92 terms

BISC 302: Unit 4 - SimU Text-Nutrient Cycling


Terms in this set (...)

Small plants and photosynthetic bacteria that live and drift in the water of oceans and lakes.
They are usually microscopic.
They photosynthesize like terrestrial green plants and are a primary food source for many aquatic animals.
There are many different kinds including diatoms, dinoflagellates, and cyanobacteria.
They are ensitive to light and nutrient availability.
Under certain conditions, some populations grow exponentially to form blooms, such as red tides in ocean waters.
Red tides can be toxic to marine animals and pose a health risk for people eating local shellfish.
Red Tides
A red, orange, or brown coloration of water caused by a bloom, or population explosion of algae (A phytoplankton); many cause serious environmental harm and threaten the health of humans and animals
Aquatic, photosynthetic bacteria.
They are also known as blue-green algae because of their superficial resemblance to green algal cells.
Unlike algae, these are prokaryotes, lacking internal organelles and a discrete nucleus.
They are unicellular, but often form large colonies. Are important primary producers in both freshwater and marine ecosystems, and many can fix atmospheric nitrogen either as free-living organisms or in symbiosis with other species.
Have been around for along time: over 3.5 billion years according to the fossil record.
Effects of Algal Bloom

(aka Blue-green algae)
(1) Massive algal blooms such as these can choke and stagnate water, establishing a barrier to the exchange of gasses between the lake and the atmosphere. Often the algal cells die en masse and in turn, increase the decomposition rate.
Both of these effects decrease the amount of dissolved oxygen.

(2) A big drop in visibility in the water. Like in Lake Victoria, vertical visibility was estimated at about 5 m in the 1930s and it has since reduced to less than 1 m in many areas of the lake.
Literally means "non-biological".

These factors that affect living organisms include physical factors such as:
- Weather
- Soil structure
- Water availability
- Light availability
- Altitude.
As well as chemical factors such as:
- pH
- Available nutrients and minerals.

All species have limited tolerances to these factors.
Chemical elements and compounds that are essential to the health and survival of an organism.
These chemicals constitute the proteins, carbohydrates, cellulose, nucleic acids, and other compounds that provide an organism's structure and physiology.
Their concentrations within an ecosystem determine which species can live there and in what abundance.
Nutrients required by all organisms in relatively large amounts.
Examples include carbon, nitrogen, phosphorus, sulfur, oxygen, and hydrogen.
The basic component of all living organisms, thus "organic compounds" are defined as those that contain this component.
The process by which autotrophic organisms use solar energy to convert water and carbon dioxide into carbohydrates and oxygen.

The reaction can be summarized as:
6 CO2 + 6 H2O → C6H12O6 + 6 O2

(C6H12O6 represents the simple sugars that store the energy captured in this process.)
Component/ Nutrient particularly important for cell and tissue structure in both plants and animals.
Most frequently, it is fixed from the atmosphere via photosynthesis.
Green plants convert this atmospheric component into carbohydrates, proteins, and fats.
Some organic compounds like sugars are used to store energy while other like proteins are used to build tissue.
Nitrogen and Phorphorus
Two essential nutrients for all plants, animals, and microorganisms.
These elements are contained in a wide variety of organic compounds that are required for life, including amino acids (and thus proteins) and nucleic acids (DNA and RNA).
One of the also occurs in ATP (adenosine triphosphate), a vital molecule involved in cell energetics in all living organisms, from bacteria to humans.
Sulfur is another important component of amino acids and proteins, particularly enzymes.
An important component of amino acids and proteins
(adenosine triphosphate)
Plants capture solar energy via photosynthesis; animals capture chemical energy via the oxidation of their food. Before that energy can be used, it is transformed into a form that can be easily transported within and between cells. This is that universal, chemical energy carrier.
It is often referred to as the 'energy currency molecule' of the cell.
Since it is required by all organisms, phosphorus in the form of phosphates is an essential nutrient for all plants and animals.
Essential nutrients are required in much smaller amounts.

