17 Biogeochemical Cycles

Dr. Vijay Shridhar

Contents

1. Introduction
2. The Oxygen cycle
3. Water cycle
4. The Carbon Cycle:
5. Nitrogen Cycle
6. Sedimentary Nutrient Cycles
7. Sulphur Cycle
8. Phosphorous Cycle
9. Suggesting Reading

Introduction

Biogeochemical and tectonic processes on the surface of the earth continuously recycle chemical elements between the atmosphere, the hydrosphere, the biosphere, and the lithosphere. The long-term geochemical cycles are driven by uplift of continents, weathering, erosion, sedimentation, etc. Superimposed on these large-scale cycles, there are faster loops that involve smaller reservoirs, including the biota. In the following module, we will concentrate on the global cycles of the elements which are most prominently involved in biological cycles: C, O, S, N, and P.

Every life form needs to actively uptake some elements called nutrients to maintain their biological activity. These nutrients provide the structural framework of organism and energy through biochemical energy process. Few of them are essential nutrients and without their supply growth cannot take place in an organism. When these nutrients are part of organism or environment, they are called as in their biotic phase or abiotic phase respectively. Certainly, by passing through different phases these nutrients complete closed circuit cyclic movement as Earth is a closed system. The supplies of these nutrients are from finite sources of Earth system. This closed circuit movement/transformation of nutrients from geochemical to biological and to geochemical is called as biogeochemical cycle of nutrients. The continuation of these cycles is dependent on a sustained energy supply into the system. Most of them is coming from sun and remaining from earth’s internal energy.  Out of 16-17 nutrients, 9 are essential nutrients which are required in larger amount and form 99% body mass of most of the organism. They are oxygen, hydrogen nitrogen, carbon, calcium, magnesium, potassium, phosphorus and sulfur.

These cycles are linked through biological and geochemical transformation steps within the system environment. Nutrient cycles of prominent compounds are varying in their transformation steps and relative importance through biological and geochemical phase. Biological transformation of Nitrogen, sulfur, carbon and oxygen’s nutrient cycle have great significant role in operation and completion of cycles. Other nutrients have essentially more geochemical operation in their relative importance. That’s why these elements are required to discuss in details.

The biogeochemical cycle of nutrient is nature’s recycling system. All forms of recycling have feedback loops that use energy in the process of putting material resources back into the use. Recycling in ecology is regulated to a large extent during the process of decomposition. Ecosystems employ biodiversity in the food webs that recycle natural materials, such as mineral nutrients. Recycling in natural systems is one of the many ecosystem services that sustain and contribute to the well-being of human societies. The Earth has a finite quantity of chemical elements from its formation. Because the chemicals on Earth function in a closed system, neither significantly increasing nor decreasing in quantity, they are recycled throughout the Earth’s biological and geological cycles. These cycles include both the living biosphere, and the nonliving lithosphere, atmosphere, and hydrosphere. Hence box models are used to portray the cycling of elements through the environment.

Plants absorb nutrients from soil and water, and in some cases, from the atmosphere. Nutrient cycle involves the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by food web pathways that decompose matter into mineral nutrients. Nutrient cycles occur within ecosystems. Ecosystems are interconnected systems where matter and energy flows and is exchanged from one step to other step as organisms feed, digest, and migrate about. Minerals and nutrients accumulate in varied densities and uneven configurations across the planet. Ecosystems recycle locally, converting mineral nutrients into the production of biomass, and on a larger scale they participate in a global system of inputs and outputs where matter is exchanged and transported through a larger system of biogeochemical cycles.

The Oxygen cycle

Earth is an oxygen rich planet where we live. Oxygen availability on earth varies from ~23% (in atmosphere) to ~85% (in oceans) on the basis of weight by weight (W/w) of oxygen molecules. Earth crust’s oxygen accounts for ~46% of total weight. Most of the oxygen in the crust in presents as silicate (tetrahedral unit) and aluminosilicate minerals. The oxygen cycle portrays how the flow of oxygen occurs through the several parts of our vast ecosystem. Oxygen is found in several parts of the ecosystem, from the air we breathe (Atmosphere), and the water bodies on the planet (Hydrosphere), inside all the biological beings (Biosphere) and inside the earth’s crust (Lithosphere).

Oxygen Cycle Steps

1. Atmosphere

Only a small percentage of the world’s oxygen is present in the atmosphere, only about 0.35 %.

This exchange of gaseous oxygen happens through Photolysis.

Photolysis This is the process by which molecules like atmospheric water and nitrous oxide are broken down by the ultraviolet radiation coming from the sun and release free oxygen.

2. Biosphere

The exchange of oxygen between the living beings on the planet, between the animal kingdom and the plant kingdom. The exchange of oxygen in the biosphere is codependent on the carbon cycle and hydrogen cycle as well. It mainly occurs through 2 processes.

