Nutrient cycling in ecosystems

Nitrogenase

Enzymes involved in reactions leading to formation of ammonium compounds from dinitrogen gas, as occurs in nitrogen fixation.

Exoenzymes

Enzymes that are secreted from cells so that they act on substrates external to the plasma membrane.

Mucigel

Formed in rhizosphere when root exudates become mixed with microbes and mineral particles.

Nitrogen entry into ecosystems

Gaseous nitrogen is very inert. Needs to be converted into more reactive forms before organisms can use it. - Most nitrogen enters ecosystems though nitrogen fixation. - smaller inputs of ammonia, nitrate and nitrite, nitrogenous gases and ammonium ions in dry (particulate) and wet (precipitation) deposits from the atmosphere - however, combustion of fossil fuels and organic matter releases increasingly large amounts of nitrogenous gases into the atmosphere. - rock weathering adds a tiny fraction. - organic inputs to individual ecosystems form outside the system (allochthonous inputs) e.g. leaf littler in a woodland street can be very important. - since the Industrial Revolution, the Haber-Bosch process (used in manufacturing fertilisers) has been an increasingly important source of available nitrogen in the environment. In natural ecosystems, plants frequently rely on nitrogen fixed by microorganisms. Nitrogen can never be a limiting factor for nitrogen fixation because it's present in such high levels in the atmosphere. Therefore, shortages of available nitrogen in ecosystems occur because of low levels of nitrogen fixation coupled with high outputs. Nitrates are very soluble anions and can be easily leached from soils; they can also be lost from soils by the processes of: - denitrification (converted back to ammonia) and - volatilisation (nitrogen is lost to he atmosphere as gases or vapours). Assimilation involves the uptake of: - nitrate - ammonium ions or - dissolved organic nitrogen-containing compounds (DON) from soil or water an their incorporation in amino acids, proteins, nucleotides and other nitrogen-containing cell components in primary producers. Nitrate is produced form ammonium ions by the process of nitrification. - nitrates needs to be reduced again (nitrification in reverse) before they can be used in biosynthesis. Heterotrophic microorganism break down nitrogen-containing compounds to release ammonium ions, which can be absorbed and immobilised. Ultimately, the mineralisation by detritivores and decomposers releases ammonium ions into the soil or water. If plants are actively metabolising at the time of this release, ammonium ions may be taken up as fast as they are released. Alternatively, ions can be moved through further immobilisation and mineralisation cycles within the decomposer subsystem.

Nitrogen - importance

Important component of proteins, which make up muscle tissue and enzymes. Rubisco - critical component of photosynthesis. - in effect, controls the carbon cycle. - less Rubisco is synthesised when nitrogen is in short supply = lower productivity. - rate-limiting factor in GPP. Nitrogen availability is the most important factor in limiting primary productivity in the vast majority of ecosystems.

Nitrogen losses

In addition to being consumed by herbivores and parasites, nitrogen and other nutrients are lost form plants: - through senescence and death - leaching from tissues and - exudation from roots into the soil. Nutrient-retention processes within plants are important mechanisms of nitrogen conservation.

Nutrient

Includes a variety of vitamins and organic compounds that organisms require - some of which they can manufacture themselves and some of which need to be obtained from external sources.