Examples include zinc, iodine, potassium, and iron.
The vitamins that humans require in our diets include many of these essential nutrients.
Limited nutrients
In many ecosystems, particular nutrients are in short supply.
Lack of availability of these limits how fast plants and animals can grow and reproduce.
Many terrestrial and aquatic ecosystems are nitrogen- and/or phosphorus-limited.
When farmers, foresters, gardeners, and fisheries managers apply fertilizers, they do so to stimulate growth of their target species by increasing the amounts of these potentially limiting nutrients.
(Ecological) Community
The association of species within a defined area that actually or potentially interact with each other.
The nature of the interactions between those species and the influence they have on each other bind this together and determine its properties.
The interactions include predation, competition, and symbiosis.
Lake Victoria
Increased nitrogen and phosphorus in the lake water resulted from deforestation and intensified agriculture throughout the drainage basin.
Although the continuing algal bloom in the lake is a logical consequence of excess nutrient loading, other factors, such as the aforementioned introduction of the Nile perch, have also been implicated.
Lac Blue and Lac Clair
In the Lac Bleu simulation, an increase in the quantity of nitrogen entering Lac Clair produced an algal bloom. As a result, the amount of decomposing algae increased and the supply of oxygen in the water decreased, subsequently causing trout in the lake to go extinct.
The rate that material or energy moves from one pool to another.
Is considered a vector, because it describes both the rate material moves and the direction of movement.
When negative it would suggest that the movement is in the opposite direction, which is often confusing and can be nonsensical. So it is usually better described as a separate process.
For example, consumption (eating food) and excretion (eliminating waste) can both be represented as nitrogen ______, but they are best described as separate, rather than representing excretion as consumption with a negative sign.
Trees take up nitrogen through their roots and use it to construct proteins and other organic compounds they need to grow. Ecologists often describe this movement of material as a ____ , measured as mass per unit time.
To ecologists, this is the amount of material located in a definable compartment.
This compartment can be a specific ecosystem or subsystem and is often expressed in units of mass.
They can also refer to specific forms of an element. Thus, we can refer to the ___ of nitrate in a forest soil or ___ of ammonium in a lake.
- Both the location and the mass of the material are described.
Pool Inputs and Outputs
Hopefully, you will notice that when the input is greater than the output, the amount stored in the pool increases. Conversely, when the input is smaller, the pool decreases.

This relationship is formalized mathematically as follows:

