9.1 Biogeochemical Cycles

Biogeochemical cycles are nature's recycling systems, moving essential elements through Earth's ecosystems. These cycles, including water, carbon, nitrogen, phosphorus, and sulfur, sustain life by circulating nutrients and energy between living and non-living components.

Understanding these cycles is crucial for grasping how ecosystems function and how human activities impact them. From carbon's role in climate change to nitrogen's influence on soil fertility, these cycles shape our planet's health and biodiversity.

Biogeochemical Cycles and Ecosystems

The Water Cycle

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• The water cycle, also known as the hydrologic cycle, describes the continuous movement of water through processes such as evaporation, transpiration, condensation, precipitation, infiltration, and runoff
• Evaporation occurs when water changes from a liquid to a gas, typically due to heat from the sun (oceans, lakes, rivers)
• Transpiration is the process by which water vapor is released into the atmosphere through the leaves of plants (forests, grasslands)
• Condensation happens when water vapor cools and transforms back into a liquid, forming clouds or fog (dew, mist)
• Precipitation is the falling of water from the atmosphere in the form of rain, snow, sleet, or hail (monsoons, blizzards)
• Infiltration is the process by which water on the ground surface enters the soil (percolation, groundwater recharge)
• Runoff is the flow of water over the surface of the Earth, often into streams, rivers, or other bodies of water (flash floods, erosion)

The Carbon Cycle

• The carbon cycle involves the exchange of carbon among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere
• Photosynthesis is the process by which plants and other organisms convert carbon dioxide and water into glucose and oxygen using energy from sunlight (forests, phytoplankton)
• Respiration is the process by which organisms break down glucose and release carbon dioxide and water (animals, decomposers)
• Decomposition is the breakdown of organic matter by decomposers, releasing carbon dioxide and other nutrients back into the environment (fungi, bacteria)
• Combustion is the burning of organic materials, such as fossil fuels, which releases carbon dioxide into the atmosphere (coal, oil, natural gas)
• Carbon can be stored in various reservoirs, such as the atmosphere, oceans, soil, and living organisms (permafrost, carbonate rocks)

The Nitrogen Cycle

• The nitrogen cycle is the biogeochemical cycle that converts nitrogen into multiple chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems
• Nitrogen fixation is the process by which atmospheric nitrogen is converted into ammonia or other nitrogen compounds that can be used by plants (legumes, cyanobacteria)
• Nitrification is the conversion of ammonia into nitrite and then nitrate by bacteria in the soil (Nitrosomonas, Nitrobacter)
• Denitrification is the reduction of nitrate back into atmospheric nitrogen by bacteria in anaerobic conditions (wetlands, sediments)
• Ammonification is the conversion of organic nitrogen compounds into ammonia by decomposers (animal waste, decaying plants)
• Plants absorb nitrate from the soil and incorporate it into their tissues, which are then consumed by animals (crops, grasslands)

The Phosphorus Cycle

• The phosphorus cycle describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere
• Phosphorus is an essential nutrient for plant growth and is often a limiting factor in ecosystems (tropical rainforests, coral reefs)
• Weathering of rocks releases phosphorus into the soil, where it can be taken up by plants (apatite, phosphorite)
• Decomposition of organic matter by microorganisms releases phosphorus back into the soil (leaf litter, animal carcasses)
• Phosphorus can be lost from ecosystems through leaching, soil erosion, and runoff (eutrophication, algal blooms)

The Sulfur Cycle

• The sulfur cycle involves the movement of sulfur between rocks, waterways, and living systems
• Weathering of rocks and volcanic eruptions release sulfur into the environment (pyrite, hydrogen sulfide)
• Sulfur-reducing bacteria convert sulfate into hydrogen sulfide in anaerobic conditions (salt marshes, hot springs)
• Hydrogen sulfide can be oxidized by bacteria, forming sulfuric acid, which contributes to acid rain (coal combustion, industrial emissions)
• Plants absorb sulfate from the soil and incorporate it into their tissues, which are then consumed by animals (cruciferous vegetables, amino acids)

Human Impact on Biogeochemical Cycles

Disruption of the Carbon Cycle

• Burning fossil fuels releases carbon dioxide into the atmosphere, intensifying the greenhouse effect and contributing to global climate change (industrial revolution, transportation)
• Deforestation reduces the amount of carbon dioxide absorbed by plants through photosynthesis, further exacerbating the greenhouse effect (Amazon rainforest, Indonesian peatlands)
• Ocean acidification occurs when excess atmospheric carbon dioxide dissolves in seawater, lowering the pH and disrupting marine ecosystems (coral bleaching, shellfish dissolution)