Nitrogen fixation

Involves the reduction of nitrogen gas to ammonia with hydrogen gas produced as a by-prodcut 8H+ + N2 + 8e- (arrow) 2NH3 + H2 Abiotic fixation occurs in relatively small amounts as a result of lighting and shortwave light. Biological nitrogen fixation (BNF) is carried out by microorganisms and is the most important process to producing nitrogen in a useable form. - occurs when atmospheric nitrogen is converted into ammonia by bacterial nitrogenase enzymes. Nitrogen fixation rates are directly proportional to temperature (they are higher in warmer temperatures). Nitrogen-fixing activity is low when soil inorganic nitrogen is high. In nitrogen-enriched soils, the roots of species that are normally well-nodulated have few nodules and nodules may break down after fertiliser has been applied - there must be a cost to the plant in maintaining the relationship. Once nitrates are freely available in soil, the nitrogen-fixing bacteria would be classified as parasites form the point of view of the host plants, taking carbohydrates and giving nothing in return. However, useable nitrogen is generally a limiting factor for production in ecosystems, so nitrogen-fixers are important in supplying the nitrogen required for growth and maintenance in an ecosystem. The triple bond in N2 is very difficult to break and takes a lot of energy. - microorganisms that fix nitrogen are mostly heterotrophs, so need a source of organic carbon to supply the energy needed to break the nitrogen bonds. - exceptions are cyanobacteria, which are nitrogen-fixing phototrophs Nitrogen fixation is also an anaerobic process. - in most bacteria the nitrogenase enzymes are very susceptible to destruction by oxygen. - however, fixation occurs in both aerobic and anaerobic environments because enzymes are protected from oxygen by either spatial or biochemical means - rhizobial root nodules contain a pigment called leghaemoglobin, which absorbs oxygen and so regulates oxygen concentrations at the enzyme site (biochemical protection). - many phototrophs have specialised cells called heterocysts that protect nitrogenase from denaturation by oxygen produced in neighbouring photosynthetic cells. - free-living chemoautotrophic nitrogen-fixer Clostridium spp. which is common in natural, wet soils. Generally, nitrogen fixation isn't favoured by low pH, but can be important in upland bogs where nitrogen is limited.

Grazing lawns

Large areas in prairie (>400 m2) favoured by bison, with shorter and more floristically diverse vegetation.

Allochthonous

Matter originating from outside the ecosystem and imported into it. Such inputs are often an important source of food for detritivores.

Leaching

Nutrients can be leached form live plant canopies, enriching the precipitation as it passes through the canopy in stem flow and through fall. About 15% of the annual nutrient return to the soil from aerial plant parts is in the form of leachates. Rain dissolves nutrients on leaf and stem surfaces and carries them to the soil as through fall or stem flow. Leachates tend to contain higher concentrations of the more soluble and abundant nutrients, and leaching is most rapid when leaves are first exposed to rain, the rate declining with time. There is no evidence of clear adaptations to minimise loss from leaching. Just as nutrients may be lose through leaching, they may also be absorbed by plant surfaces from gases, dusts and solutions that are absorbed by or deposited on plant surfaces - canopy exchange.

Phosphorus in soils

Phosphors is a macronutrient that can occur in organic or inorganic form and in a range of states, which are increasingly less soluble and therefore unavailable to plants. Phosphorus 'pools' in soil: - soluble inorganic (available for plant uptake) and organic compounds in soil, from which phosphorus is released by exoenzymes - weakly adsorbed inorganic phosphate, which could become available for plants relatively easily. - insoluble calcium phosphates in calcareous and alkaline soils of arid and semi-arid regions and iron or aluminium phosphates in acidic soils. - phosphates strongly adsorbed and/or occluded by various hydrous oxides of iron and aluminium. - fixed phosphates of silicate minerals such as apatite - insoluble organic forms of phosphorus in the soil biomass (as undecomposed plant and animal residue or as part of hummus).

Root exudates

Plants influences nutrient availability in the rhizosphere by exuding solutes of both high and low molecular weight into the soil from a zone just behind the root rip. High molecular weigh components are mainly: - exoenzymes (enzymes secreted by cells to the outside) - mucilage (a gelatinous material composed of polysaccharides) Low molecular weight components include: - organic acids - sugars - phenolics - amino acids In the natural state, the root exudates become mixed with microbes and mineral particles to form mucigel.

Wallows

Shallow depressions, about 4 - 6m across where the soil is bare and impacted, where Bison habitually scratch and roll.

Grazing patches

Small areas in prairie (20 - 50 m2) favoured by bison, having shorter and more floristically diverse vegetation.

