Water cycles continuously from the atmosphere to the earth to the oceans and back to the atmosphere again Water is indispensable to the functioning of living organisms which live or die on the basis of their ability to capture water and incorporate it into their bodies. Water evaporates from the oceans which cover ¾ of the earth’s surface into the atmosphere a process powered by energy from the sun. Over land areas, approximately 90% of the water that reaches the atmosphere comes from plants via transpiration. Following condensation, most precipitation from the atmosphere falls clean and fresh directly into the oceans but, some fall on land where it passes into the surface or subsurface bodies of fresh water. Only about 2% of all the water on earth is fixed in any form – frozen, held in the soil or incorporated into the body of organisms. This cycle of evaporation/transpiration and precipitation goes on over and over again.
THE CARBON CYCLE
The carbon cycle is based on carbon dioxide which makes up only about 0.03% of the atmosphere. Carbon is used to make carbohydrates, fats and proteins, the major sources of food energy. These compounds are oxidized to release carbon dioxide, which can be captured by plants, algae and photosynthetic bacterial to make organic compounds. The chemical reaction is powered by the light energy of the sun. This results in the fixation of about 10% of the roughly 700 billion metric tons of carbon dioxide in the atmosphere each year.
All heterotrophic organisms including non-photosynthetic bacteria, fungi, animals and few plants that have lost the ability to photosynthesize, obtain their carbon indirectly from the organisms that fix it. When their bodies decompose, organisms release carbon dioxide to the atmosphere. Once there, it can be reincorporated into the bodies of other animals.
The carbon cycle is unusual among nutrient cycles because it need not involve decomposers. Most of the organic compounds that are formed as a result of carbon dioxide fixation in the bodies of photosynthetic organisms are ultimately broken done and released back into the atmosphere or water. However, certain carbon containing compounds like cellulose are more resistant to breakdown.
The carbon in this cellulose may eventually be incorporated into fossil fuels, such as oil or coal.
In global terms, photosynthesis and respiration are approximately balanced, but the balance has been shifted recently because of the consumption of fossil fuels by man. The combustion of coal, oil and gas has released large stores of carbon into the atmosphere as carbon dioxide. This increase of carbon dioxide in the atmosphere appears to be changing global climate making it warmer.
THE NITROGEN CYCLE
The primary inorganic reservoir of Nitrogen is the atmosphere. Nitrogen gas constitutes 78% of the earths atmosphere but the total amount of fixed nitrogen in the soil, oceans and the bodies of organisms is only 0.03% of that figure.
Nitrogen cycles between organisms and reservoirs via the nitrogen cycle. This cycle can be broken down into a number of stages.
- Ammonification: When organisms excrete nitrogenous waste or die, their nitrogen is converted to ammonium ions by the action of saprotrophic fungi and bacteria. This process is known as ammonification.
- Nitrification: In warm, moist soils with a pH near 7, ammonium ions are oxidized within a few days of their formation or their addition as fertilizer (Salisbury and Ross, 1985). The oxidation benefits the bacteria performing the reactions by releasing energy which the bacteria can use for synthesis of ATP. The process takes place in two stages. First, ammonium is oxidised to nitrite, by bacteria of the genera
Nitrosomonas, Nitrosospira, Nitrosococcus and Nitrosolobus (Hamilton, 1988). Next, nitrite is oxidised to nitrate by bacteria of the genera Nitrobacter, Nitrospira and Nitrococus.
- Uptake of Nitrogen by plants: Most plants absorb the majority of their nitrogen as nitrate. However many plants also absorb ammonium like in forests on acidic soils where conversion of ammonium to nitrate is slow
- Nitrogen Fixation: Nitrogen fixation is the reduction of atmospheric nitrogen to Ammonium ion. It is of great importance to organisms. Together with Lightning, it is the natural way in which organisms gain access to the huge reserves of nitrogen in the atmosphere. Nitrogen fixation can only be carried out by certain species of bacteria and Cyanobacteria (Postgate, 1988). Some of these species are free-living, occurring in soil or in water. Others exist in symbiotic relationships with higher plants. The most well-known of the nitrogen fixing bacteria are in the genus Rhizobium. These bacteria form symbiotic associations in root nodules of many plants in the family Leguminosae which includes some important crops as peas, groundnuts, beans and clovers.