Δ Storage = Inputs - Outputs

(where Δ Storage indicates the change in the amount of material stored in a pool, which in this case is the tree's pool of nitrogen. Δ Storage is positive when the tree accumulates nitrogen and negative when it loses nitrogen.)
Simple Nutrient Cycle
Trees draw nitrogen from the forest soil through their roots, and return nitrogen to the soil when they drop needles or when they die and decompose. The flux of nitrogen from the soil to trees and back again defines a simple nutrient cycle.
One of the largest potential sources of inorganic nitrogen ~78%
Through a process called nitrogen fixation (discussed in more detail in the next section), nitrogen from the this is added to the soil nitrogen pool; thus, fixation is an input. Conversely, denitrification is an important output that removes nitrogen from the soil pool and returns it to the atmosphere.
Nutrient and Biogeochemical Cycles
Includes all of the processes by which nutrients move within and between the physical environment and living organisms, and from organism to organism.
In contrast to energy, which flows in one direction through biological systems, nutrient atoms and molecules are cycled between components of those systems.
A nutrient may be reused many times before being lost from the system.
Are pathways through which nutrients move between the biotic and abiotic components of ecosystems.
Are closed system, on a global scale, meaning they have no input or output of nutrients.
The processes involved in chemical transfers because the same nutrients are constantly reused within the biosphere.
The overall rate can vary and depends on relative pool and flux sizes.
The "living" zone on the Earth.
It is the total sum of all living organisms, also known as the 'biota', i.e. all of the plants, animals, and microorganisms that live on the planet.
One of four major, interacting components, the others being the lithosphere (rocks and sediments), the atmosphere, and the hydrosphere (water).
Global biogeochemical cycles involve all four of these components.
Tight Nutrient Cycling
When the rate of cycling between internal compartments is high (e.g., uptake and decomposition rates are large) compared to the fluxes in and out of the system (e.g., fixation and denitrification rates are small).
In this case, the average nitrogen atom is cycled between the two compartments many times before it leaves the system.
A process that acts to disrupt a population, community, or ecosystem by changing abiotic or biotic conditions, the availability of resources, or interactions between individuals of the same or different species.
Usually increase mortality rates and interfere with reproduction.
The temporal and spatial scale of these vary greatly. Catastrophic events such as floods, fires, hurricanes, and volcanic eruptions can cause widespread and significant changes.
Finer-scale ones are exemplified by animal burrowing, rain showers, and seasonal leaf drop from trees. Communities subjected to this of intermediate severity and frequency can have higher levels of species diversity compared with undisturbed or overly disturbed communities.
The process by which water vapor in the atmosphere condenses and falls to the Earth.
Can occur as rain, snow, sleet, hail, or water vapor condensation (i.e., dew).
The term can also be used for the amount of water that falls over a given area in a given time.
Evaporation of water from plants
Most readily occurs through the stomata of leaves, stems and flowers.
Draws water up the plant from the roots, along vessels in stems and trunks, and to the leaves.
Several key functions include supplying water for photosynthesis, acting as a cooling mechanism, and transporting nutrients to plant tissues.
The combined movement of water from the Earth's surface to the atmosphere via transpiration and evaporation.
The combined flux of water from the Earth's surface to the atmosphere via transpiration and evaporation.
The water that falls through the forest canopy.
It is often distinguished from precipitation falling directly on ground or water surfaces because it can wash particulates, including nutrients, from the plant's surface. As a result, its chemistry is often different than that of the original precipitation.
Some of the nutrients that have settled in the plant canopy are not washed off during this process, but are taken up directly through the plant's leaves.
The gravitational movement of water into soil. The more porous and less compacted the soil is, the higher the rate of infiltration. The texture, structure, and organic matter content of soil affect its porosity.
As water enters the soil, it enters the vadose zone, an unsaturated layer of the Earth's crust that contains both water and gases, including water vapor, that may not be present in the same proportions as in the atmosphere.
The vadose zone can expand and contract as the water table moves up and down.
Groundwater flow
The lateral movement of groundwater through the saturated zone.
Water collects in interconnected pores and fissures in sediments and rocks below the water table.
Ultimately, it is usually discharged to a spring, stream, lake or ocean, but there are some aquifers that have no natural outlets.
Surface runoff
Water from precipitation, snow melt or irrigation that does not infiltrate the soil, but flows over land surfaces into water bodies, often contributing to stream flow in the process.
During this process, water picks up and transports particulate matter and chemical compounds, making it a significant source of water pollution and soil erosion.
In urban and agricultural areas where soil has been compacted or removed and vegetation does not insulate from the physical impact of precipitation, this can cause significant environmental disturbance.
The conversion of liquid water to water vapor and is often considered the 'engine' that drives the hydrologic cycle.
It is considered a single flux when it is from water streams, lakes and oceans as well as soils and other non-vegetative surfaces.
From plants — transpiration— is considered seprarately.
Because the ocean covers more than 70% of the globe, this process from the oceans is the primary mechanism supporting the surface-to-atmosphere flux in the water cycle.
Requires energy (heat) to convert the liquid water to a gaseous phase.As a result, it tends to cool the air and/or water surface.
When ice is converted directly to water vapor, bypassing the liquid phase.
Typically, this happens when warm, dry air blows across snow fields, a relativley frequent occurance in the Rocky Mountains.
However, the total quantity of water transferred from the Earth's surface to the atmosphere via this process is relatively small compared to evaporation.
Stream flow
Water that enters and remains in a stream channel as it flows downhill.
The water in a stream is composed of surface runoff, which has been intercepted by the stream channel, and baseflow, which is the water in a stream that is supplied by groundwater effluent.
The movement of water through the unsaturated below-ground zone above the water table.
Soil can hold water like a sponge and each soil has a maximum "water-holding" capacity.Soils with finer particles and more organic matter tend to have greater water-holding capacities.
The rate depends on soil structure, texture, and organic matter content. The more compacted and exposed the soil surface, the more water will flow as surface runoff.
Soils with a balanced mix of fine and coarse particles tend to have higher rates of this.
Vadose zone
Also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure.
Physical weathering of rock and soil
Another significant nutrient input in terrestrial ecosystems.
Water dissolves phosphorus, potassium, calcium, and other nutrients from rocks. Freeze-thaw cycles, root growth, and organic acids released by plant roots and microorganisms are all processes that physically and chemically weather rock and soil.
Wetfall and Dryfall
Act as nutrient inputs in both terrestrial and aquatic ecosystems.
Rainfall, snow, fog, and sleet contain dissolved nutrients absorbed by plants via soil water or directly through leaves. Nutrients are quickly distributed throughout a floodplain via flood water. Dust particles from fires, windstorms, and volcanoes are often rich in nutrients, and during periods of low precipitation, this airborne particulate matter is deposited as dryfall.
Water flow
Surface water, leaching, and groundwater flow carry nutrients out of terrestrial ecosystems. Where bedrock is particularly porous, dissolved nutrients may leave an ecosystem in deep groundwater streams. Water flow out of aquatic systems is the main route of nutrient loss there; the nutrient budgets of water body networks are often dictated by the flow of nutrients from upstream waters.
Primary production
The creation of new biomass. New biomass that is synthesized from inorganic molecules like CO2 and H2O is called primary production.
Contrasts with secondary production, in which new biomass is created via consumption.
Plants are responsible for the bulk of this type of production. They use photosynthesis to power production. Other examples, called chemotrophs, use chemoautotrophy to build new biomass.
It is a rate (i.e., amount per unit time).
(Ecologists often distinguish between gross primary production, the rate at which photosynthesis occurs, and net primary production, the rate at which biomass accumulates.)
Explanation of Nutrient Cycle
When nutrients move in, around, and out of ecosystems, energy also moves. Green plants and algae and other primary producers capture solar energy during photosynthesis. This energy is converted into chemical energy stored in the bonds that hold molecules together. As chemicals move around an ecosystem, the energy stored within moves with them.
Organisms require energy to grow, move, reproduce, synthesize enzymes, operate nerves, and so on. Each time a chemical reaction takes place within a living cell, energy is released and a certain percentage of it is lost into the surrounding physical environment (and eventually out into space) as heat.
While energy flows into, through, and out of biological systems, nutrients move between the abiotic environment and living organisms, and nutrient mass is conserved throughout the process.
Catalysts: chemical compounds (in this case biological molecules) that speed up chemical reactions, by lowering the activation energy needed for the reaction.
Most are proteins and nearly all chemical reactions that occur in living cells require enzymes.
Is affected by temperature, chemical environmental factors such as pH, and the concentrations of the substrates whose reactions enzymes catalyze.
Consists of the rocks and sediments that make up the Earth's crust, tectonic plates, and upper mantle.
Extends from the Earth's surface down to about 80 km deep.
Elements within the this phere interact with the hydrosphere, biosphere, and atmosphere during biogeochemical cycling.
Includes all the water in liquid form that exists on Earth, including the oceans, lakes, rivers, and streams.
ater moves between the hydrosphere, atmosphere, and biosphere in the hydrologic cycle via precipitation, transpiration, and evaporation.
A layer of gases surrounding the Earth, held in place by gravity. Different distinct layers can be identified within the atmosphere, including the troposphere: the layer closest to the Earth's surface; and the stratosphere above it, which includes the ozone layer.
Extends approximately 450 km above the Earth, although there is no distinct boundary between it and outer space. The composition of the air within is made up primarily of nitrogen (approximately 78% by volume), oxygen (21%), and carbon dioxide (0.04%).
Gaseous forms of nutrients move between here and other components of biogeochemical cycles (the hydrosphere, lithosphere, and biosphere).
The primary reservoir of nitrogen exists in the air where nitrogen makes up 79% by volume of the gases present (oxygen, which obligate aerobic organisms like us need for respiration, makes up only 21% or so of air). Despite its high abundance, most living organisms cannot use atmospheric nitrogen. Nitrogen exists as dinitrogen gas (N2) in the atmosphere. The two nitrogen atoms that make up each N2 molecule are held together by a very strong triple bond, making the nitrogen in the air inert (chemically non-reactive). This means that it takes a lot of energy to split the two nitrogen atoms apart before they can be used to construct other compounds.
Obligate aerobic organism
Organisms that must have oxygen for cellular respiration. All animals, for example, are this -- though some can go a surprisingly long time without oxygen.
Many microorganisms, however, are facultatively anaerobic. This means that they can use oxygen, and typically do when it is available, but can also use other metabolic pathways for cellular respiration when oxygen is not available. Aerotolerant organisms are an usual group of microorganisms that do not use oxygen, even when it is available.
Plants and Nitrogen
Since plants cannot utilize atmospheric nitrogen, they must rely on sources of nitrogen other than the N2 in the atmosphere. Microorganisms fix atmospheric nitrogen and convert it into forms that plants can readily utilize. Most of the fixed nitrogen plants draw from the soil or water is in one of three forms: ammonium (NH4+), nitrite (NO2-) or nitrate (NO3-). Some plants are also able to take up simple organic forms of nitrogen, but this is much rarer. Many ecosystems are often nitrogen-limited due to the relatively slow rate of input of nitrogen from the atmosphere.
Nitrogen fixation
Fixation is one of five major stages of chemical transformation involved in the movement of nitrogen into, around, and out of an ecosystem:
1. Nitrogen fixation
2. Immobilization
3. Mineralization (also known as "decay")
4. Nitrification
5. Denitrification

