杏吧原创

WHEN it comes to the environment, interfaces are where the big action is.
Sometimes this action is dramatic: landslides, gale-force winds causing havoc in
forests, or rivers flooding farmland.

Of course, more often this interaction is less dramatic because the processes
occur over very long timescales, but the effects are just as important. The slow
but inexorable weathering of mountains is just one example.

And there is a vast interface zone that covers much of the global land
surface where the atmosphere, the hydrosphere, the biosphere and the geosphere
all meet. That zone is the soil; and it is both a fascinating and important
environment and also a very complex one. Not only does soil reside at the point
of overlap of the four spheres mentioned above
(Figure 1), but it owes its very
existence to the interactions that occur between those spheres.

It is made up of solids which are in part the products of the weathering of
rocks (atmosphere and hydrosphere acting on the geosphere) and in part the
products of biological activity (the decay of plant and animal debris). The soil
is a highly porous medium with typically a 50:50 mix of solids and pore spaces.
The pore spaces contain variable amounts of water and air, depending on how wet
or how dry the soil is.

Soils are formed by a combination of processes, including physical
weathering, which involves the fragmentation of rocks (Inside Science No. 107);
chemical weathering, which involves the alteration of the minerals that make up
the rocks; the decomposition of plant and animal residues deposited on and in
the soil; and the movement by percolating water of suspended solids and
dissolved materials down through the soil. These and other processes are
influenced by environmental factors, such as the nature of the underlying
geology, the local vegetation, the climate and the topography. This means that
in different places the processes combine in different ways to produce a variety
of soils.

Soils are classified by the features they exhibit when viewed in vertical
sections known as soil profiles. For example, in temperate climatic zones, sandy
materials derived from sandstone, under coniferous forest on steep slopes, may
develop into podzols, with their characteristic banded structure resulting from
the loss and redistribution of material in percolation water. However, in
tropical climates, strongly weathered basaltic rocks under rain forest may
develop into deep, red, iron oxide-rich oxisols.

Necessities of life

Required nutrients

One of the key functions of soils is to support life (鈥渟oil function鈥� is
nowadays an important concept in soil protection policies), and the soil itself
is teeming with life, mostly microorganisms which cannot be seen with the naked
eye, but more obviously the larger animals, such as earthworms, and the plant
life that live in and on it. All this biological activity results from the
ability of the soil to provide the necessities of life: shelter, food and
water.

For plants, and the many animals that in turn consume them, nutrition depends
on the intake of a number of vital elements which they require in varying
amounts. Certain chemical elements, such as nitrogen (N), phosphorus (P),
potassium (K) and calcium (Ca), they need in relatively large amounts. These are
known as plant macronutrients. Others, such as copper, zinc and boron, are
needed in smaller quantities and are called plant micronutrients.

Processes occurring in the soil, such as rock and mineral weathering and the
biological breakdown of organic materials, release these elements in soluble
forms into the soil water. They become available for use by soil organisms and
particularly for plants that absorb them through their roots.

During soil formation, the chemical weathering caused by acidic solutions in
the soil leads to the breakdown of both rocks and the minerals that they
contain. This releases some macronutrients such as K, Ca, magnesium and sodium
as soluble cations (Figure 2).
The acidic solutions which cause the chemical
weathering occur because atmospheric gases, such as carbon dioxide (CO₂),
dissolve in the natural water making it slightly acidic. This process is
speeded up by fossil fuel combustion which puts sulphur dioxide (SO₂)
and nitrogen oxides (NO_x) into the atmosphere, increasing the acidity
of rainfall. These weathering processes release chemical elements which are less
soluble, such as iron (Fe) and manganese, which are both micronutrients. But
these elements instead tend to precipitate as insoluble oxides and hydroxides,
which explains the persistence of iron oxides in many tropical soils such as the
oxisols.

Silicon (Si) and aluminium (Al), which are major constituents of rocks and
minerals, are also released in weathering, but they tend to precipitate as new
solids, such as clay minerals. Neither Si nor Al is essential for plant life.
But while Si is nontoxic, Al is potentially toxic to both plants and animals
when it is in the form of soluble aluminium cations. It becomes particularly
soluble at pH values below 5, and so is characteristic of acidic soils.
It can cause toxicity for plants growing in soils at low pH, unless
they are plant species that have adapted to the acid conditions. Additionally,
acidic water draining from soils can acidify the streams and rivers into which
it flows and kill fish.