- Denitrification: Certain proportion of the fixed nitrogen in the soil is steadily lost. Under anaerobic conditions, nitrate is often converted to nitrogen gas and nitrous oxide both of which return to the atmosphere. This process which several genera of anaerobic bacteria carry out is called Denitrification. In its absence, all nitrogen would eventually become fixed, converted into nitrate and washed into the oceans. Life would thus be possible only in marine and littoral habitats as all living organisms depend on the results of nitrogen fixation to synthesize proteins, nucleic acid and other necessary nitrogen – containing compounds. Denitrification and nitrogen fixation together constitute the mechanism for returning nitrogen from the oceans to the land.
The phosphorus cycle unlike those of carbon and nitrogen lacks an atmospheric component. Phosphorus enters ecosystems through the weathering of rocks. Plants obtain their phosphorus from the soil either as dihydrogen phosphate or more slowly as hydrogen phosphate. Once in an organism, though, phosphorus does not undergo reduction; it remains as phosphate. In this form it is found in a number of compounds including nucleic acids, phosphorylated carbohydrates and fats. Herbivores obtain their phosphorus from plants while carnivores obtain theirs from herbivores. Decomposers return phosphorus to the soil as phosphate ion. In most soils and waters, phosphorus is in short supply and limits plant growth. This is because they are relatively insoluble and are present only in certain kinds of rock. Crushed phosphate-rich rocks found in certain regions are used as fertilizer and added to agricultural lands in form of superphosphate in the belief that it becomes fixed to and enriches the soil. The absorption of phosphate along with ammonium, nitrate and the potassium ion is though, greatly aided by the presence of mycorrhizae (fungus plants).
INTERACTIONS BETWEEN THE NUTRIENT CYCLES
The various nutrient cycles are interconnected and depend on one another to a great extent. The burning of fossil fuels for example not only puts large amount of carbon into the atmosphere, it also increases the amount of atmospheric nitrogen, phosphorus and
sulphur. The interdependence of the nutrient cycles is obvious when one considers nutrient cycling through organisms. When a herbivore eats a plant or a carnivore an animal, it ingests at one go not just carbon, nitrogen and phosphorus but oxygen, calcium, potassium, chlorine and all other elements which are found in organisms.
SELF ASSESSMENT EXERCISE
What is nitrogen fixation? Explain the role played by some species of bacteria and leguminous plants in nitrogen fixation.
Explain how the phosphorous cycle differs from other nutrient cycles.
In an ecosystem which is an assemblage of organisms along with the non-living factors of the environment, nutrients circulate over and over again among organisms within the system.
- Important nutrients which are critical to the lives of organisms within an ecosystem include, water, carbon, nitrogen, oxygen and phosphorus.
- Unlike the flow of energy among the living organisms of an ecosystem which occurs on an open system, all the nutrients used in an ecosystem by living organisms operate on a closed system. That is they are recycled within organisms in the system over and over again.
- Carbon, nitrogen and oxygen have gaseous or liquid reservoirs as does water. All the other nutrients, such as phosphorus have solid reservoirs.
- The carbon cycle is unusual among nutrient cycles because it need not involve decomposers.
- The nitrogen cycle is a complex process involving the activities of many genera of bacteria.
- Phosphates are relatively insoluble and are present in most soils only in small amounts. They often are so scarce that their absence limits plant growth, hence the use of fertilizer like superphosphate in agriculture to boost crop production.
- There is an interconnection between the various types of nutrient cycles.
What are biogeochemical cycles? What are the primary reservoirs for the chemicals in these cycles?
From what source does most of the water over land reach the atmosphere? How does transpiration differ from evaporation?
Matter is a substance that has inertia and occupies physical space. It consists of various types of particles, each with mass and size. There are three states of matter: solid, liquid and gas. Gases occupy much greater space than the same amount of liquid or solid, because their particles are spaced far apart from one another and move rapidly and collide with each other often but as for the case of Solid or liquid particles are spaced much closer and cannot be compressed further.
Gases are characterized by four properties they are Pressure (P), Volume (V), Temperature (T) and Amount (n).
THE CYCLING OF PRINCIPAL GASES AND NUTRIENTS
Some of the major biogeochemical (principal gases and nutrients) cycles are as follows:
- Carbon cycle
- Nitrogen cycle.
- Oxygen cycle.
- Phosphorus cycle.
- Sulfur cycle.
- Rock cycle.
- Water cycle.
