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Limnology, Biogeochemical cycles, compiled by Schmid-Araya, QMUL, UK
BIOGEOCHEMICAL CYCLES
THE NITROGEN AND THE PHOSPHORUS CYCLES
Living organisms require between 30 to 40 elements for their normal development. Among the
most important are: carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus. Because
there is a limited amount of each of them, the continuous availability will depend on a cycle that
permits the repetitive use of the elements. The large biogeochemical cycles make possible that
they are available to use them again and again, transforming them, and re-circulating them
through the atmosphere, lithosphere, and the biosphere.
Thus for ecologists, the most important characteristic of a biogeochemical cycle is that the biotic
and the abiotic components are well interrelated. All biogeochemical cycles include organisms
(mostly micro-organisms and without life these cycles would not exist and without them life itself
would be extinct).
A biogeochemical cycle has the following characteristics:
a) The movement of the nutrient elements from the environment to the organisms and then
release back to the environment.
b) The presence of organisms (plants and/or animals and especially micro-organisms).
c) A geological storage (either the atmosphere or the lithosphere).
d) A chemical reaction which means a chemical change.
The biogeochemical cycles can be divided in two basic types:
1. Cycles of gaseous type
2. Cycles of sedimentary types
In the gaseous cycles, the main storage is the atmosphere and there is little (or no) loss of the
nutrient during the relatively fast process of re-circulation or recycling. The nutrient is distributed
widely in the atmosphere and relatively accessible to the organisms. Typical examples are the
oxygen, carbon and the nitrogen cycles.
In the sedimentary cycles, the nutrient is stored in the sedimentary rocks or earth’s crust. These
cycles are slow and have a limiting influence upon living organisms. In order that nutrients reach
for example a plant, the rocks need weathering, the nutrient is slowly wash-out, transported to the
surface soil and by means of run-off (for example a stream, river) the nutrient reach the ocean
and is deposited finally at the bottom of the ocean (or lake) as sediment. The trapped element is
not accessible to the organisms, until a disturbance (i.e. water turbulences, volcanic eruptions
occur and thus expose the nutrients back to the atmosphere or dissolve them into the water
column. This implies that these nutrients can constitute a limiting factor. Examples are the
phosphorus and sulphur cycles.
I.
NITROGEN CYCLE GENERAL OVERVIEW IN FRESHWATERS
The nitrogen cycle is a gaseous type. The main deposit of nitrogen is the earth’s atmosphere
(N2). Atmospheric nitrogen is combined (= fixed) with other substances which produce organic
compounds that are used by plants and animals (reactive nitrogen bonded to C, O, or H as in
NOx, NHx, organic N). Nitrogen is important and vital in proteins (messengers, receptors,
catalysts and structural components of cells and membranes and DNA and RNA molecules). In
aquatic systems, nitrogen supply is important in determining primary productivity and microbial
recycling of organic matter. When plants and animals die, their bodies are degraded by
bacteria (reducers) and thus produce ammonia (NH4+) another nitrogen compound. Other
bacteria transform the ammonia into nitrates (NO3-) or nitrites (NO2-), and other bacteria
degrade the nitrates releasing gaseous nitrogen (or denitrification), which return to the
atmosphere and complete the cycle.
Limnology, Biogeochemical cycles, compiled by Schmid-Araya, QMUL, UK
The atmosphere contains 79% of nitrogen, however the majority of organisms can not use (or
synthesise) this type of nitrogen. First there must be a change from nitrogen into nitrogen
compounds, which are used by plants to build up proteins. Animals consume nitrogen by
feeding on plant tissues where nitrogen has been fixed.
Examine Fig. A and follow the course of the cycle, looking at the numbers that are within the
cycle.
1. The main storage of nitrogen is the atmosphere as gas form N2. other sources include
streams and rivers entering lakes.
2. The nitrogen is transformed from gas form (N2) into nitrate (NO3-) in a process called
nitrogen fixation. There three basic ways of nitrogen fixation:
x Biological fixation: mediated by fixing bacteria that live freely on the soil, or are in the
nodules of leguminous plants (i.e. beans, peas, clover, Acacia trees). In freshwaters,
cyanobacteria (formerly blue-green algae) possess heterocysts that are specialised cells
where the nitrogen fixation occurs. Other freshwater organisms capable of fixing nitrogen are
photosynthetic and non-photosynthetic bacteria. In wetlands, non-leguminous angiosperms
(i.e. Alnus) and Azolla (an aquatic fern) that contain a symbiotic N2-fixing cyanobacterium.
x Atmospheric fixation: a physicochemical process that occurs when a lightning transform
atmospheric nitrogen into nitric acid. The acid is dissolved in the rain and precipitated in the
soil. Plants (and thus aquatic plants) can take it by absorbing water and other minerals
through the roots.
x Industrial fixation: the Haber &-Bosch process that is physicochemical based on the same
principles of the atmospheric fixation.