Also released at low pH are other potentially toxic elements (PTEs),
such as cadmium (Cd) and nickel (Ni), which become more soluble鈥攁nd hence
more available for plants to absorb鈥攁s the pH decreases. So soils
naturally high in PTEs, or polluted soils containing them as contaminants, may
cause toxicity problems for plants if the pH becomes too acidic.

Life and death saga

Biosphere at large

Biological material is chemically complex and made up of many elements; but
only four are present in large amounts, and they are carbon (C), nitrogen (N),
oxygen (O) and hydrogen (H). When biological material is returned to the soil,
for example the roots of harvested crops, it becomes a food source for organisms
that live there. In that way it is decomposed and its constituents combined into
new, generally simple, chemical compounds.

Much of the C, and some O, will finish up as CO₂ and be released
back into the atmosphere, where it can again be used for photosynthesis and
completes the carbon cycle. The H, and some more of the O, will become water and
return to the hydrosphere. The N, which is a key element in plant and
animal nutrition, will be released initially as ammonium ions (NH₄+),
which can then be transformed by specialised bacteria into nitrate ions (NO
₃-). Both ammonium and nitrate are soluble and become available for plants
to uptake when they are dissolved in the water in the soil. Nutrient elements
are therefore recycled from decaying plant residues back into growing plants
(Figure 3).

The organic residues in soil can be identified initially as plant or animal
remains, and as they undergo decomposition they become humified鈥攖耻谤苍别诲
into humus or soil organic matter (SOM). This SOM is not a single chemical
compound, but a mix of compounds of varying size. However, the compounds are
generally organic polymers containing both aliphatic (chain) and aromatic (ring)
structures. The chemical properties of the SOM are determined by the chemical
groups which are on its exposed surfaces, the so-called functional groups, which
include carboxyl, amino and phenolic groups.

Because of biological recycling, new material is constantly being added to
the SOM as plants and animals die and their remains decay and enter the soil.
However, the SOM content of soils is relatively constant in undisturbed
situations, which means that it is decaying as fast as it is being replenished.
In agricultural soils in temperate climates, for example, the rate of SOM decay
is about 5 per cent per year, which means that on average SOM remains in the
soil for about 20 years. This is called its residence time. The 20-year average
is made up from various individual components whose span of existence may last
from a few weeks, for easily degraded sugars and starches, to hundreds of years,
for more resistant lignin-like compounds.

The amount of SOM in a soil is dependent both on the nature of the vegetation
or land use, and on the climate, because the process of decay is speeded up by
increased temperature. This means that soils in hot, dry climates lose SOM
rapidly while those in cold, wet climates accumulate it. In wet climates water
determines the ease with which oxygen in its usual molecular form (O₂)
can get into the soil鈥攚et soils have most of their pores filled with water
and therefore O₂ has difficulty entering. Conversely, in dry areas, the
dry soils have pore spaces mainly filled with air. This means that the soils of
the tropical arid zones (desert regions) are well oxygenated. This, combined
with the high temperatures and small inputs of plant material, leads to them
having very low SOM contents. The soils of cold wet regions in contrast are
dominated by organic matter that decomposes only very slowly, leading ultimately
to the formation of peat.

This, of course, has implications for many of today鈥檚 concerns about
greenhouse gases and climate change because these concerns are focused on the
global carbon cycle. Clearly, soils have an important role here because they are
both a source and a sink for C (see Inside Science No. 51 鈥淭he Carbon Cycle鈥�).
How soil is managed for agriculture and forestry will influence whether overall
it is a net C source or a net C sink. It is estimated that C reserves in soils
globally are 1500 脳 10¹⁵ grams, which represents 4 per cent of the C that is
not in rocks. As CO₂ in the atmosphere increases, there will be greater
inputs of organic materials to soils from the increased plant growth and
microbial activity. But conversely the increasing global temperatures will
increase rates of decomposition in soils. So the net effect on SOM content may
be zero. However, changes in agricultural practices, particularly the reduction
in tillage, could make a significant contribution to reducing C emissions.
Disturbing the soil by ploughing introduces O₂ and exposes more SOM to
oxidation and increases CO₂ production. Reduced tillage systems not
only lead to less CO₂ release from soil, but also involve consumption
of less fossil fuel by farm vehicles. They deserve consideration as we attempt
to tackle global warming.

Because decaying organic matter releases the essential plant nutrients, the
SOM serves as a reserve of fertility鈥攊t is a measure of inherent soil
fertility. However, that inherent fertility only becomes realised when the SOM
decays, which requires not only warm conditions, but also near neutral
pH values. If soils become acid, as do podzols, then organisms cannot carry
out the decay processes and organic matter accumulates at the surface.
Fortunately many soils have the ability to slow down the acidification process,
which is known as their pH buffering capacity (see Box).

