Nitrogen Cycle, Sulphur Cycle and Hydrological Cycle


Nitrogen Cycle

Nitrogen is an essential component of protein and required by all living organisms including human beings. Nitrogen is needed for our DNA, RNA and proteins and is critical to human agriculture. Nitrogen, a component of proteins and nucleic acids, is essential to life on Earth.

Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms. Through a series of microbial transformations, however, nitrogen is made available to plants, which in turn ultimately sustain all animal life.

The steps, which are not altogether sequential, fall into the following classifications:

• Nitrogen fixation, in which nitrogen gas is converted into inorganic nitrogen compounds, is mostly (90 percent) accomplished by certain bacteria and blue-green algae (see nitrogen fixation). A much smaller amount of free nitrogen is fixed by abiotic means (e.g., lightning, ultraviolet radiation, electrical equipment) and by conversion to ammonia through the Haber-Bosch process.

• Nitrates and ammonia resulting from nitrogen fixation are assimilated into the specific tissue compounds of algae and higher plants. Animals then ingest these algae and plants, converting them into their own body compounds.

• The remains of all living things and their waste products are decomposed by microorganisms in the process of ammonification, which yields ammonia. (Under anaerobic, or oxygen-free, conditions foul-smelling putrefactive products may appear, but they too are converted to ammonia in time.) Ammonia can leave the soil or be converted into other nitrogen compounds, depending in part on soil conditions.

• Nitrification, a process carried out by nitrifying bacteria, transforms soil ammonia into nitrates, which plants can incorporate into their own tissues.

• Nitrates also are metabolized by denitrifying bacteria, which are especially active in water-logged, anaerobic soils. The action of these bacteria tends to deplete soil nitrates, forming free atmospheric nitrogen.

Human Impact on the Nitrogen Cycle:

Humans have contributed significantly to the nitrogen cycle by artificial nitrogen fertilization (primarily through the Haber process, using energy from fossil fuels to convert N2 to ammonia gas (NH3) and planting of nitrogen fixing crops. In addition, humans have significantly contributed to the transfer of nitrogen gases from Earth to the atmosphere.

N2O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources. N2O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. NH3 in the atmosphere has tripled as the result of human activities.

It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging on to water droplets, eventually resulting in acid rain. Fossil fuel combustion has contributed to a 6 or 7 fold increase in NxOx flux to the atmosphere. NxOx actively alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems.

Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants. Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species diverse heath lands.

Sulphur cycle, circulation of sulfur in various forms through nature. Sulphur is key to protein structure and is released to the atmosphere by the burning of fossil fuels. Sulphur occurs in all living matter as a component of certain amino acids. It is abundant in the soil in proteins and, through a series of microbial transformations, ends up as sulphates usable by plants.

Sulphur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. The sulphur of the amino acids is converted to hydrogen sulphide (H2S) by another series of soil microbes. In the presence of oxygen, H2S is converted to sulfur and then to sulphate by sulfur bacteria. Eventually the sulfate becomes H2S.

Hydrogen sulphide rapidly oxidizes to gases that dissolve in water to form sulphurous and sulphuric acids. These compounds contribute in large part to the “acid rain” that can kill sensitive aquatic organisms and damage marble monuments and stone buildings.

Thus cycle can be divided as:

· Sulphur Cycle in Soils

Sulphur enters the trophic cycle in terrestrial plants via root adsorption in the form of inorganic sulphates (e.g., calcium sulphate, sodium sulphate) or by direct assimilation of amino acids released in the decomposition of dead or excreted organic matter. Bacterial and fungal (Aspergillus and Neurospora) mineralization of the organic sulphhydryl in amino acids followed by oxidation results in sulphate; this adds to the sulphate pool for root adsorption.

· Sulphur Cycle in Atmosphere

Sulphur in the atmosphere comes from several different sources: decomposition and/or combustion of organic matter, combustion of fossil fuels, and ocean surfaces and volcanic eruptions. The most prevalent form of sulphur entering the atmosphere is sulphur dioxide (SO2). It, along with other atmospheric forms such as elemental sulphur and hydrogen sulphide, is oxidized to sulphur trioxide (SO3), which combines with water to form sulphuric acid (H2SO4), leading to acid rain.

Atmospheric sulphur, largely in the form of sulphuric acid, is removed by two general processes: rainout, which includes all processes within clouds that result in removal; and washout, which is the removal by precipitation below the clouds. Depending on the amount of the various sulphur compounds available to form the sulphuric acid, the degree of acidity can be strong enough to ap-proximate that of battery acid. Atmospheric inputs of sulphuric acid provide the dominant source of both hydrogen ions (H+) for cation replacement.

· Sulphur in Sediments

The sedimentary aspect of the cycle involves the precipitation of sulphur in the presence of such cations as iron (Fe) and calcium (Ca) as highly insoluble ferrous sulphide (FeS) and ferric sulphide (Fe2S3, pyrite) or relatively insoluble calcium sulphate (CaSO4).

The oxidation of sulphides in marine sediments is a key process, though poorly understood.

Human Impact on the Sulpher cycle:

The sulphur cycles are increasingly being affected by industrial air pollution. The gaseous oxides of nitrogen and sulphur are toxic to varying degrees. Normally, they are only transitory steps in their respective cycles; in most environments, they are present in very low concentrations.

