🌈Earth Systems Science Unit 10 – Biogeochemical Cycles: C, N, and P

Key Concepts and Definitions

• Biogeochemical cycles describe the movement and exchange of matter and energy between the biosphere, atmosphere, hydrosphere, and geosphere
• Carbon (C), nitrogen (N), and phosphorus (P) are essential elements for life on Earth
• Reservoirs store elements in various forms (atmosphere, oceans, soil, rocks)
• Fluxes represent the transfer of elements between reservoirs
• Residence time measures how long an element stays in a particular reservoir before moving to another
• Limiting nutrients control the growth and productivity of ecosystems (often nitrogen or phosphorus)
• Eutrophication occurs when excess nutrients lead to algal blooms and oxygen depletion in aquatic systems

The Carbon Cycle

• Carbon dioxide (CO2) in the atmosphere is absorbed by plants through photosynthesis
  • Photosynthesis converts CO2 and water into glucose and oxygen using energy from sunlight
• Respiration by plants and animals releases CO2 back into the atmosphere
• Decomposition of organic matter by microorganisms also releases CO2
• Oceans absorb CO2 from the atmosphere and store it as dissolved inorganic carbon
  • Marine organisms use dissolved CO2 to build calcium carbonate shells and skeletons
• Weathering of rocks containing carbonate minerals (limestone) consumes atmospheric CO2
• Volcanic eruptions and metamorphism release CO2 from rocks back into the atmosphere
• Fossil fuel combustion and deforestation have increased atmospheric CO2 levels

The Nitrogen Cycle

• Nitrogen gas (N2) makes up 78% of Earth's atmosphere but is unavailable to most organisms
• Nitrogen fixation converts atmospheric N2 into biologically available forms (ammonia, nitrate)
  • Biological nitrogen fixation is carried out by bacteria in root nodules of legumes (soybeans, alfalfa)
  • Lightning and industrial processes (Haber-Bosch) also fix nitrogen
• Nitrification converts ammonia to nitrite and then to nitrate by soil bacteria
• Plants absorb nitrate from the soil and incorporate it into organic compounds (amino acids, proteins)
• Animals obtain nitrogen by consuming plants or other animals
• Ammonification converts organic nitrogen back into ammonia during decomposition
• Denitrification reduces nitrate to N2 gas under anaerobic conditions, returning nitrogen to the atmosphere

The Phosphorus Cycle

• Phosphorus is a limiting nutrient in many ecosystems due to its low availability
• Weathering of rocks containing phosphate minerals (apatite) releases phosphorus into the soil
• Plants absorb phosphate from the soil and incorporate it into organic compounds (DNA, ATP)
• Animals obtain phosphorus by consuming plants or other animals
• Decomposition of organic matter releases phosphorus back into the soil
• Phosphorus is lost from ecosystems through leaching, soil erosion, and transport to the oceans
  • Marine sediments are the largest reservoir of phosphorus on Earth
• Uplift and exposure of marine sediments return phosphorus to the land through the rock cycle

Interactions Between Cycles

• Carbon, nitrogen, and phosphorus cycles are interconnected through biological processes
• Photosynthesis and respiration link the carbon and oxygen cycles
• Nitrogen fixation by legumes is enhanced by symbiotic relationships with soil bacteria
• Decomposition releases carbon, nitrogen, and phosphorus from organic matter back into the environment
• Eutrophication can occur when excess nitrogen and phosphorus from fertilizers or sewage enter aquatic systems
  • Algal blooms resulting from eutrophication can deplete oxygen levels and harm aquatic life
• Climate change affects biogeochemical cycles by altering temperature, precipitation, and ecosystem dynamics

Human Impacts on Biogeochemical Cycles

• Fossil fuel combustion and deforestation have increased atmospheric CO2 levels, contributing to climate change
• Agricultural practices (fertilizer use, livestock production) have altered the nitrogen and phosphorus cycles
  • Overuse of fertilizers can lead to eutrophication and groundwater contamination (nitrate pollution)
• Wastewater discharge and soil erosion transport excess nutrients to aquatic systems
• Urbanization and land-use changes affect nutrient cycling and ecosystem functioning
• Acid rain, caused by emissions of sulfur and nitrogen oxides, can acidify soils and water bodies
• Mining and industrial activities can release heavy metals and other pollutants into the environment

Measurement and Modeling Techniques

• Stable isotope analysis tracks the movement of elements through ecosystems
  • Carbon isotopes (12C, 13C) help distinguish between sources of CO2 (fossil fuels vs. biogenic)
  • Nitrogen isotopes (14N, 15N) trace the origin and fate of nitrogen in the environment
• Remote sensing (satellite imagery) monitors changes in vegetation cover and productivity
• Eddy covariance towers measure the exchange of CO2, water vapor, and energy between ecosystems and the atmosphere
• Nutrient budgets quantify the inputs, outputs, and storage of elements within a defined system
• Biogeochemical models simulate the complex interactions and feedbacks between cycles
  • Models help predict the responses of ecosystems to environmental changes and human activities

Environmental and Climate Implications

• Increasing atmospheric CO2 levels contribute to global warming and ocean acidification
  • Warmer temperatures can alter species distributions, phenology, and ecosystem functioning
  • Ocean acidification harms marine organisms with calcium carbonate shells or skeletons (corals, mollusks)
• Nitrogen and phosphorus pollution can lead to eutrophication, biodiversity loss, and water quality issues
• Altered biogeochemical cycles can affect soil fertility, crop yields, and food security
• Changes in nutrient cycling can influence the spread of invasive species and disease vectors
• Understanding biogeochemical cycles is crucial for developing sustainable management practices and mitigating human impacts on the environment