Photosynthesis

The process by which plants make energy by taking in carbon dioxide from the atmosphere and give out oxygen

Respiration

The process by which animals and humans take in oxygen from the atmosphere and use it to break down carbohydrates and give out carbon dioxide.

3. Lithosphere

The part of the planet containing the most of the oxygen content through biomass, organic content and mineral deposits. These deposits are formed when free radical elements were exposed to free oxygen and over time they form silicates and oxides. This trapped oxygen is released back due to several weathering processes. Also, animals and plants draw nutrient materials from the lithosphere and free some of the trapped oxygen.

4. Hydrosphere

Oxygen dissolved in water is responsible for the sustenance of the aquatic ecosystem present beneath the surface. The hydrosphere is 33% oxygen by volume present mainly as a component of water molecules with dissolved molecules including carbonic acids and free oxygen.

               Figure: Representation of Oxygen cycle

Figure: Representation of the processes involved and their magnitude

Water Cycle

Precipitation is a vital component of how water moves through Earth’s water cycle, connecting the ocean, land, and atmosphere. Knowing where it rains, how much it rains and the character of the falling rain, snow or hail allows scientists to better understand precipitation’s impact on streams, rivers, surface runoff and groundwater. Frequent and detailed measurements help scientists make models of and determine changes in Earth’s water cycle.The water cycle describes how water evaporates from the surface of the earth, rises into the atmosphere, cools and condenses into rain or snow in clouds, and falls again to the surface as precipitation. The water falling on land collects in rivers and lakes, soil, and porous layers of rock, and much of it flows back into the oceans, where it will once more evaporate. The cycling of water in and out of the atmosphere is a significant aspect of the weather patterns on Earth.

Figure: Representation of the processes involved and their magnitude

The Carbon Cycle

The carbon cycle is the circulation and transformation of carbon back and forth between living things and the environment. Hence it is convenient to consider the carbon in the cycle to be either inorganic or organic. CO2 and the related species CO32- and HCO3- constitute the vast bulk of the inorganic carbon while organic carbon is that found in organism, fossil fuels and other dispersed molecules. Carbon is often referred to as the “building block of life” because living things are based on carbon and carbon compounds.

The amount of carbon on the earth and in Earth’s atmosphere is fixed, but that fixed amount of carbon is dynamic, always changing into different carbon compounds and moving between living and nonliving things. Within the carbon cycle, one of the most important reservoirs, the atmosphere, is also one of the smallest. This reservoir is important because it influences the radiation balance and the global climate of the Earth. Because of its small size compared to the other C reservoirs, the CO2content of the atmosphere can be changed rapidly and is largely governed by exchanges and biogeochemical reactions occurring within and between the other reservoirs. Exchanges between atmosphere and these different C reservoirs regulate atmospheric CO2 on many different time scales. Carbon is released to the atmosphere from what are called “carbon sources” and stored in plants, animals, rocks, and water in what are called “carbon sinks.” This process occurs in a number of steps. In the first step, through photosynthesis (the process by which plants captures the sun’s energy and uses it to grow), plants take carbon dioxide out of the atmosphere and release oxygen. The carbon dioxide is converted into carbon compounds that make up the body of the plant, which are stored in both the aboveground parts of the plants (shoots and leaves), and the belowground parts (roots). In the next step, animals eat the plants, breath in the oxygen, and exhale carbon dioxide. The carbon dioxide created by animals is then available for plants to use in photosynthesis. Carbon stored in plants that are not eaten by animals eventually decomposes after the plants die, and is either released into the atmosphere or stored in the soil. Large quantities of carbon can be released to the atmosphere through geologic processes like volcanic eruptions and other natural changes that destabilize carbon sinks. For example, increasing temperatures can cause carbon dioxide to be released from the ocean.

In simplistic terms, plants take up carbon dioxide (CO2) from the atmosphere through photosynthesis and create carbohydrates that animals and humans use for food, shelter, and energy to sustain life. Emissions from plants and human activities return carbon to the atmosphere—thereby completing the cycle. This is the essence of the biological carbon cycle, which is nearly a closed system: about 99.9% of the carbon fixed by photosynthesis returns back to the oceans and atmosphere via respiration. Carbon cycles through the system and moves between reservoirs. The balance of carbon exchanges between the reservoirs makes up the carbon budget. When the inputs to a reservoir exceed the outputs, the amount of carbon in the reservoir is increased, and it is considered a carbon“sink”—a place where carbon is stored (or sequestered).When the outputs from a reservoir exceed the inputs, it is considered a carbon “source”—a place from which carbon is emitted. Non-atmospheric sinks are important for offsetting carbon emissions and preventing (or at least slowing) global warming. Investigating the factors affecting sources and sinks can inform management decisions aimed at getting us closer to a balanced carbon budget.

