The Earth's quadricycle

Cycles surround us (and shape us). Energy continuously enters and leaves the Earth in the form of sunlight and heat, respectively, but in general, the matter of our blue planet is conserved and recycled. Specifically, we can talk about several biogeochemical cycles that explain how elements (such as nitrogen and carbon) or molecules (such as water) move through the different layers of the Earth, including the atmosphere, the oceans, the earth and, yes, even in living organisms like you and me. From a biological point of view, these cycles make life possible on Earth, but they also play a role in aesthetics and culture, from stormy nights thanks to the water cycle, to the cempasúchil flowers that exist thanks to the nitrogen cycle. In the end, everything is cyclical!

The nitrogen cycle

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

Within the nitrogen cycle, the element must pass from one type of molecule to another depending on where it is found. For example, in the atmosphere it is gaseous nitrogen, but in the soil it is converted to nitrogen oxide and dioxide. The cycle has five stages and begins, as it were, in the atmosphere, continues in the soil and within the organisms that inhabit it, and ends again in the atmosphere.

Bacteria, fungi and plants play key roles in the nitrogen cycle to transform different types of nitrogen molecules; for example, nitrogen-rich soils can accelerate plant growth and nitrogen is a typical by-product of decay.

However, it should also be noted that an excess of nitrogen is usually not good. Many man-made products contain nitrogen or emit nitrogen as a by-product, which can sometimes alter Earth's natural nitrogen cycle. A common example is the use of fertilizers, which are very rich in nitrogen, but often leak into other cycles, such as water, 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're really talking about carbon-hydrogen bonds!

Like nitrogen, carbon is also found in the Earth's three main compartments: the atmosphere, the earth (the lithosphere) and the oceans (the hydrosphere), and the roles it plays in each of them affect us all.

This cycle also begins in the atmosphere, from where carbon is transferred to plants in the form of carbon dioxide (CO2) and, through photosynthesis, it becomes part of the plant itself (more on that below). Carbon then passes from the plants to the animals that eat them. Eventually, it moves from those plants and animals to the soil after they die. There, ideally, some of that carbon should remain for millions of years as a fossil fuel. If not, it returns to the atmosphere when fossil fuels are extracted and burned for energy. Carbon also returns to the atmosphere due to animal respiration.

The form of carbon we hear the most about is probably CO2 and how its increasing level in our atmosphere is affecting our global climate. One of the reasons for this increase has to do with the carbon found in the products of chemical reactions used for many industrial processes and the general burning of fuel (of those fossil fuels that should have remained underground). If the Earth's carbon cycle were in balance, this wouldn't be a problem. But when too much carbon is extracted from the Earth and burned for energy - and released to the atmosphere in the form of CO2 - the cycle is diverted and the result is climate change.

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

To combat this excess carbon in the atmosphere, we rely in part on something called carbon capture, which are processes that remove carbon dioxide from our atmosphere to mitigate changes in climate that are affecting organisms around the world. These processes can be based on nature with human intervention or they can be technological. In essence, We're trying to get the carbon cycle working again because he can't do it himself anymore.

Photosynthesis

Photosynthesis is a process of converting light energy into chemical energy that some organisms, such as plants, use to fuel their growth. This process plays an important role in the nitrogen cycle, the carbon cycle and the water cycle.

Plants need nitrogen for proteins and for chlorophyll, which they use to convert light energy into fuel. Specifically, 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 (food) and oxygen (by-product).

As we have explored before, thanks to 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, therefore, is an actor in the carbon cycle.

The water cycle

This life cycle deals with the H2O molecule, unlike nitrogen or carbon cycles, which deal with a specific element in several molecules. The water cycle also encompasses all regions and surface layers of the Earth and the different physical states of matter: gas, liquid and solid.

When liquid water in a body of water is converted to vapor, this is called evaporation. Subsequently, the vapor usually rises to the atmosphere and forms clouds through a process called condensation. When clouds are heavy enough, water forms droplets that fall to Earth through precipitation. Examples of precipitation include rain, snow, and hail. Snow and hail are examples of water in its solid phase after freezing. Once precipitation makes landfall, it can seep into the ground deep into aquifers or drain into a body of water, such as a lake or ocean, and the cycle is ready to repeat itself. In addition to evaporation, water can also pass into the atmosphere from plants through transpiration.

Although in a certain sense it is the simplest cycle, it is no less important than the others and constitutes one of the most defining features of the Earth that allows life. Unfortunately, like the nitrogen and carbon cycles, the water cycle is being affected by human activity and altered by climate change, giving rise to more frequent and serious meteorological phenomena, such as hurricanes and droughts, which depend on the water cycle.

As we have seen, life (and death) on Earth is cyclical, even at its most basic and fundamental levels. Chemically speaking, every part of us has existed since the beginning of time and has traveled to every corner of the planet, and will continue to do so long after our bodies perish. The water we drink today may have been the same water that a dinosaur drank, and it may be the same as it reaches a glacier in Antarctica a couple of eons from now. The carbon we exhale can end up being stored underground in the form of fossil fuels. The question is whether we will be able to rebalance the cycles so that they repeat themselves indefinitely, as they have done since life flourished on Earth.