Microorganisms play important roles in all stages of nitrogen cycling, not just fixation. Bacteria and fungi are strongly affected by moisture, temperature, and the availability of environmental resources. Nitrogen cycling is therefore also greatly influenced by these same environmental factors.
Stages of chemical transformation
1. Nitrogen fixation
2. Immobilization
3. Mineralization (also known as "decay")
4. Nitrification
5. Denitrification
Nitrogen fixation (cont.)
A few bacteria and cyanobacteria (also known as blue-green algae) convert atmospheric N2 into inorganic forms of nitrogen that plants can use. Plant life is dependent upon these microorganisms for their nitrogen and so symbiotic relationships have evolved between plants and these nitrogen fixers.

A well-known example of symbiosis is between the bacterium Rhizobium and plants of the legume family (which includes beans, peas, and peanuts). The bacteria form nodules on the roots of legume plants (see opposite) where they fix atmospheric nitrogen and convert it into ammonium. The bacteria share their ammonium with the plant in exchange for a supply of carbohydrate, which bacteria cannot synthesize. The bacteria use some of the carbohydrate they obtain from the plant as a source of energy for converting atmospheric nitrogen. This relationship is an example of mutualism. Farmers often grow legumes with other crop plants to increase the amount of nitrogen available to plants in the soil.
A similar example of symbiosis between a plant and a nitrogen fixer can be found in the relationship between the bacteria Actinomycetes and alder trees. Because of its filamentous growth form, Actinomycetes was long thought to be fungi but has recently been reclassified as a higher bacteria. Atmospheric N2 is also fixed by free-living bacteria and cyanobacteria in soil, lake and river sediments.
Nitrogen fixers
Organisms that convert atmospheric nitrogen into ammonia and ammonium: forms of nitrogen that plants can utilize.
Some are free living in soil and water, while others form symbiotic relationships with plants. Root nodules in legumes are an example of the latter. Nitrogen fixers are fungi, bacteria or cyanobacteria.
Steps of Nitrogen fixation
All five processes of nitrogen cycling include a series of chemical transformations. Nitrogen fixation involves the transformation of atmospheric N2 to plant-useable ammonium (NH4+) via the following steps:
Nitrogen Fixation Step 1:
- Nitrogen fixers need energy to split the strong bonds that hold together the nitrogen atoms in atmospheric N2 molecules. The bacteria in root nodules of legumes obtain this energy from their plant hosts.