Heterocysts

Specialized cells in which anaerobic conditions are maintained and where nitrogen fixation occurs in organisms such as cyanobacteria.

Take up

Taken up by: - plants from the soil or water - animals from food. Move with energy through ecosystems. However, energy flows through ecosystems; unlike nutrients, it can't be recycled. - whereas energy is lost form systems as heat, chemical elements can't be lost, incited they cycle within and between ecosystems. Whereas the structural electments (H, C and O) tend to cycle globally between ecosystems (e.g. released into the atmosphere as gaseous O2, CO2 or water vapour) nutrient elements cycle mainly within ecosystems. One important difference between the nutrient and carbon cycles is the movement of material from decomposers to primary producers via soil or sediments. When decomposers break down organic matter, they release inorganic nutrients such as N and P from organic compounds. Plants can then absorb the inorganic nutrients. Material in the carbon cycle passes via respiration in the decomposer subsystem (releases CO2) to the atmosphere which is taken up by primary producers. Consumer outputs - primary producers. - nutrients (mainly N) occurring in inorganic form, such as ammonium ions (NH4+) in the waste products of consumers - can be taken up directly by plants - short circuit in nutrient cycling.

Biological nitrogen fixation (BNF)

The conversion of atmospheric nitrogen gas to ammonium compounds by living organisms such as cyanobacteria (blue-greens) and members of the bacterial genus Bradyrhizobium.

Nitrogen fixation

The conversion of gaseous nitrogen into ammonia or ammonium compounds, either through abiotic means, for example lightning, or biotic means, for example symbiotic and free-living bacteria.

Mineral nutrients

The elements that make up the biomass of organisms, excluding C, H and O. Macronutrients are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). The micronutrients are iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), chloride (Cl), molybdenum (Mo), nickel (Ni), and cobalt (Co). Principle reservoirs of nutrients may be gaseous (e.g. N) or rock (e.g. P)

Leghaemoglobin

The pigment produced by Rhizobial cells in root nodules to maintain anaerobic conditions around the nitrogenase enzymes.

Resorption

The process by which nitrogen and other nutrients may be withdrawn from senescing tissues into the perennial biomass before leaves are shed; otherwise known as internal recycling.

Volatilisation

The process by which substances are lost from the soil by conversion to gases, which escape to the atmosphere.

Nitrification

The process in which ammonia or ammonium compounds are oxidized to nitrite and nitrate, yielding energy for decomposer organisms. Two groups of microorganisms are involved in nitrification. Nitrosomonas oxidizes ammonia to nitrite and water. Subsequently, Nitrobacter oxidizes the nitrite ions to nitrate.

Denitrification

The process in which nitrates are reduced to gaseous nitrogen or oxides of nitrogen. This process is used by facultative anaerobes. These organisms flourish in an aerobic environment, but are also capable of breaking down oxygen-containing compounds (e.g. NO3-) to obtain oxygen in an anoxic environment. Examples include the bacteria Pseudomonas. (converted back to ammonia)

Mineralisation

The release of cations or anions from primary litter (plants, consumers and their waste products) and secondary litter (the organisms of the decomposer subsystem.

Canopy exchange

The two-way movement of material between leaves and the wider environment, either through leaching by rainwater or absorption of nutrients dissolved from dust etc. that is deposited on leaves.

Assimilation

The uptake and retention in biomass of carbon compounds by organisms (primary producers and consumers).

Immobilisation

The use of dead biomass and waste products by microorganisms and detritivores.

Stemflow

Water that flows down stems of plants (from rain and snow, etc.).

Thermophilic

'Heat loving', for example, thermophilic bacteria that thrive at high temperatures, such as hot springs.