Out of thin air

Atmospheric inputs

The atmosphere is made up of three major components: nitrogen as N₂
(78 per cent), oxygen as O₂ (21 per cent) and CO₂ (0.04 per
cent). All of these are important in supporting life, especially the CO
₂, which is the source of C for plants through photosynthesis. Oxygen is
important for the respiration of all aerobic organisms, and is essential for the
decay of organic matter in soil, releasing plant nutrients and recycling organic
forms of C back from the soil to the atmosphere as CO₂.

Nitrogen as N₂ is of little direct use to plants, which require N in
the form of soluble NO₃鈥� or NH₄+ in solution. However, through
the action of nitrogen-fixing bacteria, such as the rhizobia, that live in the
soil, atmospheric N₂ in the soil can become available for plants. The
rhizobia convert nitrogen into organically-bound NH₂ (amino) groups.
Subsequently, during SOM decay, this fixed nitrogen can be transformed into
NH₄+ and then into nitrate ions.

But that is only part of the story of nitrogen transformations in soil.
Leaching is the loss of dissolved materials from the soil when water moves
rapidly through it. Nitrogen, as nitrate from whatever source, can be lost by
leaching from the soil into rivers and streams, causing environmental and health
risks (Inside Science No. 37 鈥淣itrates in Soil and Water鈥�). However it is also
lost by the process of denitrification, in which nitrate is chemically reduced
by specialised bacteria in waterlogged soils back to N₂ which is then
returned to the atmosphere.

Water is essential for organisms in the soil to survive because it provides
the source of H and O, which together with C and N, are the dominant elements in
the material make-up of all life forms.

The soil, being porous, has the potential to act as a reservoir for water.
Its actual water-holding capacity depends on the size of the pore spaces and
their interconnections. If the pores are greater than 0.05 millimetres in
diameter then water can drain from them under gravity. Smaller pores can hold
water without it draining away. The distribution of pore sizes in soil depends
on the soil structure鈥攖he way in which the individual particles are joined
together to form aggregates. Aggregation depends on binding agents such as clay
and organic matter. Soils with a large water-holding capacity have a mix of
particle size components (sand, silt and clay), together with organic matter.
These components can be turned into a soil with a range of pore sizes by a
combination of biological activity and physical forces, such as the swelling and
shrinking caused by wetting and drying cycles.

While the number of small pores determines the water-holding properties, the
number of large pores determines the soil鈥檚 vulnerability to leaching. In
climates with appreciable rainfall, soils which are excessively
well-drained鈥攖hose dominated by large pores, rapidly become acidified
through leaching of cations, such as Ca²⁺. This is how podzols are formed in
temperate regions.

The source of water in most soils is rainfall. This is not pure water,
because it has dissolved gases in it which lower its pH to somewhere in
the range of 4 to 5.5, depending on the level of atmospheric pollution (SO
₂ and NO_x, see above). It also contains small amounts of dissolved
salts, mostly derived from seawater. This dilute acidic solution undergoes
changes in its chemistry when it enters the soil. The concentration of dissolved
material goes up, and the acidity is in most cases partly neutralised. The
increase in concentration is caused either by water loss as plants absorb it
through their roots (transpiration) and by evaporation (Inside Science No.
18,鈥漃lants, Water and Climate鈥�), or from the addition of material by dissolving
solids in the soil and by release via microbial activity. The pH of the
soil solution is also increased by buffering reactions (see Box). The resulting
solution is richer in nutrients and a better source of essential elements for
plants than the original rainwater.

Because the soil is the source of so many of the elements of life, it is
vitally important that these elements are retained there for plants and in forms
they can use. Those forms are simple inorganic ions, such as the cations Ca²⁺,
K⁺ and NH₄⁺, and the anions
NO₃^鈥�, H₂PO₄^鈥� and
SO₄^2-. As soluble ions these tend to be lost by leaching along with any
water that drains out of the soil. Fortunately, however, the soil has a
mechanism which enables it to retain these ions because soil particles,
particularly clays and organic matter, act as ion exchangers. The particle
surfaces are electrically charged, and those charges are predominantly negative.
So most soils have a large cation exchange capacity; but there are also some
positive charges which help to retain anions too.

The cations and anions held on charged soil particle surfaces are not
vulnerable to loss by leaching in water, but they can exchange with the ions in
the soil water. It is energetically more favourable for ions to be held on soil
particle surfaces, so that about 90 per cent of the Ca²⁺ ions, for example,
will be on the surfaces and only 10 per cent in solution. If that distribution
is disturbed, say by the uptake of ions from the solution by plants, then ions
will move off the soil particle surfaces to make good the loss
(Figure 4). In
this way the plants have ready access to nutrients in a form that they can use,
while a stock is maintained in reserve on the surface of soil particles from
which the plants can later draw.

