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Unbalancing the Nitrogen Cycle

It takes a great deal of energy to convert atmospheric nitrogen into biologically useful forms; this was why ecosystems have evolved to get by on fairly modest amounts of organic nitrogen. From forest fires to farming to burning fossil fuels, human activities have been altering the natural nitrogen cycle for centuries. Human practices that add reactive nitrogen (nitrogen that has been fixed) to ecosystems can change ecological balances. Farming, for example, is a relatively nitrogen intensive activity. Crops deplete nitrogen in the soil; therefore many farmers use man-made fertilizers in order to augment nitrogen levels. Unfortunately, in its nitrate form, nitrogen is extremely soluble and is readily leached from the soils into ground water reservoirs which feed into lakes and streams. In heavily agricultural areas, fertilizers are the primary source of nitrogen pollution. There are a variety of consequences of nitrogen pollution. A major source of reactive nitrogen is atmospheric deposition which comes largely from transportation emissions, as nitrogen oxides are released through the exhaust. These emissions are a key ingredient in the formation of ground level ozone (smog). Another form of reactive nitrogen - nitric acid (HNO3) - is an important ingredient in the creation of acid rain. One of the most serious consequences of nitrogen pollution is over-nutrition, or eutrophication, of aquatic ecosystems. Nitrogen leaches into the soil, and eventually into standing bodies of water, causing an unnaturally high level of nitrogen in the water. This eutrophication harms aquatic ecosystems by fueling excessive algae growth, which overshadows the water surface and deprives other aquatic organisms of necessary sunlight. When the algae dies, the oxygen consumed in the decomposition process can further deprive other aquatic organisms of needed oxygen. In extreme cases, eutrophication can result in the total die-off of fish in lakes and ponds.

Phosphorus Cycle

Phosphorus is an important element for all forms of life. As phosphate (PO4), it makes up an important part of the structural framework that holds DNA and RNA together. Phosphates are also a critical component of ATP – the cellular energy carrier – as they serve as an energy ‘release' for organisms to use in building proteins or contacting muscles. Like calcium, phosphorus is important to vertebrates; in the human body, 80% of phosphorous is found in teeth and bones.

The largest reservoir of phosphorus is in sedimentary rock. It is in these rocks where the phosphorus cycle begins. When it rains, phosphates are removed from the rocks (via weathering) and are distributed throughout both soils and water. Plants take up the phosphate ions from the soil. The phosphates then moves from plants to animals when herbivores eat plants and carnivores eat plants or herbivores. The phosphates absorbed by animal tissue through consumption eventually returns to the soil through the excretion as well as from the final decomposition of plants and animals after death. The same process occurs within the aquatic ecosystem. Phosphorus is not highly soluble, binding tightly to molecules in soil, therefore it mostly reaches waters by traveling with runoff soil particles. Water plants take up the waterborne phosphate which then travels up through successive stages of the aquatic food chain.

While obviously beneficial for many biological processes, in surface waters an excessive concentration of phosphorus is considered a pollutant. Since phosphorus is the nutrient in short supply in most fresh waters, even a modest increase in phosphorus can set off a whole chain of undesirable events. The phosphorus cycle differs from the other major biogeochemical cycles in that it does not include a gas phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere, contributing – in some cases – to acid rain. Humans can alter the phosphorus cycle in many ways, including in the cutting of tropical rain forests and through the use of agricultural fertilizers. Rainforest ecosystems are supported primarily through the recycling of nutrients, with little or no nutrient reserves in their soils. As the forest is cut and/or burned, nutrients originally stored in plants and rocks are quickly washed away by heavy rains, causing the land to become unproductive. Agricultural runoff provides much of the phosphate found in waterways. Crops often cannot absorb all of the fertilizer in the soils, causing excess fertilizer runoff and increasing phosphate levels in rivers and other bodies of water. Phosphate stimulates the growth of plankton and plants that tend to consume large amounts of dissolved oxygen, potentially suffocating fish and other aquatic animals, while also blocking available sunlight to bottom dwelling species (eutrophication).