Carbon is the fundamental component of all organic compounds. It is one of the primary
elements of life, involved in the fixation of energy by photosynthesis. The complex mixture of carbon compounds are usually found in the biosphere, hence, they are originated, transformed and decomposed within this sphere. Carbon (C) is the fourth most abundant element in the Universe, after hydrogen (H), helium (He), and oxygen (O). The movement of carbon, in its many forms, between the biosphere, atmosphere, oceans, and geosphere is described as the CARBON CYCLE.
Carbon cycle is the process in which carbon travels from the atmosphere into organisms and the Earth and then back into the atmosphere. Carbon cycle is one of the integral parts of the biogeochemical cycles. In this cycle, there are various sinks, or stores, of carbon and processes by which the various sinks exchange carbon. Scientists consider 99.9% of all organisms on the planet to be of carbon based life.
The Earth is having a fixed amount of carbon but with recycling it becomes readily available for life. The global carbon cycle can be divided into two categories: the geological carbon cycle, which operates over large time scales (millions of years), and the biological/physical carbon cycle, which operates at shorter time scales (days to thousands of years). But, all the carbon that cycle through the Earth’s systems today was present at the birth of the solar system 4.5 billion years. In the geological carbon cycle, carbon moves between rocks and minerals, seawater and the atmosphere while Biological/physical processes play a major role in the movement of carbon in and out of the land and ocean through the processes like photosynthesis and respiration. These include the process of photosynthesis/respiration where green plants absorb solar energy and remove carbon dioxide (CO2) from the atmosphere to produce carbohydrates. Plants take CO2 from the air and use it to make food. Animals then eat the food and carbon is stored in their bodies or released as CO2 through respiration. Animals and plants die, and the organisms are
eaten by decomposers. Decomposers respire to release CO2 back into the air to be absorbed by producers again. Therefore Carbon enters the atmosphere through respiration (animals or decomposers) and combustion (fossil fuels, wood).
Carbon (C) enters the biosphere during photosynthesis:
CO2 + H2O ⇌ C6H12O6 + O2 + H2O
Carbon is returned to the biosphere in cellular respiration:
O2 +H2O + C6H12O6 ⇌ CO2 +H2O + energy
Nitrogen gas (N2) makes up nearly 80% of the Earth’s atmosphere, Nitrogen is one of the
primary nutrients critical for the survival of all living organisms, it is important to life because it is a key part of amino and nucleic acids. Also, it is an important part of proteins, DNA, chlorophyll, and ATP which is the basic energy molecule for living things. Although nitrogen is very abundant in the atmosphere, it is not largely inaccessible in this form to most organisms,’ plants nor can animals obtain it directly from the atmosphere. Instead, they depend on a process known as nitrogen fixation that is converted from dinitrogen gas into ammonia (NH3). In addition to N2 and NH3, nitrogen exists in many different forms, including both inorganic (e.g. ammonia, nitrate) and organic (e.g., amino and nucleic acids) forms. Thus, nitrogen undergoes many different transformations in the ecosystem, changing from one form to another through a process known as Nitrogen cycle. Nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The major process involves in Nitrogen cycle are: nitrogen fixation, nitrification, denitrification, anammox, and ammonification
NITROGEN FIXATION: The equation below express how nitrogen fixation have been taking place. N2 + 8H+ + 8e- ⇌ 2NH3 + H2
NITRIFICATION: The equation below express how nitrification have been taking place
1. NH3 + O2 + 2e- ⇌ NH2OH + H2O
2. NH2OH + H2O ⇌ NO2- + 5H+ + 4e-
DENITRIFICATION: The equation below express how denitrification have been taking place
1. NO3- NO2- NO + N2O N2
2. 2NO3- + 10e- +12H+ ⇌ N2 +6H2O
ANAMMOX: The equation below express how Anammox have been taking place
NH42 + NO2- ⇌ N2 + 2H2O
AMMONIFICATION: The process of ammonification is taken place when an organism
excretes waste or dies which resulted to organic nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia which becomes available to plants and other microorganisms.
Phosphorus is a primary nutrient; it is one of the major requirements for healthy plant growth.
