What Impact Have Humans Had On One Of The Biogeochemical Cycles?

Biogeochemical cycles are the pathways by which key chemical elements circulate through the Earth’s systems, including the water cycle, rock cycle, and cycles for biologically important elements like carbon, nitrogen, and phosphorus. These cycles are critical in sustaining life on Earth, as they regulate the availability and distribution of nutrients, influence environmental processes like climate change, and facilitate ecological functions. The biogeochemical cycles operate through the combined effects of biological, geological, and chemical pathways, transporting materials through the lithosphere, hydrosphere, atmosphere, and biosphere.

The carbon, nitrogen, phosphorus, and sulfur cycles are some of the most crucial biogeochemical cycles for life, as these elements are indispensable ingredients for organic matter and organisms. Disruptions to these cycles from human activities like fossil fuel emissions, agriculture, and land use changes can have major ripple effects ecologically. Therefore, it is important to understand both the natural flows of these biogeochemical cycles, as well as the ways in which humans have altered them.

Table of Contents

The Carbon Cycle

The carbon cycle is the biogeochemical cycle that describes the movement of carbon as it is recycled and reused throughout the environment. Carbon is an essential element for life on Earth and is a key component of many molecules, including carbohydrates, lipids, proteins, and nucleic acids. Carbon can exist in an organic form in living or formerly living organisms, or in an inorganic carbon form, such as carbon dioxide gas.

The carbon cycle has four main processes:

Photosynthesis – Plants, algae, and some bacteria take in carbon dioxide from the atmosphere and use sunlight to convert it into glucose, releasing oxygen as a waste product. This process stores energy from the sun in the glucose molecule.

Cellular respiration – Most living organisms undergo cellular respiration, where glucose is broken down and the stored energy is released. Oxygen is used for this process and carbon dioxide is produced as a byproduct, returning it to the atmosphere.

Decomposition – Once living organisms die, decomposers like bacteria and fungi break down their organic matter, releasing carbon dioxide gas back into the atmosphere.

Combustion – The burning of fossil fuels like coal, oil, and natural gas releases large amounts of carbon dioxide quickly into the atmosphere. This carbon was stored underground for millions of years before being released by human activities.

Human Impact on the Carbon Cycle

Human activities have significantly altered the global carbon cycle, especially in the last century. The burning of fossil fuels like coal, oil, and natural gas releases large amounts of carbon dioxide into the atmosphere. Since the Industrial Revolution began in the mid-1700s, the concentration of atmospheric carbon dioxide has increased by over 40%. Fossil fuel emissions continue to climb as countries rely heavily on coal, oil, and natural gas for energy production. Human-caused carbon dioxide emissions far outweigh natural sources like volcanoes and hot springs.

Deforestation also contributes to rising atmospheric carbon dioxide levels. Trees absorb and store carbon as they grow. When forests are cleared, carbon stored in the trees is released. Tropical deforestation is especially concerning, as rainforests store large amounts of carbon and removing them at current rates could add significantly to global carbon dioxide emissions.

In addition, some agricultural practices increase atmospheric carbon dioxide. The use of heavy machinery like tractors burns fossil fuels and releases carbon dioxide. Certain fertilizers can also emit greenhouse gases. And the drainage of wetlands for farming exposes organic matter to oxygen, causing it to decompose and release stored carbon.

The human disruption of the carbon cycle through fossil fuel emissions, deforestation, and agriculture is a major driver of climate change. The increased carbon dioxide traps more heat, causing rising global temperatures, shifting climate patterns, and severe weather events. Reversing the human impact on the carbon cycle is crucial to mitigate climate change.

The Nitrogen Cycle

The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen compounds in nature. Nitrogen is an essential nutrient for life and is a major component of nucleic acids, proteins, and other biological molecules.

Nitrogen exists in the atmosphere in the form of nitrogen gas (N2). However, most organisms cannot use nitrogen in this form. To be usable for most organisms, nitrogen must be “fixed”, or converted into forms like ammonia (NH3) or nitrate (NO3-).

There are three main processes in the nitrogen cycle:

Nitrogen fixation – conversion of N2 into more usable forms like NH3. This can occur through industrial fertilizer production, bacterial fixation by organisms like rhizobia, or lightning strikes.
Nitrification – conversion of NH3 to NO3- by nitrifying bacteria. This occurs in two steps (NH3 to NO2- then NO2- to NO3-).
Denitrification – conversion of NO3- back to N2 by denitrifying bacteria. This step closes the nitrogen cycle.

Assimilation occurs when plants take up the nitrogen compounds from the soil and incorporate them into new proteins, DNA, and other molecules. Ammonification converts organic nitrogen from decaying material back into NH3 to restart the cycle.

Human Impact on the Nitrogen Cycle

Humans have dramatically altered the global nitrogen cycle through activities like fertilizer use, growing legume crops, and burning fossil fuels. This has increased the amount of biologically available nitrogen, with significant environmental consequences.

The development of the Haber-Bosch process in the early 20th century allowed humans to convert inert nitrogen gas in the atmosphere into ammonia that can be used to synthesize fertilizer. Fertilizer use has increased over 10-fold in the last century. Excess fertilizer runs off into waterways, causing eutrophication and dead zones.

The cultivation of legumes has also increased biologically fixed nitrogen. Legumes form symbiotic relationships with bacteria that convert nitrogen gas into a usable form. Expanding agriculture devoted to legumes like soybeans and alfalfa has increased nitrogen fixation.

Finally, burning fossil fuels releases oxidized forms of nitrogen like NOx into the atmosphere. These nitrogen oxides contribute to smog, acid rain, and have increased the amount of biologically available nitrogen.