Sulfur Cycle

Sulfur (S), the tenth most abundant element in the universe, comprises many vitamins, proteins, and hormones that play critical roles in both climate and in the health of various ecosystems. The majority of the Earth's sulfur is stored underground in rocks and minerals, including sulfate salts buried deep within ocean sediments.

The sulfur cycle contains both atmosphericandterrestrial processes. Within the terrestrial portion, the cycle begins with the weathering of rocks, releasing the stored sulfur. The sulfur then comes into contact with air where it is converted into sulfate (SO4). The sulfate is taken up by plants and microorganisms and is converted into organic forms; animals then consume these organic forms through foods, thereby moving the sulfur through the food chain. As organisms die and decompose, some of the sulfur is again released as a sulfate and some enters the tissues of microorganisms. There are also a variety of natural sources that emit sulfur directly into the atmosphere, including volcanic eruptions, the breakdown of organic matter in swamps and tidal flats, and the evaporation of water.

Sulfur eventually settles back into the Earth or comes down within rainfall. A continuous loss of sulfur from terrestrial ecosystem runoff occurs through drainage into lakes and streams, and eventually oceans. Sulfur also enters the ocean through fallout from the Earth's atmosphere. Within the ocean, some sulfur cycles through marine communities, moving through the food chain. A portion of this sulfur is emitted back into the atmosphere from sea spray. The remaining sulfur is lost to the ocean depths, combining with iron to form ferrous sulfide which is responsible for the black color of most marine sediments.

Sulfur Pollution

Sulfur is prevalent in all raw materials, including crude oil, coal, and ore that contains common metals like aluminum, copper, zinc, lead, and iron. Human activities have contributed to the amount of sulfur that enters the atmosphere, primarily through the burning of fuel containing sulfur, such as coal and oil and the processing of metals. One-third of all sulfur that reaches the atmosphere – including 90% of sulfur dioxide SO2 – stems from human activities. Emissions from these activities, along with nitrogen emissions, react with other gases and particles in the atmosphere to produce tiny particles of sulfate salts which fall as acid rain, causing a variety of damage to both the natural environment as well as to man-made environments, such as the chemical weathering of buildings. However, as particles and tiny airborne droplets, sulfur also acts as a regulator of global climate. Sulfur dioxide and sulfate aerosols absorb ultraviolet radiation, creating cloud cover that cools cities and may offset global warming caused by the greenhouse effect. The actual amount of this offset is a question that researchers are attempting to answer.

Oxygen cycle

The oxygen cycle is the cycle that helps move oxygen through the three main regions of the Earth, the Atmosphere, the Biosphere, and the Lithosphere. The main mass of oxygen exists in a bound form in molecules of water, oxides, salts, and other solid substances and cannot be used directly in an ecosystem. In addition, in the form of O3, ozone, it provides protection of life by filtering out the sun's UV rays as they enter the stratosphere. In addition to constituting about 20% of the atmosphere, oxygen is ubiquitous. It also occurs in combination as oxides in the Earth's crust and mantle, and as water in the oceans.


The atmosphere contains gaseous oxygen available for photosynthesis. Approximately 1,1 x 10 15 ton of atmospheric oxygen passes through the plants within 2,500 years. During photosynthesis carbon dioxide is converted into organic form and molecular oxygen O2 is released.


The lithosphere mostly fixes oxygen in minerals such as silicates and oxides. Most of the time the process is automatic all it takes is a pure form of an element coming in contact with oxygen such as what happens when iron rusts. A portion of oxygen is freed by chemical weathering. When a oxygen bearing mineral is exposed to the elements a chemical reaction occurs that wears it down and in the process produces free oxygen.