Bison have three effects on nutrient cycling processes and patterns of nitrogen availability in the tallgrass prairie ecosystem

1. The rate of nitrogen cycling is increased because bison consume tough, recalcitrant plant biomass and return labile, easily mineralised forms of nitrogen (such as urea) to the soil. 2. The loss of nitrogen through combustion is greatly reduced because grazing prevents the build-up of large amounts of fuel in the form of plant detritus and inhibits the spread of fire by the increased habitat patchiness. 3. Bison affect the amount and quality of plant litter returned to the soil. Persistent and repeated grazing of plant shoots stimulates the reallocation of nitrogen and carbon resources to them. Consequently, root biomass and productivity are 30% and 20% lower in grazed than in ungrazed areas. The nitrogen content of roots is also higher and the C : N ratio is lower in grazed areas, which reduces microbial immobilisation and thereby enhances nitrogen availability. §

Closed nutrient cycle

A cycle in which nutrients are held tightly within an ecosystem, leading to efficient cycling (see also the leaky nutrient cycle).

Ammonification

A reaction in which organisms break down amino acids and produce ammonia (NH3).

Sorbed

A term embracing both adsorped and absorbed substances.

Garrigue

A type of scrubland composed of low, soft-leaved shrubs (e.g. Cistus spp.) found on dry limestone soils around the Mediterranean Basin.

Leaky nutrient cycle

An inefficient nutrient cycle, in which nutrients are lost from the ecosystem. Can be replaced from global resources.

Phototrophs

An organism that uses energy from light to manufacture organic compounds by photosynthesis.

Phosphorus cycling

After carbon and nitrogen, phosphorus is the next element that gets the most attention from ecosystem ecologists because: - it plays a key role in the biochemistry and physiology of living organisms (e.g. a component of ATP, nucleic acids and membranes). - second most important plant nutrient after nitrogen as measured by effects on limiting NPP in terrestrial and freshwater ecosystems. Tropical ecosystems are particularly susceptible to phosphorus limitation because of their rapid rates of weathering. Unlike carbon and nitrogen, phosphorus has no significant gaseous phase - the atmospheric component of phosphorus cycle is very small. The little phosphorus that is present in the atmosphere enters is small quantities as windblown dust form soil or sea spray and returns to the lad or sea dissolved in rainwater or as a dry deposit. Such inputs can be important in some particularly phosphorus-deficient ecosystems, such as deserts. Phosphorus is released from the phosphorus-containing mineral apatite by carbonic acid generated from soil respiration. The phosphorus produced may be taken up by plants and microorganisms or it may be adsorbed or precipitated. Consequently, phosphorus is bound up by the process of sedimentation in the oceans, which leads to rock formation, and is released through either: - natural weathering - anthropogenic activity such as mining. The major losses of phosphorus from ecosystems are usually as particles through erosion and run-off. Terrestrial ecosystems rely on weathering of rocks for their inputs of phosphorus. Therefore the rates of inputs are very slow as are the rates at which soil phosphorus becomes available to plants, and the overall timescales involved in the phosphorus cycle are much longer than for the nitrogen or carbon cycles. Unlike nitrogen, phosphorus isn't oxidised or reduced in soils in the same way as nitrogen. A high proportion of the phosphorus present in soils is in a form that is unavailable to plants. Phosphorus cycle is closed compared with the nitrogen cycle in natural ecosystems. In closed ecosystems, inputs and outputs are less significant.

Nitrification

Ammonia produced by fixation or mineralisation may be converted to nitrate by nitrifying microorganisms. 2-step process that is mainly carried out by two genera of chemoautotrophic bacteria: - Nitrosomonas - Nitrobacter They use energy from these reactions to fix CO2 Nitrosomonas NH4+ + O2 (arrow) NO2- + 4H+ + 2e- Nictrobacter NO2- + H2O (arrow) NO3- + 2H+ + 2e- Most important factor controlling the rate of nitrification is the supply of ammonium ions. Environmental conditions required for maximising nitrification rates - high pH (7-8) - 15 degrees C temperature optimum. - most nitrifies are aerobic. Acid soils are especially limiting to bacteria carrying out the first step, but if mineralisation release enough NH4+ and OH-, the pH increases locally and provides a more favourable environment for the first step. The second step is slightly favoured by acid conditions. As the products of step 1 build up, the pH falls and the second step proceeds. High concentrations of the substrates for these reactions lead to fast nitrification. Limitation of nitrate production by nitrification in winter occurs because water logging and cold conditions inhibit the active of nitrifying bacteria. Nitrate is also lost more readily than ammonium from soils by leaching because of the relative shortage of positively changed binding sites.