Nitrogen Fixation Step 2:
- The enzyme nitrogenase present in nitrogen-fixers reduces N2 to ammonia (NH3). Nitrogenase is inhibited by O2, therefore nitrogen fixation requires an anaerobic environment, at least locally.

Nitrogen Fixation Step 3:
- The nitrogen in ammonia is further reduced to form plant-useable ammonium (NH4+).
Some ammonia can be released into the atmosphere during this transformation via the process of volatilization. This tends to occur more readily under high pH conditions where there are fewer hydrogen (H+) ions present to reduce the nitrogen in ammonia.
Examples of Nitrogen fixers
(1) Atmospheric nitrogen is not only fixed by microorganisms, but also during thunderstorms. Lightening can provide the energy needed to break the bonds that hold nitrogen atoms together in atmospheric N2 to form nitric oxide (NO) and nitrogen dioxide (NO2), which are collectively known as NOx.

(2)Fires also release fixed nitrogen to the atmosphere, but since this nitrogen comes from the combustion of organic matter the fire is not actually responsible for fixing new nitrogen from atmospheric N2.

(3) Humans are also nitrogen fixers. Burning fossil fuels releases NOx into the atmosphere where it often reacts with other molecules in the atmosphere to form nitrate and nitric acid, a contributor to acid rain. Although some of the fixed nitrogen released from fossil fuel combustion originates in the fuel, much of it comes from N2 in the atmosphere. As the fuel burns it releases enough energy to break the triple bond in the atmospheric N2, producing NOx.1 Regardless of where the nitrogen originates, NOx released during combustion is considered a new source of fixed nitrogen, since it would have remained unavailable to the biota in the absence of human activity. Industrial nitrogen fixation involves the manufacture of ammonium compounds which are used to fertilize agricultural crops and timber plantations. Application of artificial nitrogen fertilizers allows crops and trees to grow in locations where they would otherwise be unable to grow due to nitrogen deficiency. However, anthropogenic nitrogen fixation and artificial fertilization can have serious negative environmental impacts.2
The process by which nutrients are converted from inorganic to organic forms. It is the opposite of mineralization.

For example, ammonium compounds combine with organic acids to form amino acids and proteins. Organic nitrogen is stored in both non-living organic matter and living tissue.
Nitrogen Cycle
Organic nitrogen is stored in both non-living organic matter and living tissue. Much of the nitrogen stored in terrestrial and aquatic ecosystems is in organic form that is not available for uptake by plants (e.g., up to 95% of nitrogen in a grassland can be stored as soil organic matter).
Nitrogen enters soil via dead plant and animal matter, plant residues, and animal waste. The amount of plant-available nitrogen in the soil affects the rate of net primary productivity, the amount of nitrogen in plant tissue, and hence the nitrogen available to herbivores and other consumers.
A feedback loop occurs in the cycling of nitrogen between soil and plants when nitrogen is limited. With lower available soil nitrogen, plants store less in their tissues. This leads to lower litter quality and less nitrogen available to decomposers. Microbial decomposers immobilize more nitrogen when it is more limiting in the soil. Hence, the amount of ammonium and nitrate available to plants is further reduced. When soil nitrogen content is high in an undisturbed, intact terrestrial ecosystem, a feedback exists that further increases plant-available soil nitrogen. Both of these feedback loops are self-reinforcing, meaning that the feedback makes the effect larger.
The process by which a system is changed or controlled by the output or product it produces.
Can be positive or negative and are common in both physical and biological systems.
Positive feedback
The feedback process involved is accelerated or amplified by the product or output of that process.

Exponential population growth is an example: as the population grows, it produces more individuals, which allows the population to grow faster.
Negative feedback
The feedback process is slowed or stabilized by the product or output of the process.

An example can be seen in predator-prey systems: an increase in the abundance of the prey population produces more food for the predator, which leads to an increase in the predator population. An increase in the predator population subsequently leads to a decrease in the prey population, and so forth.
Litter quality
Refers to the resources available in litter to decomposers. High quality litter is easy to break down by decomposer organisms (bacteria, fungi, and microarthropods) and has a high proportion of nitrogen and phosphorus. Low quality litter is more resistant to decomposition, has lower N and P concentrations, and higher lignin content.
Often expressed as the ratio of carbon to nitrogen, with higher quality litter having a lower ratio (i.e., relatively more N).
Aka microbes, are a diverse group of microscopic organisms.
These include fungi, bacteria, cyanobacteria, phytoplankton, single-celled organisms such as amoeba and Paramecium, and zooplankton.