Alteration of the Nitrogen and Phosphorus Cycles

• Agricultural practices, such as the use of nitrogen-based fertilizers, can lead to nutrient runoff into waterways (eutrophication, dead zones)
• Excessive nitrogen and phosphorus in aquatic ecosystems stimulate algal growth, depleting oxygen levels and harming aquatic life (Gulf of Mexico, Lake Erie)
• Wastewater discharge and sewage leaks introduce additional nutrients into water bodies, further contributing to eutrophication (urban areas, septic systems)

Impact on the Sulfur Cycle and Acid Rain

• Mining activities and fossil fuel combustion release sulfur compounds into the atmosphere (coal, oil refineries)
• Sulfur dioxide and other sulfur oxides react with water vapor to form sulfuric acid, a major component of acid rain (Appalachian Mountains, Black Forest)
• Acid rain can harm plant and animal life, degrade infrastructure, and leach nutrients from the soil (forest decline, corroded buildings)

Urbanization and Land-Use Changes

• Urbanization and land-use changes alter the water cycle by increasing surface runoff and reducing groundwater recharge (impervious surfaces, stormwater management)
• Modification of local precipitation patterns can occur due to changes in surface albedo and heat island effects (urban microclimates, flash flooding)
• Habitat fragmentation and destruction disrupt the flow of nutrients and energy through ecosystems (wildlife corridors, biodiversity loss)

Plastic Pollution and Biogeochemical Cycles

• The production and disposal of plastics introduce microplastics into marine ecosystems (nurdles, plastic bags, synthetic fibers)
• Microplastics can be ingested by marine organisms, potentially disrupting food chains and biogeochemical cycles (zooplankton, fish, seabirds)
• Plastic debris can also serve as a substrate for the growth of microorganisms and the transport of invasive species (great Pacific garbage patch, rafting communities)

Interconnectedness of Biogeochemical Cycles

Cascading Effects of Cycle Disruptions

• Biogeochemical cycles are interconnected, and a disruption in one cycle can have cascading effects on other cycles and ecosystem processes
• Changes in the carbon cycle affect the nitrogen and phosphorus cycles, as carbon is essential for the growth and development of organisms that participate in these cycles (plant productivity, microbial activity)
• Alterations in the water cycle can influence the availability and distribution of nutrients, impacting the functioning of terrestrial and aquatic ecosystems (drought, flooding)

Climate Regulation and Greenhouse Gases

• Biogeochemical cycles play a crucial role in regulating Earth's climate by controlling the concentrations of greenhouse gases in the atmosphere
• The carbon cycle helps maintain the Earth's temperature within a habitable range by regulating the amount of carbon dioxide in the atmosphere (photosynthesis, ocean absorption)
• Feedback loops between the climate system and biogeochemical cycles can amplify or mitigate the effects of climate change (permafrost thaw, ocean carbon uptake)

Nutrient Cycling and Ecosystem Productivity

• Nutrient cycling through biogeochemical processes supports primary productivity and the growth of organisms at the base of food webs (phytoplankton, plants)
• The availability of essential nutrients, such as nitrogen and phosphorus, can limit the productivity of ecosystems (oligotrophic lakes, desert soils)
• Efficient nutrient cycling sustains higher trophic levels and maintains the stability and resilience of ecosystems (predator-prey interactions, keystone species)

Soil Health and Ecosystem Functioning

• Biogeochemical cycles contribute to the formation and maintenance of soil health, which is essential for supporting plant growth and ecosystem productivity
• The decomposition of organic matter by microorganisms releases nutrients back into the soil, improving fertility and structure (humus, mycorrhizal fungi)
• Healthy soils provide ecosystem services, such as water filtration, carbon sequestration, and erosion control (wetlands, grasslands)

Global Redistribution of Nutrients and Energy

• The coupling of biogeochemical cycles and physical processes, such as ocean circulation and atmospheric transport, helps redistribute nutrients and energy across the globe
• Ocean currents transport nutrients and dissolved organic matter, supporting the diversity of marine life (Gulf Stream, upwelling zones)
• Atmospheric circulation patterns, such as the Hadley cell, influence the distribution of moisture and nutrients on a global scale (Intertropical Convergence Zone, dust storms)