The Cycles of Nature

Every single thing in the universe is made up of matter. The atoms that make up matter can never be created or destroyed, just rearranged in different forms. That means that the atoms that were created during the Big Bang make up the tree in your backyard, your hand, the chair you’re sitting in, and everything else! The important nutrients and molecules that we are made of all move through the food web (yum!). But the food web is just one part of our world’s natural cycles. The carbon, nitrogen, phosphorus, and water cycles are some of the main ways that ecosystems recycle and transform matter. These cycles provide us with food, energy, and fresh water. They fertilize the soil and regulate our climate. Whether it is the rain from the sky, the carbon in our bones, or the dirt under our feet, we rely on the balance of these cycles that make life possible on Earth.

Nutrients and molecules like oxygen, carbon, and water can be stored in “reservoirs.” Some reservoirs hold nutrients for a short period; others hold them for centuries. For example, a relatively short-term reservoir for water is a lake—water molecules (H20) just need a hot day to cycle back up into the clouds via evaporation and later come back down as rain. Meanwhile, glaciers act as long-term reservoirs, storing water in its frozen form for centuries. Releasing too much of a locked-away resource too quickly can have negative effects on our global ecosystems. We need to understand these different reservoirs and responsibly preserve the delicate balance of these important cycles.

The Carbon Cycle

Every living thing you can think of is made of carbon. You, your dog, the grass on your lawn, and the worms in the ground are all carbon-based life forms. Not only is every living thing on Earth made of carbon, but we also depend on the carbon cycle for cellular respiration, breathable air, and climate regulation. The carbon cycle relies on algae and plants (aka producers), which absorb carbon dioxide (CO2) from the atmosphere and use photosynthesis to turn it into sugars. During this process, carbon dioxide is absorbed and oxygen is released into the atmosphere. The sugars in plants are a form of stored energy. When plants are eaten, their stored energy and carbon compounds begin their journey through the food web.

Carbon is stored in the bodies of plants and animals for some time. Some of that carbon becomes poop or other waste. Eventually living things die and their carbon is broken down by decomposers. Both waste and dead matter are a part of the food web, and when broken down by bacteria and fungi, their carbon becomes part of the nutrient-rich soil that plants need. This is one reason that farmers use manure or compost to help crops grow.

Carbon is a vital part of sugar (glucose) molecules, which are a form of stored energy. Living things use this energy to do work in a complicated process called cellular respiration. During cellular respiration, carbon dioxide is released back into the atmosphere. Photosynthesis is the process that creates stored energy, and cellular respiration is the process of using that energy. Photosynthesis is only done by plants and other producers, and uses carbon dioxide, while releasing oxygen into the air as a byproduct. Meanwhile, cellular respiration is done by all living things and uses oxygen, while releasing carbon dioxide into the air as a byproduct.

The cycling of oxygen and carbon keeps our air breath-able, regulates global temperatures, balances the pH of the ocean, and helps to keep our soil fertile. Certain human activities are upsetting to the balance of the carbon cycle. The rapid burning of fossil fuels is releasing more carbon dioxide into the atmosphere than ever before, causing global temperatures to rise and altering ecosystems worldwide (see this page). Understanding the balance of the carbon cycle is important to protecting our planet.

The Nitrogen Cycle

Nitrogen makes up about 78 percent of our air and is an important building block for proteins and the nucleic acids that make up the DNA of living things. Although there is atmospheric nitrogen all around us, it is impossible for plants and animals to absorb it directly. Nitrogen is usually found as N2, in which two nitrogen atoms are strongly bound together. Luckily, certain bacteria can “fix” this very strong molecule so that plants and animals can use it.

All living things rely on a process called “nitrogen fixation” that turns nitrogen gas (N2) into compounds that plants can absorb. This transformation is done by certain types of microscopic bacteria that live in soil, certain types of blue-green algae found in water, and microbes that live on the root nodules of certain legume plants. Through several transformative processes, microbes turn nitrogen (N2) into molecules that plants love, like nitrates (NO3-). Certain plants, like rice, can also absorb nitrogen in the form of ammonium (NH4+).

Once Nitrogen is absorbed by plants it becomes available to the rest of the food web. As consumers eat plants (and then are eaten by other animals), the nitrogen is also passed along and used. Nitrogen compounds return to the soil when bacteria decompose dead organic matter and waste. Plants absorb this recycled and decomposed nitrogen as well.

The nitrogen cycle is completed when different types of de-nitrifying bacteria turn nitrates back into pure atmospheric nitrogen (N2). Those strong nitrogen molecules return to the atmosphere until the cycle starts all over again.