DON

Dissolved organic nitrogen found in soils (for example amino acids).

Nitrogen mineralisation (ammonification)

Many soil microorganisms, including bacteria and fungi, mineralise organic, nitrogenous compounds. Key limiting factor is the supply of DON which is released from insoluble organic matter by decomposer microorganisms. Mineralisation involves removing amino (NH2) groups from amino acids which is often called ammonification. - ammonia is subsequently reduced by acquiring a proton, which releases ammonium and hydroxyl ions NH3 + H20 (arrow) NH4+ + OH- C : N ratio of soil organic matter (SOM) is important because mineralisation is an energy-requiring process. Ammonium ions are only released if the supply of carbon is limiting (low C : N ratio of about 25 : 1) Most ammonium ions are immobilised within the microbial biomass at higher C : N ratios, where nitrogen is limiting. Ammonium ions released by mineralisation can be: - taken up by plants - absorbed and retained on negatively charged clay particles or SOM, - volatilised or - used in the nitrification process. Volatilisation is generally only a significant process in improved agricultural soils (with very high nitrogen inputs from fertilisers or manure). Ammonium ions released into the soil before the spring growth starts are usually relatively immobile. In contrast to nitrate ions, positively charged ammonium ions have a strong attraction for soil. However, in soils containing very high amounts of sand and low organic matter, ammonium ions, like nitrate ions can be leached.

Denitrification

In addition to leaching, denitrification is one of the major causes of nitrogen loss from ecosystems. Production of dinitrogen gas (N2), dinitrogen oxide or nitrous oxide (N2O) and nitrogen oxides (NO3) by the reduction of nitrate (NO3-) and nitrite (NO2-) Acid rain (acid precipitation) has been particularly associated with the increased generation of NOx gases since industrialisation. Nitrogen returned to the atmosphere may have been denitrified either abiotically or biotically. Abiotic All 3 types of gas - N2, N2O and Nix are released through burning. - slow burning (at cooler temperatures) produces relatively more NOx - hot flames generate a higher proportion of N2. - in tropical ecosystems, release of nitrogen through burning is extremely important and is equivalent to 12-46% or more of the total mass of fixed nitrogen lost each year. Biotic Result of activity by common facultative aerobic bacteria (e.g Clostridium, Bacillus app.) - these organisms behave as heterotrophs in aerobic conditions but, when soil conditions are anaerobic, they switch forming oxygen as the terminal electron acceptor in respiration to using nitrate. - consequently, oxygen availability can strongly influence the rate of denitrification. - however, it is the concentration of oxygen at the cell's surface that is most important, which may be significantly different from eh bulk soil concentration. - rates of denitrification reactions are also greatly affected by factors such as the kind of substrate, the ambient temperature, and the system pH. - high concentrations of readily available carbohydrates favour denitrification (bacteria responsible for denitrification are heterotrophs) After oxygen, denitrification requires a good source of nitrate, but as nitrification is primarily an aerobic process, low-oxygen conditions that are optimal for denitrification frequently limit nitrate supply. Denitrification is highest in ecosystems that: - receive nitrate from outside the system - have an aerobic zone above an anaerobic zone, or - go through cycles of flooding and drainage Denitrification processes are slower at low temperatures, yet active denitrification occurs in environments as divers as the polar regions, oceanic upwelling zones and hot springs. However, as in the case of burning, the ambient temperature affects the relative proportions of N2 and N2O produced. Salt marshes and peat bogs are typical, naturally occurring examples of rapid denitrification environment. Clay soils after heavy rain are prone to it. Sewage is an anthropogenic, anaerobic ecosystem, and sewage treatment works manipulate environmental conditions to maximise the rate of denitrification. Denitrification in coastal waterways in densely populated parts of the world can permantely remove nitrogen form the system as nitrogen gas and, therefore can help counteract the eutrophication process.

