Biogeochemical cycles

##Key terms

Term|Meaning

• | -
  Carbon | An essential element that forms the base of all organic matter, including the bodies of living organisms, and is a key component of fossil fuels
  Carbon dioxide $\ce{(CO2)}$ | A compound in the atmosphere that is taken in by photosynthetic organisms to make organic molecules and is later released back into the atmosphere through processes such as respiration and decomposition
  Photosynthesis |The process by which energy from sunlight is used to transform carbon dioxide from the atmosphere into the organic matter that fuels food webs
  Cellular respiration | The process by which living organisms break down organic compounds to produce usable energy, releasing carbon dioxide into the atmosphere as a by-product
  Carbon cycle | The complex flow of carbon between inorganic and organic sources within the environment

##The carbon cycle and carbon reservoirs

The carbon cycle describes the continuous flow of carbon between organic and inorganic carbon reservoirs, or areas of Earth where large amounts of carbon are stored. Most of Earth’s carbon is found in inorganic reservoirs such as rocks, water, and sediments. Only a small portion is stored in organic reservoirs, such as in the bodies of living organisms.

The carbon cycle is represented in the following diagram:

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To help us understand the carbon cycle, we can think of it as two interconnected subcycles: a biological carbon cycle and a geological carbon cycle.

##Biological carbon cycle

The biological carbon cycle occurs through biological processes and describes the relatively quick flow of carbon between the atmosphere and Earth’s ecosystems.

Carbon is introduced into food webs by photosynthetic organisms, which convert gaseous $\ce{CO2}$ from the atmosphere into biomass. As organisms carry out cellular respiration, they break and re-form molecular bonds, producing usable energy and cycling $\ce{CO2}$ back into the atmosphere. The decomposition of dead organisms and other nonliving organic matter also returns $\ce{CO2}$ to the atmosphere.

Organic matter that is not immediately broken down accumulates and is buried. Given enough time and pressure, this matter turns into fossil fuel deposits and rock, which brings us to the geological carbon cycle.

##Geological carbon cycle

The geological carbon cycle occurs by geologic processes and describes the much slower flow of carbon between Earth’s nonliving carbon reservoirs.

During the geological carbon cycle, carbon moves from rocks on land into the oceans via weathering and rainwater runoff. Carbon also enters oceans from the atmosphere as carbon dioxide dissolves into the water, forming carbonic acid and bicarbonate ions.

Bicarbonate is part of the shells of many marine creatures, and these shells and other sunken organic materials ultimately form the sediment and rocks lining the ocean floor. Carbon may remain stored deep in sediments and rocks for millions of years until an event such as a volcanic eruption returns it to the surface.

##Human impacts on the carbon cycle

The amount of carbon in the atmosphere has been increasing rapidly over the past century, and this increase is largely the result of human activity. For example, the burning of fossil fuels releases a significant amount of carbon into the atmosphere, and deforestation removes photosynthetic organisms that would normally help trap excess $\ce{CO2}.$

Although photosynthetic organisms and oceans absorb some $\ce{CO2},$ they simply can’t keep up. As a result, carbon is entering the atmosphere faster than it can cycle back into reservoirs for long-term storage. This excess atmospheric $\ce{CO2}$ contributes to global warming and, ultimately, climate change.

##Key points

• The element nitrogen is essential for life. DNA, RNA, and protein molecules all contain nitrogen atoms.

• The largest reservoir of nitrogen on Earth is the atmosphere, with nitrogen gas ($\ce{N2}$) making up about 78% of air. However, plants and animals cannot directly use this nitrogen gas. To get the forms of nitrogen they need, these organisms rely on the nitrogen cycle.

• The nitrogen cycle is the cyclic movement of nitrogen in different chemical forms between living organisms and the environment. The steps of the nitrogen cycle are described below.

• Nitrogen fixation: During this step, atmospheric nitrogen gas is fixed, or converted into a form that can be used by plants and animals. Two natural processes are responsible for most nitrogen fixation.

• In the atmosphere, energy from lightning helps recombine the atoms in nitrogen gas and oxygen gas ($\ce{O2}$), forming nitrate ($\ce{NO3-}$). This nitrate falls to Earth's surface in precipitation.

• In the soil, nitrogen-fixing bacteria use enzymes to combine nitrogen gas with hydrogen gas ($\ce{H2}$) to produce ammonia ($\ce{NH3}$). The bacteria can use this ammonia as a nutrient, or convert it into ammonium ions ($\ce{NH4+}$).

• Fixed nitrogen can be taken up by plants and incorporated into biological molecules in the plants’ tissues. This allows nitrogen to move up the food chain as plants are eaten by animals, who are themselves eaten.

• Ammonification: Nitrogen-containing biological molecules may stay in organisms’ tissues, or be broken down and released as waste. As dead organisms and waste decompose, ammonia and ammonium ions are returned to the environment through ammonification. This process is carried out by certain bacteria and fungi.

• Nitrification: During nitrification, groups of nitrifying bacteria convert the products of ammonification into nitrites ($\ce{NO2-}$), and then nitrates.

• Denitrification: During denitrification, denitrifying bacteria convert nitrites and nitrates back into nitrogen gas, returning nitrogen to the atmosphere.

• The nitrogen cycle tends to occur faster than other major nutrient cycles because living, active bacteria control many of the chemical transformations of nitrogen.

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##Key points

• Phosphorus is an essential nutrient for living organisms. DNA, RNA, and cell membranes all contain phosphorus atoms.