Nitrogen (N2) bonds are so strong there is only one other way to break them apart: Lightning! The energy in a bolt of lightning can “fix” a small amount of atmospheric N2 that plants can use. We also have learned how to artificially break apart N2 to create fertilizer to help plants grow and create large farms to feed our growing population.

Like nitrogen, phosphorus is needed by all organisms to build DNA, the genetic code that tells our cells what to do. Phosphorus is locked underground in sedimentary rocks that formed from dead animals and plants over the course of millions of years. Eventually these rocks surface and are eroded by the weather or are eaten away by specialized bacteria called lithotrophs. The phosphorus then dissolves into water or seeps into the soil, where it can be absorbed by plants and become available to the rest of the food web. Animals and people eat food and the phosphorus becomes part of their DNA. Eventually plants and animals die and are decomposed by bacteria. Most of that phosphorus returns to the soil to be absorbed by plants again. A phosphorus atom can cycle through just the biological system of the food web and decomposition for over a hundred thousand years. Occasionally, dead animals or plants end up in deep holes in the ocean, where the environment is so extreme that there are no decomposers to break it down. Time and pressure turn dead organic matter into sedimentary rocks. Over millennia, rocks surface and erode, and the cycle begins again. As you can see, phosphorus rocks!

Phosphorus and nitrogen are vital to life on Earth but are hard for plants to access. This is why people have created fertilizers to artificially reinvigorate the soil and ensure that plants will grow. Fertilizers have helped us feed our growing population, which is fantastic! But too much of a good thing can be dangerous. Fertilizer runoffs into waterways have thrown off the balance of ecosystems and have created dead zones in the oceans. We need to change how fertilizers are used by farms and prevent agricultural runoff so we can minimize this kind of pollution.

The Water Cycle

Whether you’re having a refreshing glass of water on a sunny day or you’re stuck inside because of the rain, you’re seeing the water cycle in action. Water (H20) covers over 70 percent of the earth’s surface and makes up about 60 percent of our bodies. Even though water is all around us, drinkable water is actually a scarce resource. We all depend on the water cycle to filter and distribute fresh water around the globe.

Eventually, all water molecules reach the ocean. As the sun heats the ocean’s surface, water molecules evaporate into the air, leaving behind the salt or minerals that make it undrinkable. This fresh water condenses to form clouds. These fluffy reservoirs of fresh water float around the entire world. When the clouds get too heavy, gravity brings that fresh water back down to Earth in the form of rain, snow, or even hail, which distributes fresh water. Now plants, animals, and people have something to drink!

Some of that water evaporates right away because of the sun’s heat. Some of it is frozen on the top of mountains as glaciers. Some is pulled down by gravity even farther, sinking into the ground. Over time, that soil moisture is used by plants or animals, or slowly moves underground and makes its way back to the ocean. The water in mountain snow slowly melts to feed streams and rivers that lead to the ocean. River and groundwater runoff draws salt and other minerals to the ocean. Mineral runoff, constant evaporation of fresh water, and erosion of rocks all contribute to making ocean water salty.

The water that is sucked up by plants and slurped up by animals and humans is also a part of the water cycle. What isn’t released as urine will either evaporate from our bodies when we sweat or be released as water vapor on our breath. Water leaves plants in the form of gas in a process called “transpiration.”

In certain parts of the world it can seem like drinkable water is everywhere; however, globally over two billion people do not have regular access to clean water. Water shortages are caused by scarcity in dry regions combined with the lack of funds to import water. Other water shortages are purely economic, when communities are surrounded by water but do not have the resources to dig wells or sanitize it. Together we need to think about how we are using water sustainably and how water can be equitably distributed.

Plants

We all depend on our leafy friends. Whether the mighty oak tree or a microscopic cell of algae, plants are the only living things that can get energy directly from the sun. Through photosynthesis, plants combine sunlight, carbon dioxide, and water to create glucose, a type of sugar. The plant uses this sugar for energy (food!) and helps to build its own structure. The waste that is released during photosynthesis is oxygen! Plants naturally create the oxygen-rich air that we must have to breathe.

Plants’ ability to create their own food with sunlight makes them the start of almost every food web. They also cycle important nutrients into the food web by absorbing them from the soil. Energy and nutrients are passed along to us when we eat plants or animals that have eaten plants. Roots of plants also help stabilize the soil beneath our feet, prevent erosion, and protect coastlines from floods. The world we live in, the food we eat, and the air we breathe all exist thanks to plants!