The first cycle that we will go over is one that you are likely fairly familiar with, but that is nonetheless absolutely essential for life. Life on Earth could not exist without water; it is needed for a wide variety of biochemical reactions, is an essential part of many ecosystems, and plays an important role in a variety of environmental and geological processes. The hydrologic cycle, also known as the water cycle, is a process through which water moves between different sources and sinks. The key components of this cycle are the state of matter that water is in (solid, liquid, or gas) as well as where water is moving.

The largest reservoir of water is by far the ocean, which holds over 97% of Earth's water. The remaining 3% is the freshwater that we humans need, and about 2% is in the form of ice caps and glaciers; less than 1% of Earth's water is groundwater, surface fresh water, or atmospheric water vapor that will become rain.

The major thing powering this cycle is the sun; solar energy drives the progress of the water cycle. The heat from the sun causes evaporation - the liquid water will change into gaseous water vapor and enter the atmosphere. It can also cause transpiration, a process where water will move through a plant and water vapor will enter the atmosphere through their leaves. Evapotranspiration, the combined effect of these two processes, is the main source of water in the atmosphere. These two processes work to distill water - the solutes that were dissolved in water are left behind as water becomes water vapor.

Water vapor in the atmosphere will condense back into liquid water droplets. Gravity will pull these droplets back to Earth's atmosphere, where they will fall as precipitation. Some water will quickly be used by plants and animals, but most will become runoff. Runoff is the flowing water on the ground that occurs when there is too much water for the soil to absorb. This runoff will flow into streams, rivers, lakes, ponds, wetlands, and oceans. From here, it can be evaporated and repeat the process again. Some water, however, undergoes infiltration - it will sink through permeable soil and rock in the ground and enter an aquifer, an underground reservoir of groundwater. Other water will freeze to become ice in glaciers.

All of the land that drains into a common outlet is known as a watershed. Sometimes called drainage basins, these areas of land will vary in how much water infiltrates vs runs off into a body of water by things like its size, slope (steeper slope = faster water = more runoff), soil composition, and vegetation (more vegetation = more infiltration = more groundwater recharge.

Humans are interfering with the water cycle in a number of ways. In some areas, we withdraw fresh water from reservoirs faster than it is naturally replaced, causing the depletion of water sources. When we clear land for creating farms, cities, roads, etc., we oftentimes reduce the amount of infiltration that can happen and therefore increase runoff and flooding.

Carbon is the basic building block of all organic molecules required for life, including the four major biomolecules - carbohydrates, lipids, proteins, and nucleic acids.

One of the key reservoirs of carbon is the atmosphere. The amount of carbon found in the atmosphere has drastic effects on Earth's climate and it is continually being added and removed from the atmosphere in the form of carbon dioxide (CO2). Photosynthesis and Cellular Respiration are both relatively quick processes that work to cycle CO2 in the biosphere. Photosynthesis is a process carried out by autotrophs, or producers. They will pull the CO2 out of the atmosphere and use it to create sugars. Cellular Respiration, carried out both by producers and heterotrophs, or consumers,  does the opposite. It breaks down the sugars that were made for energy and, as a byproduct, releases CO2 back into the atmosphere. When decomposers break down waste and dead organic matter, even more CO2 is released. If you are interested in more details about how these processes work at a molecular level, please click the following links for Photosynthesis or Cellular Respiration.

CO2 from the atmosphere can diffuse into and be absorbed into oceans. Carbon-containing compounds can also enter oceans through processes such as runoff or the eruption of undersea volcanoes. Algae and phytoplankton in the ocean will remove CO2 from the ocean and atmosphere through the process of photosynthesis, much like organisms will add it via cellular respiration. Some marine organisms, such as corals, will use carbon-containing molecules in order to form calcium carbonate exoskeletons. As marine organisms die, their bodies sink to the ocean floor and are broken down into carbon-containing sediments in a process known as sedimentation.

Burial is the process through which carbon is stored in underground carbon sinks, either as sediments that formed via sedimentation or as fossil fuels (such as oil, gas, or coal). This will happen with the remains of organic matter in both marine and terrestrial ecosystems. The carbon trapped in these deposits can eventually be released naturally via geological processes such as erosion, volcanic eruptions, or uplift. However, we can also release it ourselves through extraction (the digging up and mining of fossil fuels) and combustion (the burning of fossil fuels as energy). Burial is a slow process and takes an extremely long time. We have been extracting and burning fossil fuels at a much greater rate than the sinks are being replaced, leading to an imbalance - more CO2 is being added to the atmosphere than is being removed.

As the levels of CO2 in the atmosphere continue to increase, the planet warms and climate change becomes more and more severe. 