3. Nitrogen fixed by photosynthetic and heterotrophic microbes becomes part of the particulate
organic nitrogen pool. Dissolved nitrate (NO3-) is taken by photosynthetic organisms
(assimilative nitrate reduction). Here photosynthetic energy is used to reduce oxidised (NO3-)
or (NO2-) to the reduced nitrogen of the protoplasm (i.e. protein molecules). The animals
acquire their nitrogen compounds from feeding upon these photosynthetic and heterotrophic
organisms. The contribution of animals to the total nitrogen cycle is small, only under certain
circumstances their grazing activities can influence microbial populations and photosynthetic
organisms.
4. The oxidised form of nitrate (NO3-) is reduced to gaseous N2 or N2O by a process known as
denitrification. A great proportion of nitrates undergo loss or removal of nitrogen by the
action of the heterotrophic, facultative anaerobic bacteria ad fungi. denitrifying- bacteria
5. Another process (dissimilative ammonia production) reduces remaining nitrate to ammonia
(NH4+).
6. A great fraction of ammonia (NH4+) results from breakdown and mineralization of organic
matter (when all types of organisms die or when animals produce waste products such as
urea. This process is called amonification and is carried out by aerobic and anaerobic
bacteria. Phagotrophic protozoans (i.e. bacterivorous flagellates) can accelerate the
regeneration of ammonia by the consumption of bacteria.
7. Other bacteria will transform the ammonia (NH4+) to nitrites (NO2-), and then into nitrates
(NO3-) (nitrification). Streams and rivers transport other nitrates to the sea, where together
with nitrates derived from the ocean food webs proceed to form part of the ocean sediments,
and are lost from the lake/stream system.
The five special processes that take place in the nitrogen cycle can be summarised as:
x Nitrogen fixation (Fig A2): Transforming atmospheric nitrogen into nitrates. It can occur by
physicochemical processes and by nitrogen-fixing bacteria.
x Reduction of nitrates to ammonia (assimilative nitrate reduction & dissimilative ammonia
production) (Fig. A3, A5): Oxidised nitrate or nitrite is transform into reduced nitrogen of the
protoplasm.
Limnology, Biogeochemical cycles, compiled by Schmid-Araya, QMUL, UK
x
x
x
Ammonification (Fig. A6): The largest source of ammonia (NH4+) by breakdown and
mineralization of organic matter.
Denitrification (Fig A4): Transform nitrates into gaseous N2.
Nitrification (Fig A7): Biological oxidation of ammonia to nitrite and nitrate.
Fig. A. General nitrogen transformations in freshwaters.
Nitrogen in lakes is supplied by the other aquatic systems in the drainage basin (streams/rivers)
and the atmosphere and can be produced in situ via nitrogen fixation. Nitrogen can be lost from
the lake via outflows, however if this process does not occur then nitrogen in denitrified and not
stored in sediments. Denitrification increases with longer water residence and decreasing water
depths.
The nitrogen cycle will directly affect other nutrient cycles (i.e. phosphorus and sulphur).
Fig. B. Nitrogen transformations in the sediment-water interface within a lake (modified from Kalff 2002).
Numbering follows Fig. A. PON: particulate organic nitrogen, DON: dissolved organic nitrogen.
Limnology, Biogeochemical cycles, compiled by Schmid-Araya, QMUL, UK
II.
PHOSPHORUS CYCLE GENERAL OVERVIEW IN FRESHWATERS
The phosphorus cycle is a typical cycle of sedimentary type. The main deposit of phosphorus is
the earth’s crust or sedimentary rocks that intervene in the basic cycle in a reduced amount, as
a result of weathering. The cycle has no gaseous phase and it moves very slowly. Phosphorus
is a vital component of DNA, RNA and ATP, therefore necessary for all living cells. Phosphorus
is the most important ecologically element, and its deficiency tends to limit the productivity of
any region, particularly in aquatic systems. Phosphorus occurs in the biosphere almost
exclusively in a fully oxidised form (orthophosphate PO4-3).