The Carbon cycle can be studied as a series of nested loops recycling Carbon through the atmosphere at different rates:

• Changes in atmospheric CO2 on millennial time scales (geological time scale) are controlled by tectonically driven exchange of C with the lithosphere or Entire Ocean, the upper sediment layer and the upper crust.
• Changes in atmospheric CO2 on decadal to centennial time scales involve exchanges between the atmosphere, intermediate-depth ocean water, and soil organic carbon.
• Changes on the decadal to annual time scales involve exchanges between atmosphere, Upper Ocean, leaf litter, and land biomass.
• Changes on a seasonal timescale involve the cycle of respiration/photosynthesis of land biomass.

The cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere. In August, the green areas of North America, Europe, and Asia represent plants using carbon from the atmosphere to grow. In December, net primary productivity at high latitudes is negative, which outweighs the seasonal increase in vegetation in the southern hemisphere. As a result, the amount of carbon dioxide in the atmosphere increases.

Source: NASA earth observatory

The process involved

1) Photosynthesis

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel called glucose and other sugars for building plant structures. This process forms the foundation of the fast (biological) carbon cycle.

Carbon dioxide + water + sunlight -> carbohydrate + oxygen

CO2 + H2O + sunlight -> CH2O + O2

 2) Respiration

Oxygen + carbohydrate -> energy + water + carbohydrate

O2 + CH2O -> energy + H2O + CO2

3)  Weathering (Slow carbon cycle)

The carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On an average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year.

Carbonate Rocks

a) Carbon dioxide is removed from the atmosphere by dissolving in water and forming carbonic acid

CO2 + H2O -> H2CO3 (carbonic acid)

b) Carbonic acid is used to weather rocks, yielding bicarbonate ions, other ions, and clays H2CO3 + H2O + silicate minerals -> HCO3- + cations (Ca++, Fe++, Na+, etc.) + clays

c) Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater by marine organisms like coral

Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O (the carbon is now stored on the seafloor in layers of limestone)

Metamorphism of Carbonates

Some of this carbon is returned to the atmosphere via metamorphism of limestone at the depth in subduction zones.

CaCO3 + SiO2 -> CO2 + CaSiO3

Explanation

The movement of carbon from the atmosphere to the lithosphere (rocks) begins with the rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks which is called chemical weathering and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean. Rivers carry calcium ions (the result of chemical weathering of rocks) into the ocean, where they react with carbonate which dissolved in the water. The product, calcium carbonate is then deposited onto the ocean floor, where it becomes limestone. In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on faucet if one lives in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.

The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide. When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.

Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.

Nitrogen Cycle

Nitrogen is one of the primary nutrients essential for the life or survival of all living organisms. It is a essential component of many bio-molecules, including fundamental amino acids, DNA, RNA, protein, base pair in nucleic acids, and chlorophyll. Although nitrogen is very abundant in the atmosphere as dinitrogen gas (N2), it is largely inaccessible in this form to most organisms, making nitrogen a scarce resource and often limiting primary productivity in many ecosystems. Their accessible or absorbable form is ammonia (NH3) or nitrate. In order for plants and animals to be able to use, Nitrogen must be in the form of ammonium and nitrate or organic nitrogen (urea).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 as organisms use it for growth and, in some cases, energy. The major transformations of nitrogen are nitrogen fixation, nitrification, denitrification, and ammonification. The transformation of nitrogen into its many oxidation states is key to productivity in the biosphere and is highly dependent on the activities of a diverse assemblage of microorganisms, such as bacteria, archaea, and fungi.

Steps involved in Nitrogen Cycle

To convert the atmospheric nitrogen into usable nitrogen, five steps are involved.

Nitrogen Fixation

Nitrogen fixation is the process by which gaseous nitrogen (N2) is converted to ammonia (NH3 or NH4+) via biological fixation or nitrate (NO3-) through high-energy physical processes. N2 is extremely stable and a great deal of energy is required to break the bonds that join the two N atoms. N2can be converted directly into NO3- through processes that exert a tremendous amount of heat, pressure, and energy. Such processes include combustion, volcanic action, lightning discharges, and industrial means. However, a greater amount of biologically available nitrogen is naturally generated via the biological conversion of N2 to NH3/ NH4+. A small group of bacteria and cyanobacteria are capable using the enzyme nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen.

Nitrogenase only functions in the absence of oxygen. The exclusion of oxygen is accomplished by many means. Some bacteria live beneath layers of oxygen-excluding slime on the roots of certain plants. The most important soil dwelling bacteria, Rhizobium, live in oxygen-free zones in nodules on the roots of legumes and some other woody plants.

