soils is the New Zealand mean organic
carbon content less than the global mean for similar soils
(Vitrands). The lower carbon content in Pumice Soils may
be explained by the relative youth (<2,000 years) and large
extent of New Zealand pumice materials (Chap. 12) compared to global soils formed in pumice.
Although most New Zealand soils are relatively high in
carbon, few have thick enough topsoils to qualify for the
definition of “Black Soils” (as currently being used internationally, by organisations such as the FAO (Food and
Agriculture Organisation) in the United Nations). Black
Soils include the Mollisols/Chernozems of the Northern
Hemisphere that feature deep, organic matter-rich, topsoils,
and are currently of interest due to the desire to prevent them
from losing carbon to the atmosphere. It is good that carbon
is relatively high in New Zealand soils, however, that means
that on average, there is little potential for our soils to act as
carbon sinks. However, like the ‘Black Soils’, it is important
to ensure that we maintain, or improve, existing soil carbon
levels.
Why are New Zealand soils relatively high in carbon
compared to world averages? The main factors include the
dominant cool, moist, temperate New Zealand climate, and
the preponderance of soil acidity, which support plant growth
but impede organic matter decomposition. The existence of
soil carbon at depth in some soils has been attributed to the
relatively short time since native forested lands were converted to pasture. Under mostly evergreen native forest (in
contrast to deciduous forests), the soils had slow leaf turnover, low rates of decomposition, and low nutrient availability as a nutrient-conserving mechanism. Lower carbon
levels in other countries may also be because the New
Zealand soils are predominantly now under pasture, which is
more carbon conservative than intensive arable use. Also,
New Zealand soils have not had the accumulated impacts of
thousands of years of cultivation by ancient civilisations
which has likely led to soil carbon loss in many regions.
The Allophanic Soils are a special case (Chap. 2) as they
may contain relatively large amounts of organic matter
(generally matching levels in many of their counterparts
overseas). The high organic carbon content is usually correlated with allophane content and the high specific surface
areas associated with tiny allophane spherules. Allophanic
Soils thus not only adsorb substantial organic carbon but
also protect it from degradation, including in tight nanopores
within allophanic micro- and nanoaggregates. Also, Allophanic Soils commonly contain carbon-enriched, buried A
or buried AB horizons (within 1-m-deep profiles) so that the
overall carbon content for the soils is higher than that
associated with mineral soils of other orders that do not
contain buried soil horizons.
18.3 Pathways of Soil Formation in New
Zealand
What would become of a Raw Soil, on a fully formed parent
material with no, or negligible, additions of geological
materials to the land surface, given an extensive period of
time, a stable site, and continuity of climate, landform, and
uninterrupted ecological succession? Under such conditions,
the Raw Soil will likely age and develop through ‘classical’
topdown pedogenesis (soil formation). Topdown pedogenesis comprises soil-forming processes operating mainly from
the land surface (driven by the organic and water cycles) that
result in the gradual deepening of the profile as a downward
moving front on a static parent material. Initially, a
well-developed topsoil (A horizon) will form and, subsequently, subsoil horizons develop beneath it driven by the
environmental conditions (soil-forming factors). The properties and arrangement of consequent soil horizons provide
clues to the processes that have driven the pathways experienced by the maturing soil over hundreds to thousands of
years, or more.
However, in many landscapes in New Zealand, ongoing
deposition of new materials, such as river-flood alluvium,
tephras, or loess, is common. The evolution of soils in such
landscapes therefore has an additional complexity because
the impact from topdown (pedogenic) processes is modified
by the rates at which new materials are added to the land
surface via geological processes. The resultant soils are
formed by upbuilding pedogenesis. Upbuilding pedogenesis
is the ongoing formation of soil via topdown processes,
while tephras or sediment (such as loess or alluvium) are
intermittently added to the land surface as the result of
normal geological processes. The resultant soils may show
distinctive layering and buried horizons (sometimes referred
to as paleosols), forming multi-layered profiles. The frequency and thickness of the accumulating materials determine how much impact topdown processes have on the
ensuing soil profile, and if either developmental or retardant
upbuilding, or both, will take place.
Developmental upbuilding occurs when the rate of
addition of tephra or loess, or other geological materials, to
the land, is incremental and slow enough to allow topdown
soil formation to keep pace as the ground surface gradually
rises (a corollary is that each part of the profile has been an A
horizon at one time). In New Zealand, the most extensive
soils formed by developmental upbuilding pedogenesis
include many Allophanic Soils (Chap. 2) formed on tephra
and essentially all the Pallic Soils (Chap. 10) that are formed
in loess deposits. Other soils, such as the Perch-gley Podzol
Soils, are also formed by developmental upbuilding as are
many Recent Soils (Chap. 14) and some Brown Soils
(Chap. 4).