They occur in all parts of the biosphere and play essential roles in decomposition, primary production, and nitrogen fixation.
The process by which nutrients are converted from inorganic to organic forms. It is the opposite of mineralization.
The process by which nutrients are converted from organic to inorganic forms. This usually takes place during decomposition as microbes break down dead and decaying organic matter. It is the opposite of immobilization.
A suite of chemical compounds found in all vascular plants.
What we refer to as the 'fiber' in our diets.
It provides plants with structural support and plays an important role in water balance by allowing plants to transport water along their vessels during transpiration.
It is particularly abundant in the wood of trees.
C:N ratio
The ratio of carbon to nitrogen is a key indicator of litter quality and soil fertility. The lower the ratio (that is, the higher proportion of nitrogen), the higher the litter quality and the more fertile the soil.
With much of the stored nitrogen locked away in organic matter, plants once again rely on microorganisms to supply them with the nitrogen they need. Mineralization (or "decay") is the opposite of immobilization. When dead organisms and animal waste decompose, the organic forms of nitrogen contained in that material are converted back into ammonia and ammonium.
The rate at which nitrogen is made available to primary producers is determined largely by the rate of mineralization, at least in terrestrial ecosystems. Temperature, moisture, litter quality, and environmental chemistry determine the rate at which organic matter decomposes.
Litter quality reflects how easily dead matter is decomposed. It is determined by nitrogen and phosphorus concentrations, lignin content of plant material, and the ratio of carbon to nitrogen. Litter of higher quality is considered to have higher nitrogen, lower lignin, and a lower C:N ratio. High quality litter is easier for decomposers to break down.
A process in the nitrogen cycle in which microorganisms convert ammonium into nitrates and nitrites. This conversion is most commonly performed by free-living bacteria.
For biologists, it is the term most frequently used to identify groups of organisms that are capable of interbreeding and producing fertile offspring.

However, this term can also be used to refer to the different types of chemical compounds that contain a particular element. Thus, NH4+, NO, and NO3- are all nitrogen species.
A process in the nitrogen cycle in which denitrifying bacteria convert nitrates and nitrites back into gaseous forms of nitrogen such as nitric and nitrous oxides.

A series of chemical transformations occur during this process:
1. Nitrate (NO3-) is transformed back into nitrite (NO2-).
2. Nitrite is converted into nitric oxide (NO) and then into nitrous oxide (N2O), some of which escapes into the atmosphere.
3. Nitrous oxide is converted into atmospheric nitrogen (N2).
Conditions, or processes that do not involve or require oxygen or air.
Generally, the larger the proportion of runoff that flows through the soil, the greater the likelihood that nutrients, generally, will be taken up by plants or that nitrogen, specifically, will be removed by denitrifying bacteria in the soil. Indeed, this is the rationale for establishing riparian buffers. These strips of vegetation help retain nutrients and prevent them from entering stream water by increasing the amount of water flowing through the root zone.1
The chemical properties of nutrients affect how they move via water flow. In the case of nitrogen, ammonium ions (NH4+), are positively charged. Soil contains negatively-charged clay particles. Since positives attract negatives, ammonium tends to bind tightly to these clay particles. In contrast, nitrites (NO2-) and nitrates (NO3-) are negatively charged ions that do not bind to clay particles. Nitrites and nitrates are also easily dissolved in water. These forms of nitrogen are therefore more mobile. Water moving through the soil carries free nitrites and nitrates. High rates of streamflow can therefore, lead to losses of nitrogen from the ecosystem.
The process of converting nutrients from a solid to dissolved form that leads to loss from the system is called leaching. Nitrites and nitrates are susceptible to loss via leaching. Leaching can thus lead to decreases in soil fertility and nitrogen enrichment of the water bodies located downstream. In most undisturbed ecosystems, nitrogen is in high enough demand that little is lost via leaching. Disturbances, whether naturally occurring phenomena or human-caused disruptions of the ecosystem, can produce large increases in the amount of nitrogen that water carries out of the ecosystem.