Phosphates react with and bind tightly to soil particles and are not readily lost by leaching, its addition to soil can reduce the bioavailability of metal ions because metal ions usually form insoluble compounds when react with phosphorous ions. Organic phosphorus compounds include nucleotides, nucleic acids and phospholipids. Phosphorous nutrients are usually originated from death and decaying of plants and animals in soil and water whereby some are absorbs by plants and other moves into other water bodies such as Ocean through leaching and run-off later drawn away as Ocean sediments through sedimentation and return into soil and water through erosion and weathering from rocks and fossil
Organic sulfur compounds such as proteins is an living organisms cycle with inorganic sulfate ions in soil or water, sulfur is usually bound up in rocks and salts or buried in deep ocean sediments, unlike phosphorous, sulfate ions are easily leached out of the soil. Decomposers break down organic sulfur compounds to gases such as hydrogen sulfide which enter the atmosphere and are oxidized to sulfur dioxide Sulfur dioxide dissolves in water droplets in the atmosphere to give solutions of sulfurous acid (H2SO3) which is a weak acid and only partially ionizes in water (HSO3−).
SO2 (g) + H2O(l) ⇌ H2SO3(aq)
H2SO3 (aq) ⇌ H+ (aq) + HSO3− (aq)
Sulfur compound undergo several process, for instance, Photosynthetic sulfur bacteria are able to form free sulfur from hydrogen sulfide gas. Industrial processes, including burning fossil fuels and volcanoes liberate hydrogen sulfide and sulfur dioxide gases. Sulfuric acid can also be formed from dimethyl sulfonioproprionate (DMSP) which is formed by plankton species and gives the seaside it characteristic smell.
Oxygen (O) is a gas as well as nutrient form through many process but a few are the breathing of plants and animals and the decomposition of dead plant and animal material. The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water: The oxygen cycle is the biogeochemical transitions of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. It is a cycle that helps move oxygen through the four main reservoirs of the Earth; the terrestrial biosphere (green), marine biosphere (blue), lithosphere (brown), and atmosphere (grey), whereby atmosphere serve as the largest reservoirs of free oxygen on earth. Oxygen cycle is critical to life on Earth.
Humans, and most other organisms, need oxygen to survive. Humans need it to breathe, oxygen is needed for decomposition of organic waste, Water can dissolve oxygen and it is this dissolved oxygen that supports aquatic life. That is why it is so important to help someone who cannot breathe by providing them with oxygen. Without oxygen at the bottom of the water body, anaerobic bacteria (those that live without oxygen) produce acid. Plants and other organisms that perform photosynthesis rely on animals for carbon dioxide which also contain oxygen.
Because all rocks are made up of minerals they undergo various process like most Earth
materials, hence they are created and destroyed through the process known as cycle Rock cycle is a model that describes the formation, breakdown, and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes. This cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: Each rock type is altered when it is forced out of its equilibrium conditions as such, this determine how rocks are form and so, the rock cycle.
The rock cycle begins with molten rock (magma below ground, lava above ground), which cools and hardens to form igneous rock. Exposure to various processes through weathering and
erosional forces, break the original rock into smaller pieces. Therefore, igneous rock metamorphosed directly into metamorphic rock, and metamorphic rock turn directly to
sedimentary rocks which also get back to igneous rocks through metamorphic activities. But, generally these rocks change over hundreds of years in the six rock cycle steps:
- Weathering & Erosion.
- Compaction & Cementation.
- Rock Melting.
Water cycle is the process of evaporating and transpiration of water from the surface of the earth, into the atmosphere then cools and condenses into rain or snow in clouds form, and falls again to the surface as precipitation. Therefore, water cycle consist of four processes the evaporation which involve the changing of the liquid’s surface to a gas, transpiration that is the process through water on plants turn into gas, condensation the solidification of the gas water and finally the melting of the solid water into liquid form again that fall as precipitation, dew or snow. In general the following stages are considered as the processes of water cycle:-
- Step 1: Evaporation
- Step 2: Condensation
- Step 3: Sublimation
- Step 4: Precipitation
- Step 5: Transpiration
- Step 6: Runoff
- Step 7: Infiltration
Based on this, some are of the view that water cycle has no starting point. Yet, we can say that the process begin in the oceans, since that is where most of Earth’s water exists. The sun, which drives the water cycle, heats the ocean water in the oceans which then evaporates as vapor into the atmosphere. History revealed that, Bernard Palissy who was the first published thinker to assert that rainfall alone was sufficient for the maintenance of rivers in 1580 CE, was credited as the “discoverer” of the modern theory of the water cycle. Water cycle is very important to plants and animals because sometimes this water collects nutrients from the soil it runs over, making the valley good for plant growth, the cycling also makes water to be available to all corner of our environment, hence, available for humans, animals, and plants survival and the process of evaporation and infiltration help to remove impurities from water.