This system works well for cations, but less well for anions because there
are fewer positive charges to hold them to the soil particle surfaces. They are
therefore more vulnerable to leaching loss, and the problems of nitrate moving
from agricultural land into water supplies is well known (Inside Science No. 37,
鈥淣itrates in Soil and Water鈥�). Other anions, notably phosphate, don鈥檛 suffer
this problem as they are less soluble and therefore less liable to leaching.

Natural ecosystems are sustained by the cycling of nutrients. Nutrients
supplied to plants from the soil are eventually recycled through litter back to
the soil. On the other hand, in agriculture, crops are removed from the land and
with them the nutrients obtained from the soil. In order to maintain the supply
of nutrients farmers add manufactured fertilisers to the soil, as well as
recycling as much organic material as possible, such as manure produced on the
farm. The manufactured fertilisers they use are salts such as ammonium nitrate
and potassium chloride. These have the advantage of providing nutrient elements
in soluble form, which can be readily utilised by the crops, but unfortunately
they are prone to loss by leaching. However, much of the K or NH₄
fertilisers that are added, while initially going into the soil solution, will
then be rapidly exchanged and held as cations on the surface of soil
particles.

Some of the fertilisers are created by processing natural mineral deposits.
For example, soluble phosphate is manufactured by acid treatment of rock
phosphate. Unfortunately, these natural deposits contain small amounts of
impurities, which finish up in the fertiliser product. Addition of the
fertiliser over many years can lead to the accumulation of measurable amounts of
the impurities, which may pollute the soil. For example, Cd, a potentially toxic
element, is sometimes associated with phosphate fertilisers.

Other sources of soil pollution include: atmospheric pollution, such as the
fallout of radioactive caesium from the Chernobyl nuclear reactor accident, or
lead from vehicle emissions; the application of contaminated sewage sludge,
which often contains heavy metals and organic chemicals originating in
industrial effluent; and persistent pesticides. Some of these contaminants can
cause human health problems if they get into crops or into water supplies,
problems for other parts of the environment if the contaminant is mobile, such
as nitrate and some pesticides, and problems in the functioning of the soil, for
example metal contaminants reducing microbial activity.

Soil quality can also be affected by physical degradation, the most dramatic
instances of which are soil erosion by wind and water. These often occur as a
result of mismanaging the soil, and particularly by not maintaining a vegetation
cover at times when wind or water can remove soil from the surface.

Water erosion of soils is dependent on there being rainfall of sufficient
intensity especially on slopes down which the water can run, carrying soil
particles with it. With serious erosion, the water collects in gullies as it
runs across the field. These can be eroded to considerable depth, taking tonnes
of soil off each hectare of land.

It is important therefore to protect the soil against contamination and
physical degradation. In this way soil quality can be maintained so that neither
our health nor that of the environment is threatened and soils can continue to
function properly in both natural and managed environments.

SOIL can resist a reduction in pH by a variety of mechanisms, even
though it may have undergone an input of acidic contaminants. Some soils have
naturally occurring carbonate minerals in them because they formed over chalk or
limestone deposits. The carbonates neutralise the acidity and maintain the
pH at 7 or above. For soils with no carbonate minerals the pH is
buffered at a lower level by ion exchange processes
(Figure 4). Incoming H⁺
ions are exchanged for ions such as Ca²⁺ or K⁺ on soil surfaces, the H⁺ being
removed from solution and replaced by the other cations. This process buffers
the pH in the range between 5 and 7. Once the surfaces have lost all
their base cations, this mechanism can no longer operate, and the acidity is
buffered by dissolution of silicate minerals. This is a chemical weathering
reaction which releases base cations into solution
(Figure 2) and maintains the
pH in the range between 3 and 5.FIG-mg21608702.JPG

The ability to resist changes in pH also operates when increases in
pH are required, particularly in the treatment of acid soils with lime,
an alkali. The amount of lime to be added must take into account not only the
soil pH, but also the soil鈥檚 ability to buffer pH change. The
amount of buffering capacity depends on the clay and organic matter contents of
the soil, because those components carry the negative charges on their surfaces
which hold H⁺ cations in an acid soil. So to raise the pH to 7, the
amount of lime required for a sandy soil initially at pH 4, with few
H⁺ ions held on soil particle surfaces, will be much less than for a clay-rich
soil at the same pH.