Water Cycle

Earth is the water planet with more than two-thirds of its surface covered by water. Most of life on Earth is also primarily composed of water; our cells, and those of plants and animals are composed of approximately 70 percent water. Vast quantities of water also cycle through the Earth's atmosphere, oceans, land, and biosphere over both short and long time scales. This grand cycling of water is called the hydrologic cycle. The hydrologic cycle is a conceptual model that describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere.The cycling of water shapes our weather and climate, supports plant growth, and makes life itself possible. The water cycle is dominated by the oceans, where 97 percent of the water on Earth is found and where 86 percent of global evaporation occurs.

When rain and other precipitation falls on land, some of it runs off into surface waters such as lakes and streams. Much of it, however, seeps into the ground. This process – the movement of water into and through the soil and rocks – is called infiltration. It is primarily during this stage of the hydrologic cycle that water is purified. The extent to which the water is “cleaned” depends on the state of the environment and the amount of pollution in the water. Passing through layers of sediment and rock helps to filter pollutants out, allowing the pure water to pass through. Generally, the deeper groundwater is found, the cleaner it will be.

Water that is not absorbed into the soil flows across the landscape to rivers, lakes, streams, and eventually to the oceans, as runoff. While some runoff waters originate from precipitation, others stem from melting snow or ice, and are called melt water runoff. The area where precipitation that reaches the land drains into a common body of water is called a “watershed,” and can range in size from a few acres to many square miles Some water returns to the air in gaseous form (water vapor) through evaporation. Ocean evaporation is the most prevalent, consisting of about 86 percent of total global evaporation. For land-based evaporation, roughly half occurs on the surface area of plants and is called transpiration. The process in which water vapor is converted back into liquid is called condensation. A more important type of condensation within the hydrologic cycle takes place in the atmosphere. As water vapor moves upward in the atmosphere it cools. This process – the loss of heat through vertical movement – is called convection. The droplets formed from atmospheric condensation gather together as a result of their gravitation pull to form clouds. Depending on the temperature of the surrounding air, this cloud moisture will take either frozen or liquid form. Water in the atmosphere, after condensing and forming into clouds, returns to Earth through precipitation, which can take many forms. Although some water is transmitted directly to Earth through the condensation of ambient water vapor, it is primarily through precipitation that water moves from the atmosphere to the Earth.

Reservoirs (a)

Reservoir Size (volume of water in cubic km x 10,000,000) Percent of Water in Hydrologic Cycle  
Polar Ice and Glaciers
Groundwater 9.5 0.7
Lakes 0.125 0.01
Soils 0.065 0.005
Atmosphere 0.013 0.001
Rivers and Streams 0.0017 0.0001
Biosphere 0.0006 0.00004

Water is stored for periods of time in various types of reservoirs. The primary reservoirs are (in order of size) the oceans, polar ice and glaciers, the atmosphere, groundwater, lakes, soils, atmosphere, rivers and streams, and the biosphere (plants and animals). The planetary water supply is dominated by the oceans. Approximately 97% of all the water on the Earth is in the oceans (see the Table). The other 3% is held as freshwater in glaciers and icecaps, groundwater, lakes, soil, the atmosphere, and within live organisms. The amount of time that water stays in the reservoirs varies: deep groundwater can be held for up to 10,000 years, while glaciers retain their water for an average of about 40 years. At the other end of the spectrum, the retention time for rivers, soil moisture, and seasonal snow cover is typically less than 6 months.


Global warming

Global warming is the increase in the average temperature of the Earth's near-surface air and the oceans since the mid-20th century. Global surface temperature increased 0.74 ± 0.18 °C during the 100 years ending in 2005.

Climate model projections summarized in the IPCC report indicate that global surface temperature will rise further 1.1 to 6.4 °C during the 21st century.

Although most studies focus on the period up to 2100, warming is expected to continue beyond 2100, even if emissions have stopped, because of the large heat capacity of the oceans and the lifespan of CO2 in the atmosphere.

Date: 2015-12-24; view: 3640

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