In forestry plantations, a repeated disturbance occurs when timber is harvested. Clearcutting obviously has a major impact on the forest ecosystem. A virtual tree-removal experiment in a simulated forest will allow you to investigate how nitrogen cycling is impacted by such a disturbance,
The loss of nutrients, such as nitrates, from the soil. These negatively-charged compounds do not bind with negatively-charged clay particles in the soil and are therefore free to be dissolved in water passing through the soil column. These dissolved nutrients end up in groundwater or streams and flow out of the ecosystem.
Soil fertility
Refers to the ability of the soil to allow abundant plant growth. Soils with high fertility are rich in essential nutrients, contain ample organic matter, have a neutral pH, are well drained, and have a balanced texture.
Nitrogen cycling in aquatic ecosystems
The amount of nutrients flowing into a water body is strongly influenced by the nutrient dynamics of the surrounding landscape or watershed. For example, large amounts of nitrogen can come from fertilizer runoff, sewage, and animal waste.
Internal cycling of nitrogen in aquatic ecosystems is similar to that in terrestrial systems. Aquatic plants and algae can use nitrites, nitrates and ammonium dissolved in the surrounding water. Cyanobacteria fix atmospheric nitrogen and convert it into ammonium, and aquatic nitrifying bacteria convert it into nitrites and nitrates. Nitrogen is immobilized and stored in aquatic sediments in a similar way as it is in soil.
In streams and rivers, water flow is the distinctive feature. Water currents move dissolved and suspended nutrients downstream. As they move, these nutrients may be chemically transformed and pass through different plants, animals, and microorganisms, in a similar way to nutrient cycling in terrestrial systems. Aquatic ecologists often to refer to "nutrient spiraling": an atom of a nutrient travels through a cycle, but is displaced downstream by the current as it goes. The faster the current, the more elongated the spiral.
Nutrient Spiraling
A representation of nutrient dynamics in streams, which, because of downstream displacement of organisms and materials, are better represented by a spiral than a cycle.
Gene Likens and Herbert Bormann at the Hubbard Brook Experimental Forest in New Hampshire, USA.
Likens and Bormann's study was designed to investigate how a forest and its vegetation affects the loss of nutrients from the ecosystem. They studied two adjacent tracts of land for three years to gather baseline data, and then clearcut one of them. The clearcut and undisturbed forest areas were compared through time to see how nutrient dynamics differed between the two. Likens and Bormann discovered that the loss of nitrates in streamflow from the clearcut area was 40 to 50 times greater than from the intact forest.
When crops or trees are harvested, a large pool of nitrogen remains in the soil. With most of the vegetation no longer present to uptake this nitrogen, nitrite and nitrate leaching into groundwater, surface water, and outgoing stream flow increases. Lakes, estuaries, and ocean waters located downstream often receive increased nitrogen inputs via the watershed system. In extreme cases of nitrogen enrichment, water bodies can become eutrophic.
This Hubbard Brook study was one of the first to demonstrate that the vegetation in forest ecosystems has a significant effect on the rates and stability of nutrient cycling. When a disturbance (in this case a clearcut) significantly decreases the amount of standing vegetation, nutrient cycling is severely perturbed and nutrient retention greatly reduced.
Occurs when an ecosystem receives a larger than typical input of nutrients from an outside source.
Causes particularly significant changes in aquatic systems receiving large concentrations of nitrogen or phosphorus.
Takes place when water bodies receive large inputs of nutrients, stimulating plant growth, particularly algae. The increased plant growth in turn increases inputs into decomposer systems (with more plants present, there are more plants dying) which leads to deoxygenation of the water.
Deoxygenation can cause aquatic animals to die, thus causing significant change to the local community.
The widespread removal of trees from a forest ecosystem. This can occur through anthropogenic activities such as harvesting of timber, can result from acid rain, or via natural catastrophic events such as insect infestations, earthquakes, and forest fires.
The Impacts of Acid Rain on Nutrient Cycling
Acidic water dissolves nutrients in the soil and washes them out via streamflow. Prolonged exposure to acid precipitation leaches aluminum from forest soil particles, and so acidic forest soils contain more solubilized aluminum than neutral soils. Aluminum in this form not only flows more readily into aquatic environments, affecting organisms there, but also inhibits trees' ability to take up nutrients. Trees are subjected to a double whammy: their supply of nutrients declines, and their ability to use remaining nutrients is impeded. Tree health gradually declines, resistance to drought, disease and pests decreases, defoliation occurs, and eventually death ensues. A forest's capacity to withstand these effects depends on how well its soil buffers
Deforestation and Acid rain
Deforestation due to acid rain has occurred in Sweden, Norway, Germany, Switzerland, and Canada, where huge areas of forests have been severely impacted.
Coniferous forests are often the hardest hit.

