Biogeochemical cycles continuously move essential elements through living organisms, the atmosphere, land, and water. The nitrogen and phosphorus cycles are foundational for supporting life. Nitrogen, a key component of proteins and genetic material, cycles through the atmosphere, soil, and water, transformed by microorganisms into usable forms. Phosphorus, vital for energy transfer, DNA, and cell membranes, primarily moves through rocks, soil, and water, as it lacks a significant atmospheric component. These natural movements ensure a balanced supply of nutrients necessary for growth and overall ecosystem health.
Human Influence on the Nitrogen Cycle
The natural nitrogen cycle involves processes like nitrogen fixation, converting atmospheric nitrogen into ammonia, and denitrification, returning nitrogen gas to the atmosphere. Human activities have substantially altered this balance, dramatically increasing reactive nitrogen in the environment. A significant disruption comes from agricultural practices, especially the widespread use of synthetic nitrogen fertilizers. This industrial method supplies nutrients for crops but leads to excess nitrogen runoff into waterways.
Burning fossil fuels is another major contributor, releasing large quantities of nitrogen oxides into the atmosphere. These compounds contribute to air pollution and acid deposition, increasing nitrogen inputs to ecosystems. Industrial processes and untreated or inadequately treated wastewater from human sewage and industrial effluents also introduce considerable nitrogen into aquatic environments. These combined activities have more than doubled global nitrogen fixation since the mid-20th century.
Human Influence on the Phosphorus Cycle
The phosphorus cycle begins with the weathering of rocks, releasing phosphate into soils and water. Plants absorb this dissolved phosphate, which then moves through the food web before returning to the soil or water through decomposition and excretion. Human activities, particularly in agriculture, have accelerated this cycle, leading to environmental pollution. Farmers extensively use phosphorus-based fertilizers, derived from mined phosphate rock, to enhance crop growth.
When these fertilizers are applied excessively, surplus phosphorus can be washed into water bodies during rainfall, a process known as agricultural runoff. Untreated or poorly treated wastewater also discharges phosphorus into aquatic systems. Additionally, deforestation and other land-use changes can increase soil erosion, leading to higher rates of phosphorus loss from land into waterways.
Environmental Outcomes of Altered Cycles
The human-induced overload of nitrogen and phosphorus in ecosystems leads to several detrimental environmental outcomes. A prominent consequence is eutrophication, where excess nutrients stimulate the rapid overgrowth of algae in aquatic environments. These dense algal blooms block sunlight, and their subsequent decomposition by bacteria consumes vast amounts of dissolved oxygen. This oxygen depletion creates “dead zones” in lakes, rivers, and coastal waters, where oxygen levels are too low to support most aquatic life, leading to fish kills and ecosystem disruption.
Nitrogen oxides released from fossil fuel combustion also contribute to acid rain, which forms when these compounds react with water, oxygen, and other chemicals in the atmosphere to create sulfuric and nitric acids. This acidic precipitation can harm forests, acidify lakes and streams, and damage infrastructure. Furthermore, some nitrogen compounds, particularly nitrous oxide (N2O), are potent greenhouse gases, contributing to climate change. The alteration of nutrient cycles impacts biodiversity, as changes in nutrient availability can lead to shifts in species composition, reducing the overall richness of plant and animal life.
Addressing Human Impact on Nutrient Cycles
Mitigating the human impact on nitrogen and phosphorus cycles requires a multi-faceted approach focusing on sustainable practices and improved management. In agriculture, precision farming techniques allow for more accurate application of fertilizers, matching nutrient delivery to crop needs and reducing excess runoff. Practices such as crop rotation and the use of cover crops can also naturally enhance soil fertility and reduce the need for synthetic inputs.
Improvements in wastewater treatment technology are crucial, with advanced methods capable of removing significant amounts of nitrogen and phosphorus before discharge. Environmental policies and regulations, such as setting limits on nutrient discharges and promoting best management practices, play a role in driving these changes. Individuals can also contribute by properly disposing of pet waste, using phosphorus-free detergents, and fertilizing lawns responsibly with low-phosphorus or no-phosphorus options.