• Phosphorus is often the limiting nutrient in ecosystems. This means that phosphorus is the nutrient in shortest supply, so it puts a limit on plant growth. Phosphorus is scarce in ecosystems because plants use available phosphorus quickly, but natural processes replenish phosphorus slowly.

• Phosphorus moves between living organisms and the environment during the phosphorus cycle.

• The largest reservoirs of phosphorus are buried in rock and deep ocean sediments. When rocks and sediments weather, or wear down, phosphate ions ($\ce{PO4^3-}$) are released. These ions leach into surface water and soils where they are more accessible to plants and other organisms.

• In soil, phosphate ions can be taken in by plants and incorporated into biological molecules in the plants’ tissues. This allows phosphorus-containing compounds to move up the food chain as plants are eaten by animals, who are themselves eaten.

• Phosphorus-containing biological molecules may stay inside the bodies of organisms, or be broken down and released as waste. As the bodies and wastes of organisms break down, they become compacted into layers of soil and rock, returning phosphorus to the environment.

• Humans influence the phosphorus cycle by using animal waste and phosphate sediments to make fertilizer, detergent, and other products. When these products are released into the environment they can enter aquatic ecosystems as runoff. This can accelerate plant growth in these ecosystems, leading to harmful algal blooms.

• The phosphorus cycle is slow compared to other nutrient cycles because there is no atmospheric component to move phosphorus directly from the ocean to land.

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##Key points

• The vast majority of Earth's water is saltwater found in oceans. Only a tiny fraction is readily accessible freshwater, which is what humans need.

• Water found at the Earth's surface can cycle rapidly, but much of Earth's water lies in ice, oceans, and underground reservoirs; this water cycles slowly.

• The water cycle is complex and involves state changes in water as well as the physical movement of water through and between ecosystems.

• Groundwater is found underground between soil particles and in cracks of rocks. Aquifers are groundwater reservoirs often tapped by wells.

##Water: Why does it matter?

Water is pretty darn important for living things. Your body is more than one-half water, and if we were to take a look at your cells, we’d find they were over 70% water! So, you—like most land animals—need a reliable supply of fresh water to survive.

Of the water on Earth, 97.5% is salt water. Of the remaining water, over 99% is in the form of underground water or ice. All told, less than 1% of fresh water is found in lakes, rivers, and other available surface forms.

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Many living things depend on this small supply of surface fresh water, and lack of water can have serious effects on ecosystems. Humans, of course, have come up with some technologies to increase water availability. These include digging wells to get at groundwater, collecting rainwater, and using desalination—salt removal—to get fresh water from the ocean. Still, clean, safe drinking water is not always available in many parts of the world today.

Most of the water on Earth does not cycle—move from one place to another—very rapidly. We can see this in the figure below, which shows the average time that an individual water molecule spends in each of Earth’s major water reservoirs, a measurement called residence time. Water in oceans, underground, and in the form of ice tends to cycle very slowly. Only surface water cycles rapidly.

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##The water cycle

The water cycle, or hydrologic cycle, is driven by the Sun’s energy. The sun warms the ocean surface and other surface water, causing liquid water to evaporate and ice to sublime—turn directly from a solid to a gas. These sun-driven processes move water into the atmosphere in the form of water vapor.

Over time, water vapor in the atmosphere condenses into clouds and eventually falls as precipitation, rain or snow. When precipitation reaches Earth's surface, it has a few options: it may evaporate again, flow over the surface, infiltrate into the soil, or percolate—sink down—into the ground.

In land-based, or terrestrial, ecosystems in their natural state, rain usually hits the leaves and other surfaces of plants before it reaches the soil. Some water evaporates quickly from the surfaces of the plants. The water that's left reaches the soil and, in most cases, will begin to move down into it.

In general, water moves along the surface as runoff only when the soil is saturated with water, when rain is falling very hard, or when the surface can't absorb much water. A non-absorbent surface could be rock in a natural ecosystem or asphalt or cement in an urban or suburban ecosystem.

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Water in the upper levels of the soil can be taken up by plant roots. Plants use some of the water for their own metabolism, and water that's in plant tissues can find its way into animals’ bodies when the plants get eaten. However, most of the water that enters a plant's body will be lost back to the atmosphere in a process called transpiration. In transpiration, water enters through the roots, travels upwards through vascular tubes made out of dead cells, and evaporates through pores called stomata found in the leaves.

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If water is not taken up by plant roots, it may percolate down into the subsoil and bedrock, forming groundwater. Groundwater is water found in the pores between particles in sand and gravel or in the cracks in rocks, and it’s an important reservoir of freshwater. Shallow groundwater flows slowly through pores and fissures and may eventually find its way to a stream or lake, where it can become part of the surface water again.

Some groundwater lies deep in the bedrock and can stay there for millennia. Groundwater reservoirs, or aquifers, are usually the source of drinking or irrigation water drawn up through wells. Today, many aquifers are being used up faster than they're renewed by water that moves down from above.

##The water cycle drives other cycles.

The water cycle is important in itself, and patterns of water cycling and rainfall have major effects on Earth's ecosystems. However, rainfall and surface runoff also play important roles in the cycling of various elements. These include carbon, nitrogen, phosphorus, and sulfur. In particular, surface runoff helps move elements from terrestrial, land-based, to aquatic ecosystems.

We'll take a closer look at how this works in the following articles, where we'll examine different elements' biogeochemical cycles.

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