Nitrogen and phosphorus are essential elements for life, cycling through ecosystems in complex ways. These cycles involve various biological and chemical processes, transforming nutrients from inorganic to organic forms and back again.

Human activities have significantly altered these cycles, leading to environmental issues like eutrophication. Understanding these processes is crucial for managing ecosystems, agriculture, and water quality in a sustainable manner.

Nitrogen Cycle Processes

Nitrogen Fixation and Biological Processes

• Nitrogen fixation converts atmospheric nitrogen (N₂) into biologically available forms
• Occurs through natural processes (lightning) and biological nitrogen fixation
• Biological nitrogen fixation performed by specialized bacteria and archaea
• Symbiotic nitrogen-fixing bacteria form root nodules on legumes (soybeans, peas)
• Free-living nitrogen-fixing bacteria found in soil and aquatic environments
• Nitrogen-fixing microorganisms use nitrogenase enzyme to break N₂ triple bond
• Converted nitrogen forms include ammonia (NH₃) and ammonium (NH₄⁺)

Nitrification and Ammonification

• Nitrification oxidizes ammonia to nitrite (NO₂⁻) and then to nitrate (NO₃⁻)
• Two-step process carried out by different groups of chemolithotrophic bacteria
• Nitrosomonas bacteria oxidize ammonia to nitrite
• Nitrobacter bacteria oxidize nitrite to nitrate
• Ammonification decomposes organic nitrogen compounds into ammonia
• Performed by various microorganisms in soil and aquatic environments
• Releases nitrogen from dead organic matter, making it available for other organisms

Denitrification and Nitrogen Cycle Completion

• Denitrification reduces nitrate to atmospheric nitrogen (N₂)
• Carried out by denitrifying bacteria in anaerobic conditions
• Occurs in waterlogged soils, wetlands, and deep ocean sediments
• Completes the nitrogen cycle by returning nitrogen to the atmosphere
• Intermediate products include nitric oxide (NO) and nitrous oxide (N₂O)
• Nitrous oxide acts as a potent greenhouse gas
• Denitrification can lead to nitrogen loss from ecosystems

Phosphorus Cycle and Limitation

Phosphorus Weathering and Cycling

• Phosphorus weathering releases phosphate from rocks and minerals
• Primary source of phosphorus in terrestrial ecosystems
• Weathering of apatite minerals releases inorganic phosphate
• Phosphorus cycles between organic and inorganic forms in ecosystems
• Organic phosphorus found in living organisms and dead organic matter
• Inorganic phosphorus exists as phosphate ions in soil and water
• Phosphorus moves through food chains and returns to soil through decomposition

Phosphorus Limitation and Ecosystem Impacts

• Phosphorus limitation often restricts primary productivity in ecosystems
• Typically the limiting nutrient in freshwater ecosystems
• Phosphorus scarcity due to its low solubility and tendency to bind with soil particles
• Plants absorb phosphorus as phosphate ions (PO₄³⁻) through their roots
• Mycorrhizal fungi form symbiotic relationships with plants to enhance phosphorus uptake
• Phosphorus limitation influences species composition and ecosystem structure
• Affects agricultural productivity, leading to the widespread use of phosphorus fertilizers

Human Impacts on Nutrient Cycles

Eutrophication and Water Quality Issues

• Eutrophication enriches water bodies with excess nutrients, primarily nitrogen and phosphorus
• Caused by agricultural runoff, sewage discharge, and urban development
• Leads to algal blooms, including harmful algal blooms (cyanobacteria)
• Algal decomposition depletes dissolved oxygen, creating hypoxic "dead zones"
• Impacts aquatic ecosystems, causing fish kills and biodiversity loss
• Affects water quality for human consumption and recreational use
• Cultural eutrophication accelerates natural nutrient enrichment processes

Fertilizer Use and Environmental Consequences

• Fertilizers increase crop yields but disrupt natural nutrient cycles
• Nitrogen-based fertilizers produced through the Haber-Bosch process
• Phosphorus fertilizers derived from mined phosphate rock (finite resource)
• Excessive fertilizer use leads to nutrient runoff and groundwater contamination
• Nitrate contamination in drinking water poses health risks (methemoglobinemia)
• Fertilizer production and use contribute to greenhouse gas emissions
• Sustainable fertilizer management practices include precision agriculture and crop rotation