Earth’s Quadricycle

Cycles are all around us (and in us). Energy continuously flows in and out of Earth as sunlight and heat respectively, but generally speaking, the matter on our blue planet is conserved and recycled.

Cycles are all around us (and in us). Energy continuously flows in and out of Earth as sunlight and heat respectively, but generally speaking, the matter on our blue planet is conserved and recycled. Specifically, we can talk about several biogeochemical cycles that explain how elements (like nitrogen and carbon) or molecules (like water) move through different layers on Earth, including the atmosphere, the oceans, land, and yes, even in living organisms like you and me. From a biological perspective, these cycles make life on Earth possible, but they also play a role in aesthetics and culture, from stormy nights thanks to the water cycle to blossoming cempasúchil flowers (Mexican marigold) made possible by the nitrogen cycle. In the end, everything is cyclical!

The Nitrogen Cycle

Nitrogen is a very abundant element on Earth and necessary for life. Interestingly enough, roughly three quarters of our atmosphere is nitrogen. Aside from the air, it is also present in our soil and in our oceans, and in many different molecules, such as nitrates, nitrites, and ammonia.

In the nitrogen cycle, the element must pass from one type of molecule to another depending on where it is. For example, in the atmosphere it is nitrogen gas, but in the soil, it becomes nitrogen oxide and nitrogen dioxide. The cycle has five stages and begins, so to speak, in the atmosphere, continues in the soil and within organisms that inhabit the soil, and ends again in the atmosphere.

Bacteria, fungi, and plants play key roles in the nitrogen cycle to transform the differing types of nitrous molecules; for example, nitrogen-rich soils can accelerate plant growth and nitrogen is a typical waste byproduct in decomposition.

One thing to also consider, however, is that too much nitrogen is not a good thing, usually. Many of our human-made products contain nitrogen or have nitrogen as a byproduct, which can sometimes disrupt the natural nitrogen cycle on Earth. A common example is the use of fertilizers, which are very nitrogen-rich, but oftentimes leak into other cycles, like the water cycle, and can pollute and alter the ecosystem of the affected region.

The Carbon Cycle

Like nitrogen, carbon is a vital element for life on Earth. In fact, chemically speaking, whenever we talk about “organic” we are really talking about carbon-hydrogen bonds! 

Like nitrogen, carbon can also be found in the three main compartments of Earth, namely, the atmosphere, on land (the lithosphere), and in the oceans (the hydrosphere) and the roles that it plays in each of these affect us all. 

This cycle also begins in the atmosphere, from where carbon moves to plants as carbon dioxide (CO2) and through photosynthesis becomes part of the plant itself (more on that below). Then the carbon moves from the plants to animals which eat them. Then it moves from those plants and animals into the soil after they die. There, ideally some of that carbon will stay there for millions of years as fossil fuels. If not, it moves back into the atmosphere from fossil fuel burning. It also moves back into the atmosphere from living animals that breathe.

The form of carbon that we hear the most about is probably CO2 and how its rising level in our atmosphere is affecting our global climate. One reason for this rise has to do with the carbon found in the products of chemical reactions used for many industrial processes and general burning of fuel (from those fossil fuels that should have stayed underground). If the carbon cycle on Earth was in balance, then this would not be an issue. But when too much carbon is being taken from the land and burned for energy—therefore being released into the atmosphere as CO2—then the cycle is thrown off course and the result is climate change. 

There are sections within the carbon cycle that are called carbon sinks that in a balanced cycle can absorb carbon excess without negative impacts. For example, important carbon sinks are forests and the ocean, which is the last place that carbon can go because it naturally absorbs carbon from the air. Unfortunately, within the off-balance carbon cycle that humanity has induced, this has resulted in the acidification and warming of the ocean, which in turn is also wreaking havoc on marine ecosystems. 

To combat this problem of too much carbon in the atmosphere, we rely in part on something called carbon sequestration, which are processes that take out carbon dioxide from our atmosphere in order to mitigate the changes in climate that are affecting living organisms around the globe. These processes can be nature-based with human intervention or can be technological. In essence, we are trying to get that carbon cycle back on track because it cannot do it on its own anymore.


Photosynthesis is a process of converting light energy into chemical energy which some organisms, like plants, use to fuel their growth. This process plays a major role in the nitrogen cycle, the carbon cycle, and the water cycle.

Plants require nitrogen for proteins and for chlorophyll, which they use to convert light energy into fuel. Specifically speaking, the photosynthetic apparatus consists of the reaction of carbon dioxide and water (where those two respective cycles come into play) with light energy to create sugar and oxygen.

As we have previously explored, because of photosynthesis, plants (mainly in the form of forests) serve as carbon sinks when they absorb carbon dioxide from the atmosphere for respiration and release oxygen as a product in the chemical reaction. Photosynthesis, then, is a player within the carbon cycle.

The Water Cycle

This vital cycle deals with the molecule H2O, as opposed to the nitrogen or carbon cycles, which deal with a specific element in various molecules. The water cycle also encompasses all the regions and surface layers on Earth and the different physical states of matter, namely: gas, liquid, and solid. 

When liquid water from a body of water becomes vapor, this is called evaporation. Water vapor then typically rises in our atmosphere and forms clouds through a process called condensation. When the clouds get heavy enough, the water forms droplets that fall back to Earth via precipitation. Examples of precipitation include rain, snow, and hail. Snow and hail are examples of water in its solid phase after undergoing freezing. Once precipitation makes landfall it can seep into the soil deep into water reservoirs or drain into a body of water, such as a lake or ocean, and the cycle is ready to repeat. In addition to evaporation, water can also move into the atmosphere from plants through transpiration.

Although this may be in a sense the simplest cycle, it is no less important than the other ones and constitutes one of the most defining features of Earth that allows for life. Unfortunately, like the nitrogen and carbon cycles, the water cycle is being impacted by human activity and altered by climate change, resulting in more frequent and severe weather events like hurricanes and draughts, which all depend on the water cycle.

As we can see, life (and death) on Earth is cyclical, even at its most basic and fundamental levels! From a chemical perspective, every part of us has been around since the dawn of time and has travelled to all the nooks and crannies of the planet, and will continue to do so long after our bodies perish. The water we drank today may have been the same water that a T-rex was drinking, and it may be the same water to make it into a glacier on Antarctica in a couple number of eons. The carbon that we breathe out may eventually be stored underground as fossil fuels. The question is if we will manage to bring the cycles back into balance so that they may repeat themselves indefinitely, as they have done since life on earth first flourished.


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