Allelopathy

Mechanism by which some plants inhibit the growth of competitors. This involves the release of toxic substances into the soil from roots, leaves or litter.

Diazotrophs

Microorganisms that fix nitrogen. Free-Living nitrogen-fixers are widespread in the soil, on the surfaces of plants, especially roots, and in marine and fresh water. Several types of organisms, including some autotrophs (such as legumes and some lichens) and some animals (such as termites) have formed symbiotic associations with diazotrophs. The majority of nitrogen is fixed by symbiotic, heterotrophic bacteria in the rot nodules of higher plants in terrestrial environments.

Mycorrhizas and phosphorus

Mycorrhizal also play a role in phosphorus cycling. - the fungal partner can take in and store phosphorus, which is important because phosphate diffuses very slowly through soil to the nutrient-depleted zone around the root. - Hyphae growing through the soil increase the volume of soil explore compared with the volume explored by a root without mycorrhizas - it is more efficient to invest a given amount biomass in many fine hyphae rather than fewer thicker roots because the volume of soil exploited by the hyphae is much greater than that explored by even the finest roots. - mycorrhizas have additional adaptations for phosphate uptake, including their ability to produce a range of phosphatase exoenzymes. In tropical forests, which are particularly phosphate-limited, mats of mycorrhizal roots form in the litter layer and produce phosphatase that release phosphate from inorganic matter.

Nutrient conservation and recycling within plants

Nitrogen and other nutrients may be withdrawn from senescing tissues into the perennial biomass before leave are shed - internal recycling or resorption. Resorption of nitrogen is an important conservation mechanism that allows plants to use the same nitrogen repeatedly. Also means that litter has a higher C : N ratio than living leaves (living leaves should decompose more easily as they have a lower C : N ratio than the litter) Plants resorb about 50% of their nitrogen and phosphorus form leaves before they are shed. All species are fairly similar in their resorption efficient, which doesn't appear to correlate closely with any one environmental factor. However, resorption efficiency seems to be inhibited by water stress.

Chelates

Organic molecules that can bind to metal ions. E.g. carrots secrete chelates which combine reversibly with iron from insoluble iron phosphate complex. The phosphate released diffuses into roots.

Chemoautotroph

Organism that uses energy derived from the oxidation of inorganic chemicals (rather than from light) to synthesise organic compounds.

Assimilation, immobilisation and mineralisation of phosphorus

Phosphate is the main form of available phosphorus in soils, but the precise form of phosphate depends primarily on the pH of the soil solution. It matters which form of phosphate predominates because the more highly charged the species of phosphate, the less mobile and therefore less available to plants they are ph 4 - H2PO4- ph 10 - H+ + HPO42- pH14 - 2H+ + PO43 Situation is complicated by the fact that at low pH, aluminium, iron and manganese are also quite soluble and react with dihydrobgen phosphate to form insoluble compounds. Furthermore, phosphorus can be sorbed onto the surfaces of clays and oxides of iron and aluminium. Can also bind with soluble minerals such as iron oxides to form insoluble precipitates. Therefore, highly weathered soils, such as those found in the tropics (and very rainy areas of upland Britain) are much more phosphate-limited than temperate soils in which clay minerals predominate. In calcareous soils, calcium phosphate precipitates out, which lowers phosphate availability to plants. Consequently, very little of the soil phosphorus is soluble at any particular time. As microorganisms break down organic matter containing phosphorus, the amount that is mineralised into a soluble form available for plant uptake depends on the C : P ratio of the SOM. - if it is high (300 : 1 or more - there is a small amount of P relative to the amount of C), the microorganisms incorporate all the phosphorus into their own biomass. - they also need to take up extra inorganic phosphorus to take advantage of the carbon in the SOM, thereby immobilising the phosphorus in their own biomass. - however, is the C : P is lower (200 : 1 or less), the excess phosphorus in the organic matter is not used by the organisms and is released through mineralisation. Because there is little phosphorus entering the ecosystem from outside (via the atmosphere or weathering) the recycling of phosphorus in organic matter is one on the main controls of the availability of phosphorus to plants. The turnover of phosphorus pools is relatively rapid, which is important for the maintenance of ecosystem productivity.