For example, soils in much of eastern Canada lack the natural alkalinity that protect forests from acidification. As much as 15% of Canada's most productive softwood forests have been damaged by acid deposition.
Experimental Lake 226
In the early 1970's a series of classic experiments was conducted on lake eutrophication on the Canadian Shield that led to two publications in the journal Science (Schindler 1974 and Schindler 1977). In these studies, Schindler demonstrated that phosphorus was the nutrient most responsible for the eutrophication of lakes in northwestern Ontario. In a now-famous experiment, he strung a curtain across Lake 226, dividing it in two. He fertilized one side but not the other with phosphorus, nitrogen and carbon, producing an extensive algal bloom which is clearly visible (Schindler 1974). In a subsequent set of experiments he was able to demonstrate that it was the phosphorus and not nitrogen or carbon that caused the algal bloom. When sufficient phosphorus was added in the absence of nitrogen, he stimulated growth of nitrogen-fixing cyanobacteria (Schindler 1977). These species did not compete well with other organisms when nitrogen was plentiful, but as long as there was plenty of phosphorus and little nitrogen, their ability to use atmospheric nitrogen gave them a competitive advantage.
Largely due to this work, phosphorus has been removed from many detergents. As a result, many lakes in the United States and Canada that had experienced significant eutrophication saw marked improvements in water quality.
Phosphorus Cycling
Most phosphorus exists in sediments and mineral deposits. Weathering of rocks slowly releases phosphorus into terrestrial and aquatic ecosystems. Since rock weathering is a slow process, most undisturbed terrestrial ecosystems are normally phosphorus-limited. Like the case with nitrogen, phosphorus cycles between inorganic and organic forms within an ecosystem, and some is lost via leaching and streamflow.
Human impact on Phosphorus Cycling
Humans also have a significant impact on the phosphorus cycle. Phosphate rock is mined for use in fertilizers, industrial processes, and the manufacture of detergents. Globally, the artificial transfer of phosphorus from rocks to soil is about five times faster than natural weathering. High phosphorus content fertilizer is added to agricultural, horticultural, and forest soils to increase yields. Soil erosion and agricultural runoff caused by disturbances such as clearcutting and urban development lead to much of the phosphorus applied as fertilizer ending up in streams, rivers, lakes, and estuaries. Agricultural and human sewage, even after being treated, still contains high concentrations of phosphorus, which also flows into aquatic ecosystems.
Phosphorus enrichment of aquatic systems
Phosphorus enrichment of aquatic systems can lead to sudden blooms of phytoplankton and aquatic vegetation. Indeed, phosphorus has long been thought the main culprit responsible for the eutrophication of lakes,1 as exemplified in a famous 1970s study of Experimental Lake 226. When lakes become eutrophied, water quality declines and dissolved oxygen is depleted. Under extremely eutrophic conditions, many fish and aquatic invertebrates cannot survive. Although we now recognize that limiting both phosphorus and nitrogen pollution is critical for maintaining healthy aquatic ecosystems, how best to accomplish this goal remains the focus of much current research.
Lake Washington
A famous example of phosphorus pollution causing eutrophication occurred in Lake Washington in the 1950s. Lake Washington is a 30 square-mile freshwater lake near Seattle in the state of Washington, in the United States. In the 1940s and 1950s, the local human population grew rapidly and huge amounts of phosphorus-rich sewage effluent (20 million gallons per day at one stage!) were deposited into the lake. In the 1950s, local residents noticed that the lake water was becoming progressively cloudier and smellier, fish were dying, and phytoplankton were blooming every summer. These are classic symptoms of phosphorus enrichment.
In the early 1960s, steps were taken to treat sewage more effectively and divert some of the effluent into tidally-flushed areas nearer the coast. The lake responded well to these changes and by the mid 1970s, annual algal blooms had ceased and the water quality of Lake Washington was greatly improved.
Carbon Cycling
The increase in global atmospheric carbon due to human activity is one of the most prominent contemporary environmental issues.

All living things are built mostly from carbon. But the living world accounts for only a tiny fraction of the total carbon on Earth. More than 99% of Earth's carbon is tied up in rocks, particularly limestone, and cycles very slowly. So the major fluxes of carbon involve less than 1% of the total carbon on Earth, and of that, most involve carbon dioxide (CO2).
Although we think of CO2 as a part of the air we breathe, CO2 from the atmosphere can also dissolve in water. Because oceans are so huge, the biggest pool of CO2 is actually in the deeper ocean waters, and, on the timescale of decades to millennia, oceans are the primary regulator of atmospheric CO2. Ocean pools of carbon are depicted in the diagram on the right. As shown, the ocean comprises two separate carbon pools: one on the surface, in contact with the atmosphere, and a second in deeper waters that, for physical reasons, mixes only slowly with surface waters. Thus CO2 in the deep ocean tends to stay there, while CO2 in the atmosphere and the ocean surface tends to equilibrate.
Carbon (cont.)
Carbon dioxide that diffuses into the surface waters of oceans from the atmosphere is used by photosynthesizing plankton and macrophytes. This carbon moves through the food web of the marine community from lower to higher trophic levels. Marine organisms such as clams, oysters, coral, and some algae store carbon in their shells and cells as calcium carbonate. When these organisms die, their bodies and shells drop to the ocean floor where they accumulate as carbon-rich deposits. Over long periods of time, these deposits form the sedimentary rocks like limestone where most of the Earth's carbon is stored. But over shorter time periods (decades to centuries) this is an important flux of carbon from the surface to deep ocean.

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