294
18 Conclusion: Global Context, Formation Pathways …
Only in the Pumice Soils is the New Zealand mean organic
carbon content less than the global mean for similar soils
(Vitrands). The lower carbon content in Pumice Soils may
be explained by the relative youth (<2,000 years) and large
extent of New Zealand pumice materials (Chap. 12) compared to global soils formed in pumice.
Although most New Zealand soils are relatively high in
carbon, few have thick enough topsoils to qualify for the
definition of “Black Soils” (as currently being used internationally, by organisations such as the FAO (Food and
Agriculture Organisation) in the United Nations). Black
Soils include the Mollisols/Chernozems of the Northern
Hemisphere that feature deep, organic matter-rich, topsoils,
and are currently of interest due to the desire to prevent them
from losing carbon to the atmosphere. It is good that carbon
is relatively high in New Zealand soils, however, that means
that on average, there is little potential for our soils to act as
carbon sinks. However, like the ‘Black Soils’, it is important
to ensure that we maintain, or improve, existing soil carbon
levels.
Why are New Zealand soils relatively high in carbon
compared to world averages? The main factors include the
dominant cool, moist, temperate New Zealand climate, and
the preponderance of soil acidity, which support plant growth
but impede organic matter decomposition. The existence of
soil carbon at depth in some soils has been attributed to the
relatively short time since native forested lands were converted to pasture. Under mostly evergreen native forest (in
contrast to deciduous forests), the soils had slow leaf turnover, low rates of decomposition, and low nutrient availability as a nutrient-conserving mechanism. Lower carbon
levels in other countries may also be because the New
Zealand soils are predominantly now under pasture, which is
more carbon conservative than intensive arable use. Also,
New Zealand soils have not had the accumulated impacts of
thousands of years of cultivation by ancient civilisations
which has likely led to soil carbon loss in many regions.
The Allophanic Soils are a special case (Chap. 2) as they
may contain relatively large amounts of organic matter
(generally matching levels in many of their counterparts
overseas). The high organic carbon content is usually correlated with allophane content and the high specific surface
areas associated with tiny allophane spherules. Allophanic
Soils thus not only adsorb substantial organic carbon but
also protect it from degradation, including in tight nanopores
within allophanic micro- and nanoaggregates. Also, Allophanic Soils commonly contain carbon-enriched, buried A
or buried AB horizons (within 1-m-deep profiles) so that the
overall carbon content for the soils is higher than that
associated with mineral soils of other orders that do not
contain buried soil horizons.
18.3 Pathways of Soil Formation in New
Zealand
What would become of a Raw Soil, on a fully formed parent
material with no, or negligible, additions of geological
materials to the land surface, given an extensive period of
time, a stable site, and continuity of climate, landform, and
uninterrupted ecological succession? Under such conditions,
the Raw Soil will likely age and develop through ‘classical’
topdown pedogenesis (soil formation). Topdown pedogenesis comprises soil-forming processes operating mainly from
the land surface (driven by the organic and water cycles) that
result in the gradual deepening of the profile as a downward
moving front on a static parent material. Initially, a
well-developed topsoil (A horizon) will form and, subsequently, subsoil horizons develop beneath it driven by the
environmental conditions (soil-forming factors). The properties and arrangement of consequent soil horizons provide
clues to the processes that have driven the pathways experienced by the maturing soil over hundreds to thousands of
years, or more.
However, in many landscapes in New Zealand, ongoing
deposition of new materials, such as river-flood alluvium,
tephras, or loess, is common. The evolution of soils in such
landscapes therefore has an additional complexity because
the impact from topdown (pedogenic) processes is modified
by the rates at which new materials are added to the land
surface via geological processes. The resultant soils are
formed by upbuilding pedogenesis. Upbuilding pedogenesis
is the ongoing formation of soil via topdown processes,
while tephras or sediment (such as loess or alluvium) are
intermittently added to the land surface as the result of
normal geological processes. The resultant soils may show
distinctive layering and buried horizons (sometimes referred
to as paleosols), forming multi-layered profiles. The frequency and thickness of the accumulating materials determine how much impact topdown processes have on the
ensuing soil profile, and if either developmental or retardant
upbuilding, or both, will take place.
Developmental upbuilding occurs when the rate of
addition of tephra or loess, or other geological materials, to
the land, is incremental and slow enough to allow topdown
soil formation to keep pace as the ground surface gradually
rises (a corollary is that each part of the profile has been an A
horizon at one time). In New Zealand, the most extensive
soils formed by developmental upbuilding pedogenesis
include many Allophanic Soils (Chap. 2) formed on tephra
and essentially all the Pallic Soils (Chap. 10) that are formed
in loess deposits. Other soils, such as the Perch-gley Podzol
Soils, are also formed by developmental upbuilding as are
many Recent Soils (Chap. 14) and some Brown Soils
(Chap. 4).
294
18 Conclusion: Global Context, Formation Pathways …