The Phosphorus Cycle

Phosphorus is an essential nutrient for all life on Earth. The phosphorus cycle describes how phosphorus moves through the lithosphere, hydrosphere, and biosphere.

Phosphorus exists in many different forms, but the most common form is phosphate (PO43-). Phosphates are found in rocks and sediments on land, as well as dissolved in ocean water. Weathering and erosion of rocks releases phosphate into rivers, streams, and groundwater, which eventually flows into the ocean.

Plants and animals take up phosphates from soil and water to build biomass. Phosphorus is a key component of important biological molecules like DNA, RNA, and ATP. When organisms die, their biomass decomposes and releases phosphates back into the environment. Some phosphates get buried in ocean sediments or incorporated into new rock.

On land, phosphates cycle between organisms, soil, and water. In the ocean, upwelling from deep water and river inputs replenish surface phosphates. Overall, the phosphorus cycle recycles phosphorus slowly over millions of years between the land, oceans, and organisms.

Human Impact on the Phosphorus Cycle

Human activities have significantly altered the natural phosphorus cycle in various ways. Two major human impacts on the phosphorus cycle are through agricultural runoff and detergents.

Phosphorus is an essential nutrient for plant growth. In agriculture, phosphorus fertilizers are applied to cropland to increase crop yields. However, excess phosphorus from fertilizers often runs off from fields into nearby waterways. This agricultural runoff introduces high concentrations of phosphorus that can cause algal blooms and eutrophication in lakes and rivers. Algal blooms reduce water quality and can be toxic to aquatic life.

Another human impact comes from phosphates present in detergents. When detergents containing phosphates are used in households and industry, they end up flowing into wastewater. This phosphate-rich wastewater is discharged into waterways, adding additional phosphorus. This extra phosphorus again promotes algal blooms and eutrophication.

To reduce these impacts, many jurisdictions have restricted or banned the use of phosphates in detergents. Agricultural practices are also evolving to include vegetated buffer strips around fields to filter out phosphorus before it reaches waterways. However, human alterations to the phosphorus cycle remain a significant environmental issue in many areas.

The Sulfur Cycle

The sulfur cycle describes the movement of sulfur through the atmosphere, lithosphere, hydrosphere, and biosphere. Sulfur is an essential element for all life and is a component of many proteins, vitamins, and other molecules. The sulfur cycle is complex due to the many oxidation states sulfur can exist in (-2 to +6).

The key processes in the sulfur cycle include:

Volcanic emissions – Sulfur gases, like hydrogen sulfide (H2S), sulfur dioxide (SO2), and carbonyl sulfide (OCS) are released into the atmosphere and oceans from volcanic activity.
Chemical weathering – Sulfide minerals in rocks are oxidized to sulfuric acid, releasing sulfate (SO42-) into waterways.
Decomposition – Bacteria and fungi release sulfur from decaying organic matter.

Assimilation – Plants, algae, and other producers take up and assimilate sulfur into proteins and other organic molecules.
Desassimilation – Sulfur is released back into the environment through excretion and decomposition.
Bacterial metabolism – Bacteria carry out redox reactions, converting between different sulfur compounds.

These processes maintain a balance of sulfur species in reservoirs like the atmosphere, biosphere, lithosphere, and hydrosphere. The length of time sulfur remains in a particular reservoir ranges from days in the atmosphere to millions of years locked in sedimentary rocks.

Human Impact on the Sulfur Cycle

Humans have had a major impact on the global sulfur cycle, primarily through the burning of fossil fuels and agricultural practices. Fossil fuel emissions are the largest source of sulfur introduced into the environment by human activity. When oil, coal, and natural gas are burned, sulfur dioxide (SO2) is released into the atmosphere.

Once in the atmosphere, SO2 reacts with other compounds to form sulfate aerosols and sulfuric acid. These sulfur compounds contribute to acid rain formation, which can damage forests,crops, and aquatic ecosystems when deposited on land and in water bodies. Acid rain also corrodes buildings and monuments.

Another way humans have altered the global sulfur cycle is through agricultural practices and changes in land use. The application of sulfate fertilizers increases the levels of sulfur in soils. The removal of sulfur-containing crops also reduces the recycling of sulfur back into the soil. Furthermore, the clearing of sulfur-rich forests and wetlands for agriculture increases sulfur emissions into the atmosphere.

Overall, human activities have at least doubled the amount of sulfur flowing through the global sulfur cycle relative to pre-industrial times. Reducing fossil fuel burning and modifying agricultural practices could help decrease sulfur emissions and mitigate their environmental impacts.

Conclusions

Overall across the major biogeochemical cycles, human activity has significantly altered cycling rates and processes. Through processes such as fossil fuel burning, fertilizer use, waste disposal, and deforestation, humans have greatly increased the flux of key elements like carbon, nitrogen, phosphorus, and sulfur through their respective cycles.

The increased flux of carbon into the atmosphere via fossil fuel emissions is the primary driver of climate change. The additional nitrogen and phosphorus from fertilizers has caused problems like eutrophication and ocean dead zones. More sulfur emissions have led to acid rain. Each of these human impacts has negative consequences for ecosystems and human health.

Because biogeochemical cycles are complex and interrelated, changes to one cycle affect the others. Disrupting these natural elemental flows can damage environmental stability. It is critical that humans take steps to mitigate their impacts, through actions like transitioning to renewable energy, reducing deforestation, improving waste management, and optimizing fertilizer application. With proactive measures, we may be able to restore balance to biogeochemical cycles and build a more sustainable future.