The combustion of fossil fuels, however, has greatly increased the concentrations of these volatile oxides in the air, especially in urban areas and in the vicinity of power plants, to the point where they adversely affect important biotic components and processes of ecosystems. When plants, fish, birds, or microbes are poisoned, humans eventually are also adversely affected.

Coal-burning emissions and automobile exhaust are major sources of SO2 and SO4 production and, along with other industrial combustion, a major source of poisonous forms of nitrogen. Sulphur dioxide is damaging photosynthesis, as was discovered in the early 1950s when leafy vegetables, fruit trees, and forests showed signs of stress in the Los Angeles Basin. The destruction of vegetation around copper smelters is largely caused by SO2.

Furthermore, both sulphur and nitric oxides interact with water vapour to produce droplets of dilute sulphuric and nitric acid (H2SO4 and H2NO3) that fall on Earth as acid rain, a truly alarming development.

Acid rain has the greatest impact on soft-water lakes or streams and already acidic soils that lack pH buffers (such as carbonates, calcium, salts, and other bases). Acid rain damages building materials. Our heritage monuments (such as Taj Mahal at Agra) are threatened by the corrosive action of acid deposition. Acid rain adversely affects terrestrial and aquatic vegetation. Most planktons, mollusks and fry young fish cannot tolerate water having pH below 5.0. Low pH conditions also damage soil microbial community.

Hydrological Cycle

Water cycle, also called hydrologic cycle, cycle that involves the continuous circulation of water in the Earth-atmosphere system. Of the many processes involved in the water cycle, the most important are evaporation, transpiration, condensation, precipitation, and runoff. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

• A brief treatment of the water cycle follows. For full treatment, see hydrosphere: The water cycle.

• Evaporation, one of the major processes in the cycle, is the transfer of water from the surface of the Earth to the atmosphere. By evaporation, water in the liquid state is transferred to the gaseous, or vapour, state.

• This transfer occurs when some molecules in water mass have attained sufficient kinetic energy to eject themselves from the water surface. The main factors affecting evaporation are temperature, humidity, wind speed, and solar radiation.

• The direct measurement of evaporation, though desirable, is difficult and possible only at point locations. The principal source of water vapour is the oceans, but evaporation also occurs in soils, snow, and ice.
Evaporation from snow and ice, the direct conversion from solid to vapour, is known as sublimation.

• Transpiration is the evaporation of water through minute pores, or stomata, in the leaves of plants. For practical purposes, transpiration and the evaporation from all water, soils, snow, ice, vegetation, and other surfaces are lumped together and called evapotranspiration, or total evaporation.

• Water Vapour is the primary form of atmospheric moisture. Although its storage in the atmosphere is comparatively small, water vapour is extremely important in forming the moisture supply for dew, frost, fog, clouds, and precipitation. Practically all water vapour in the atmosphere is confined to the troposphere (the region below 6 to 8 miles [10 to 13 km.] altitude).

• The transition process from the vapour state to the liquid state is called condensation. Condensation may take place as soon as the air contains more water vapour than it can receive from a free water surface through evaporation at the prevailing temperature. This condition occurs as the consequence of either cooling or the mixing of air masses of different temperatures. By condensation, water vapour in the atmosphere is released to form precipitation.

• Precipitation that falls to the Earth is distributed in four main ways: some is returned to the atmosphere by evaporation, some may be intercepted by vegetation and then evaporated from the surface of leaves, some percolates into the soil by infiltration, and the remainder flows directly as surface runoff into the sea. Some of the infiltrated precipitation may later percolate into streams as groundwater runoff. Direct measurement of runoff is made by stream gauges and plotted against time on hydrographs.

• Most groundwater is derived from precipitation that has percolated through the soil. Groundwater flow rates, compared with those of surface water, are very slow and variable, ranging from a few millimetres to a few metres a day. Groundwater movement is studied by tracer techniques and remote sensing.

• Ice also plays a role in the water cycle. Ice and snow on the Earth’s surface occur in various forms such as frost, sea ice, and glacier ice. When soil moisture freezes, ice also occurs beneath the Earth’s surface, forming permafrost in tundra climates. About 18,000 years ago glaciers and ice caps covered approximately one-third of the Earth’s land surface. Today about 12 percent of the land surface remains covered by ice masses.

In the natural system, material circulation is driven by energy from the sun and, to a much lesser extent, from radioactive decay of elements in the earth’s interior and motions of its tides. This is a mechanical and inorganic view of the earth. In another and more realistic sense, the earth has a natural metabolism; materials have circulated about its surface for millions of years in a complex, interconnected web of biogeochemical cycles.

An array of physical, chemical, and biological processes weather and erode rocks and transfer materials in and out of the atmosphere, from the atmosphere to the biota and back again, to the oceans via rivers, and to the continents by uplift. Each element has a natural biogeochemical cycle. It is these cycles and their relationship to the physical climate system that have led to the development of a relatively stable and resilient surface system during geologic time. Life has evolved in this system and plays a strong role in the development and maintenance of the system through processes, and fluxes.

Human activities have contributed materials to the biogeochemical cycles. Some of these materials enter element cycles already naturally in operation; they are the same chemical species that have circulated for millions of years.

Other materials are synthetic compounds and are foreign to the natural environment. These anthropogenic fluxes are leading to a number and variety of environmental issues, including the possibility of global climate change. There is no doubt that human activities have interfered in biogeochemical cycles and have modified the composition of the atmosphere. Understanding the consequences of this interference requires better quantitative descriptions of these cycles, their interconnections, and, in particular, their coupled response to perturbations, such as a change in climate.


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