Nitrogen is needed by all living things for the formation of their proteins and nucleic acids (DNA or RNA). The atmosphere is a massive reservoir for nitrogen, with about 78% of the volume of the atmosphere being N2, but nitrogen in this form is not usable by plants or animals. Nitrogen needs to be converted into a different form via lightning, bacteria, or human meddling, in order to be cycled through and used by plants and animals.

Nitrogen fixation is the process through which N2 becomes ammonia (NH3) or its water-soluble ion counterpart, ammonium (NH4+), two biologically available forms of nitrogen that are useable by plants. One of the major ways this happens is through nitrogen-fixing bacteria. These are bacteria in the soil or water that convert N2 into ammonia or ammonium. They are oftentimes found in mutualistic relationships with various plants where the plants are provided essential nutrients in the form of useable nitrogen. Lightning strikes are capable of converting atmospheric nitrogen into usable forms due to their high-energy nature. In addition, through synthetic fixation, humans are capable of burning fossil fuels in order to convert nitrogen into a usable form, which we often use to make fertilizers.

Nitrification is the process where ammonium is turned into nitrite (NO2-) and nitrate (NO3-). These as water-soluble and are easily taken up by plants and phytoplankton, which use them as a resource for growth, through the process of assimilation. Animals will take in the nitrogen as they eat the plants or phytoplankton. Decomposers obtain nitrogen from breaking down waste and dead organisms. They release ammonium ions as a byproduct from doing so, through a process known as ammonification. Just like there are nitrogen-fixing bacteria, there are also denitrifying bacteria. They convert nitrates in the soil or water into N2 which is then released back into the atmosphere, through a process known as denitrification.

Humans are affecting the nitrogen cycle in a variety of ways. As we create fertilizers, we are increasing the amount of nitrogen that moves from the atmosphere to Earth's surface. Nitric oxide (NO) is produced through the combustion of fossil fuels. Nitrous oxide (NO2) is a greenhouse gas that is produced from NO in the atmosphere. It damages the ozone layer and can lead to the warming of the Earth. NO can also become nitric acid vapor (HNO3). This can fall down to Earth as acid rain, damaging buildings and rocks, and causing harm to plants and animals. Acid rain can also be caused by ammonia volatilization - a process where excessive fertilizer in the soil will form ammonia which enters the atmosphere; this can also cause respiratory issues in humans and other animals. Fertilizers can also lead to leaching - a process through which nitrates from synthetic fertilizers are carried out of the soil by water runoff. This can lead to eutrophication, which causes algae blooms in water sources that kill other aquatic species.

Phosphorus is needed by living things in order to make nucleic acids, such as DNA, RNA, and ATP. Unlike the other three cycles, the phosphorus cycle does not have the atmosphere as a reservoir as none of the phosphorus-containing compounds of the cycle are gaseous. There is some windblown dust and sea spray, but not enough to make the atmosphere a significant reservoir. The main reservoir for phosphorus is phosphate-containing rocks and sediments. Due to the nature of where phosphorus is found, and the fact that it relies on the breakdown and formation of rocks, the phosphorus cycle is a very slow cycle in comparison to the other three. Phosphorus is often the limiting nutrient in an ecosystem because of this.

Weathering, the gradual breaking down of rocks by physical, chemical, or biological means, is the main way phosphate is released. It will release phosphate ions, which will dissolve into water. These ions can remain in the water or can be carried into the soil. Plants will absorb the phosphate through assimilation and animals will take in their needed resource through the organisms that they eat. As organisms die or produce waste, decomposers will break down the phosphorus-rich matter and return phosphorus to the soil. Assimilation, excretion, and decomposition will form a small loop of their own within the cycle.

Phosphates that are dissolved in bodies of water can precipitate into solid bits of phosphate. These will settle at the bottom of water as sediment, which over long periods of time will be compressed by the pressure of the overlying water into sedimentary rocks through the process of sedimentation. Phosphorus-containing rocks that are found at the bottom of the ocean can be brought to the surface through geological uplift - a process where tectonic plates collide, forcing rock layers up, and forming mountains. This allows weathering of the rock to occur above the surface.

Humans will mine phosphate minerals from rocks in order to create synthetic fertilizers and detergents. Fertilizers can be carried into nearby bodies of water through runoff, which can lead to eutrophication. In eutrophication,  excess nutrients (particularly nitrogen and phosphorus) will fuel algae growth. Algae blooms will form, covering the surface of the water, which blocks the sunlight and kills plants below the surface. The dead plants will be broken down by decomposers, which use oxygen to do so. Low oxygen levels will result in the death of aquatic animals, which will then be decomposed, using more oxygen, resulting in more death, and so on...