Nitrification

Nitrification is a two-step process in which NH3/ NH4+is converted to NO3-. Firstly, the soil bacteria, Nitrosomonas and Nitrococcus convert NH3 to NO2-, and then another soil bacterium, Nitrobacter, oxidizes NO2- to NO3-. These bacteria gain energy through these conversions, both of which require oxygen to occur.

                                              Figure: The representation of steps of nitrogen cycle

Assimilation

Assimilation is the process by which plants and animals incorporate the NO3- and ammonia formed through nitrogen fixation and nitrification. Plants take up these forms of nitrogen through their roots, and incorporate them into plant proteins and nucleic acids. Animals are then able to utilize nitrogen from the plant tissues.

Anammox

Traditionally, all nitrification was thought to be carried out under aerobic conditions, but recently a new type of ammonia oxidation occurring under anoxic conditions was discovered. Anammox (anaerobic ammonia oxidation) is carried out by prokaryotes belonging to the Planctomycetes phylum of Bacteria. The first described anammox bacterium was Brocadiaanammoxidans. Anammox bacteria oxidize ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen .Anammox bacteria were first discovered in anoxic bioreactors of wasterwater treatment plants but have since been found in a variety of aquatic systems, including low oxygen zones of the ocean, coastal and estuarine sediments, mangroves, and freshwater lakes. In some areas of the ocean, the anammox process is considered to be responsible for a significant loss of nitrogen. Anammox and denitrification is responsible for most nitrogen loss in the ocean, and it is clear that anammox represents an important process in the global nitrogen cycle.

Denitrification

Denitrification is the reduction of NO3- to gaseous N2 by anaerobic bacteria. This process only occurs where there is little to no oxygen, such as deep in the soil near the water table. Hence, areas such as wetlands provide a valuable place for reducing excess nitrogen levels via denitrification processes. Denitrification is the only nitrogen transformation that removes nitrogen from ecosystem(essentially irreversibly) and roughly balances the amount of nitrogen fixed by the nitrogen fixers.

Ammmonification

Assimilation produces large quantities of organic nitrogen, including proteins, amino acids, and nucleic acids. Ammonification is the conversion of organic nitrogen into ammonia. The ammonia produced by this process is excreted into the environment and is then available for either nitrification or assimilation.

Sedimentary Nutrient Cycles

Sulphur Cycle:

Sulphur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulphur atoms. Sulphur is important for the functioning of proteins andenzymes in plants, and animals. Plants absorb sulphur when it is dissolved in water. Animals consume these plants; so that they take up enough Sulphur to maintain their health.

Most of the earth’s sulphuris tied up in rocks and salts or buried deep in the ocean in oceanic sediments. Sulphur can also be found in the atmosphere. It enters the atmosphere through bothnatural and human sources. Natural recourses can be for instance volcanic eruptions, evaporation from water, or decaying organisms. Sulphur enters the atmosphere through anthropogenic activitywhich is mainly a consequence of industrial processes where sulphurdioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.

When sulphur dioxide enters the atmosphere, it reacts with oxygen to produce sulphur trioxide gas (SO3), or with other chemicals in the atmosphere, to produce sulphur salts. Sulphur dioxide may also react with water to produce sulphuric acid (H2SO4). Sulphuric acid may also be produced from dimethylsulphide, which is emitted to the atmosphere by plankton species.

                                                               Figure: The sulphur cycle (Source: www.Lenntach.com)

Phosphorous Cycle

Phosphorus is an essential nutrient for plants and animals in the form of ions PO43- and HPO42-. It makes up an important part of DNA, RNA molecules that store energy (ATP and ADP) and of fats of cell membranes. Phosphorus is also a building block of certain parts of the human and animal body, such as bones and teeth.

Phosphorus can be found on earth in water, soil and sediments. Unlike the compounds of other matter cycles phosphorus cannot be found in air in the gaseous state. This is because phosphorus is usually liquid at normal temperature and pressure. It is mainly cycling through water, soil and sediments. In the atmosphere phosphorus can mainly be found as very small dust particles.

Phosphorus moves slowly from deposits on land and in sediments, to living organisms, and then much more slowly back into the soil and water sediment. Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts. Phosphate salts that are released from rocks through weathering usually dissolve in soil water and will be absorbed by plants. Because the quantity of phosphorus in soil is generally small, it is often the limiting factor for plant growth. That is why humans often apply phosphate fertilizers on farmland. Phosphates are also limiting factors for plant-growth in marine ecosystem, because they are not very water-soluble. Animals absorb phosphates by eating plants or plant-eating animals.

Phosphorus cycles through plants and animals much faster than it does through rocks and sediments. When animals and plants die, phosphate will return to the soil or oceans again during their decay. After that, phosphorus will end up in sediments or rock formation again, and remain there for millions of years. Eventually, phosphorus is released again through weathering and the cycle starts over.

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