Phosphorus enters the lake via runoff and also directly from atmospheric precipitation. Plants
(aquatic macrophytes, phytoplankton) incorporate phosphorus into their cells forming more
complex cells. Animals obtain their phosphorus by feeding on these primary producers. 90% of
the phosphorus in freshwaters occurs as (a) organic phosphates and cellular constituents in the
biota and (b) adsorbed to inorganic and dead particulate organic materials.
Figure C is a simplified description of the complete cycle in a lake. The numbers can be
explained as it follows:
1. A large portion of the phosphorus in freshwaters is bound to organic phosphates and cellular
constituents of organisms (bacteria, phyto- and zooplankton, fish) as well as detritus. When
the organisms die or excrete their waste products, microbes degrade the organic
compounds/waste products into a soluble reactive P or dissolved organic phosphates.
2. Another large portion of the phosphorus can be found within or adsorbed to organic colloids
or
3. Inorganic compounds (i.e. clay and minerals).
Phosphate is extremely reactive interacting with cations such as iron (Fe) and calcium (Ca+2),
that under oxidising conditions form the insoluble compounds that precipitate to the deep lake
sediments. Phosphate is also easily adsorbed to inorganic colloids, clays, carbonates and
hydroxides. Recent studies of phosphorus cycling in the trophogenic zone have shown that
exchange of phosphorus between its various forms is rapid and complex (Fig. C1). The organic
phosphorus of the seston is rapidly cycled (Fig. C2), while the dissolved organic and colloidal
phosphorus material is recycled much slower (Fig. C3).
4. Biological recycling and desorption/adsorption of the sedimenting organic phosphate occurs
at the tropholytic zone.
5. If water above sediments is oxygenated, an oxidized micro-zone is formed below the
sediment-water interface (0-5cm), below which sediments are extremely reducing. The
oxygenated micro-zone prevents the phosphorus to migrate up the water the water column.
Phosphorus moves into the sediments.
6. If the hypolimnion becomes anoxic (especially in eutrophic lakes) the oxidised micro-zone is
lost and phosphorus and ferrous iron are release when reducing conditions reach a redox
potential of +200mV.
7. Soluble phosphorus can accumulate in the anaerobic hypolimnia. When the autumn
circulation takes place, ferrous iron is rapidly oxidised and precipitates much of the
phosphate as ferric phosphate.
8. Movement of phosphorus from sediment interstitial water can be accelerated by physical
turbulences and by a process name bioturbation by the biota.
The phosphorus cycle seems simpler than the nitrogen cycle, but it is somehow more complex.
It depends on a fragile equilibrium, because phosphorus is not so widely distributed as the
nitrogen, and because the main pools conserve it strongly than the atmosphere does with the
nitrogen.
Limnology, Biogeochemical cycles, compiled by Schmid-Araya, QMUL, UK
Intensive agricultural practices exhaust rapidly the available phosphate pools. For example in
temperate climates 50 years of cultivating practices can reduce easily the available phosphate
in more than a third. This limits seriously the earth’s fertility. Agriculture can be more damaging
in tropical climates. Diverse land from the Middle East and India were fertile sometime ago, but
impoverished due to the exhaustion of phosphates. Man has learned to use natural resources
such as fish and animals faeces as fertilisers in order to supply the soil with phosphates.
However, modern man extract the non-soluble phosphates and use as fertilisers (that means
utilises phosphates that are “artificially washed”).
Another crucial contribution are sewage which carry domestic detergents that contain great
amount of phosphates and increase the speed of eutrophication in lakes, ponds, streams and
rivers.
Fig. C. General phosphorus cycle in freshwaters.
Both the extraction of phosphates from sedimentary rocks plus the use of fertilisers increase the
amount of phosphorus in the cycle and at the same time the speed of the phosphorus cycle.
All biogeochemical cycles are well related to the hydrological cycle and to the energy flow in the
biosphere. Then, water constitutes the mean of transport and circulation of nutrients. Solar
energy absorbed by the plants, supply the energy necessary for the process of “pumping” and
of transpiration needed to maintain the movement of nutrients within the cycle.
Bibliography
Kalff, J. Limnology. Inland Water Ecosystems. Prentice Hall, New Jersey, 592pp.
Wetzel, R.G. 2001. Limnology. Lake and River Ecosystems. Academic Press, California, 1006pp.