Mucilage

Plant root exudates from a zone just behind the root tip; solutes include exoenzymes, polysaccharides and low molecular weight components e.g organic acids, sugars, phenolics and amino acids. Protects root tips from desiccation and abrasion, and can make elements more available by affecting soil structure, lowering the pH near the root surface, mobilising organic sources of nutrients (e.g. exoenzymes can included acid phosphatase enzymes that hydrolyse organic phosphorus) and providing sources of carbon and energy that stimulate soil microbe activity.

Nitrogen assimilation

Plants that form symbiotic associations with nitrogen-fixing microorganisms can assimilate the ammonium ions (NH4+) produced directly. However, some plant and microbial species take up nitrate ions (NO3-), and some take up both. Therefore, nitrate production is essential to the functioning of many ecosystems, even through there is a considerable metabolic cost to organisms because nitrate production requires a high input of energy. Plants can take up nitrogen as DON (e.g. amino acids) but rarely have access as microorganisms compete more effectively for it. Nitrate ions (NO3-) are produced in the soil by the oxidation of ammonium ions (NH4+) in the top-step process of nitrification. - Ammonium is the predominant form in low pH environments with typically low nutrient availability - Nitrates are predominant in more alkaline environments with higher pH and nutrient concentrations. Very specialised plants grown in the environments at the extremes of this range. They are usually adapted to absorb either ammonium ions or nitrate ions and consequently exploit different pools of nutrients. These differences in nitrogenous resources and physiological mechanisms lead to very different patterns of nutrient dynamics in these contrasting ecosystems. Ecosystems vary greatly in the promotions of soil nitrogen that is nitrified. - only 4% of soil nitrogen is nitrified in coniferous forest - nearly 100% of it can be nitride in tropical forests.

Rate of nutrient cycling and retention

Rate of nutrient cycling - critical when nutrient supply limits plant growth. Equally as important - capacity of an ecosystem to retain nutrients released by decomposers or entering from outside the system. - uptake by mycorrhizal and other fungi and bacteria - soil or sediments (clay and organic matter bind ions so they are not leached by percolating water and lost from the system). - some ions bind only loosely by soil (e.g. nitrate NO3-) and are readily leached - other ions such as phosphate (PO4-) and ammonium (NH4+) are not leached because they bind highly. Nutrients may be retained at different tropics levels of autotrophic or heterotrophic biomass for longer periods to time, often as a result of internal recycling or storage in perennial biomass. Nutrient cycling involves nutrients alternating between organic and inorganic states. The release of carbon from organic compounds in decomposition is linked to the cycling of other nutrients. - proteins, amino acids, enzymes and other carbon-based molecules release mineral ions such as nitrogen (as ammonium), phosphorus (as phosphates), iron or manganese when they are broken down - *so the decomposition sequence for carbon is an integral part of almost all other major mineral cycles* Because different nutrients have different cycling pathways, an ecosystem may be closed for one nutrient and leaky for another. An ecosystem may also be closed during one phase of its development and leaky during another. E.g. forests tend to be very leaky after catastrophic deforestation (natural storm or human) but become closed within two to three years or regeneration. Some nutrients are held very tightly and are recycled many times become being lost as outputs from the system. Essential nutrients tend to be cycled more tightly and lost less easily from ecosystems than non-essential elements.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)

The most abundant protein in the world. It is the enzyme responsible for converting carbon dioxide into sugar in photosynthetic organisms.

Throughfall

Water that drops from the canopy of plants.