2.2 Nitrogen cycle

The nitrogen cycle is a complex biogeochemical process that transforms nitrogen between various forms in the environment. It involves interactions between atmospheric, terrestrial, and aquatic systems, playing a crucial role in ecosystem functioning and nutrient availability.

This cycle encompasses nitrogen fixation, ammonification, nitrification, denitrification, and assimilation. Human activities, such as fertilizer use and industrial emissions, have significantly altered the natural nitrogen cycle, leading to environmental issues like eutrophication and increased greenhouse gas emissions.

Overview of nitrogen cycle

• Nitrogen cycle represents the biogeochemical processes that transform nitrogen between various chemical forms in the environment
• Plays a crucial role in ecosystem functioning, nutrient availability, and global climate regulation
• Involves complex interactions between atmospheric, terrestrial, and aquatic systems, making it a key focus in geochemistry studies

Nitrogen reservoirs

Atmospheric nitrogen

Top images from around the web for Atmospheric nitrogen

• 

  The Nitrogen Cycle | Biology for Majors II View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles | Microbiology View original

  Is this image relevant?

• 

  The Nitrogen Cycle | Biology for Majors II View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Atmospheric nitrogen

• 

  The Nitrogen Cycle | Biology for Majors II View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles | Microbiology View original

  Is this image relevant?

• 

  The Nitrogen Cycle | Biology for Majors II View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

1 of 3

• Comprises approximately 78% of Earth's atmosphere as dinitrogen gas (N₂)
• Serves as the largest reservoir of nitrogen on the planet
• Remains largely inert due to the strong triple bond between nitrogen atoms
• Requires significant energy input for conversion into biologically available forms

Terrestrial nitrogen pools

• Includes soil organic matter, plant biomass, and microbial communities
• Soil organic nitrogen represents the largest terrestrial pool
• Plant-available forms include ammonium (NH₄⁺) and nitrate (NO₃⁻)
• Microbial biomass acts as both a source and sink for nitrogen in terrestrial ecosystems

Aquatic nitrogen pools

• Dissolved inorganic nitrogen (DIN) includes ammonium, nitrate, and nitrite
• Dissolved organic nitrogen (DON) comprises amino acids and other organic compounds
• Particulate nitrogen exists in suspended organic matter and sediments
• Aquatic plants and algae incorporate nitrogen into their biomass

Nitrogen fixation

Biological fixation

• Carried out by specialized microorganisms called diazotrophs
• Converts atmospheric N₂ into biologically available ammonia (NH₃)
• Utilizes the enzyme nitrogenase to break the N₂ triple bond
• Occurs in both terrestrial (legumes) and aquatic (cyanobacteria) environments

Industrial fixation

• Haber-Bosch process artificially fixes N₂ into ammonia for fertilizer production
• Requires high temperatures (400-500°C) and pressures (200-300 atm)
• Consumes significant amounts of fossil fuels, contributing to greenhouse gas emissions
• Has dramatically increased global nitrogen availability for agriculture

Lightning fixation

• High-energy lightning strikes break N₂ bonds in the atmosphere
• Produces nitrogen oxides (NOx) that can be deposited as nitric acid in rainfall
• Contributes a relatively small but significant amount to the global nitrogen budget
• Plays a role in the formation of tropospheric ozone

Ammonification

Organic nitrogen decomposition

• Breakdown of complex organic nitrogen compounds (proteins, nucleic acids)
• Carried out by heterotrophic bacteria and fungi in soil and aquatic environments
• Releases simpler organic nitrogen compounds and eventually inorganic ammonium
• Rate influenced by factors such as temperature, moisture, and substrate quality

Ammonium production

• Final step in the mineralization of organic nitrogen to inorganic forms
• Results in the release of ammonium (NH₄⁺) into the soil or water
• Ammonium can be directly assimilated by plants and microorganisms
• Excess ammonium may undergo further transformations (nitrification, volatilization)

Nitrification

Ammonia oxidation

• First step of nitrification, carried out by ammonia-oxidizing bacteria and archaea
• Converts ammonia (NH₃) to nitrite (NO₂⁻)
• Utilizes the enzyme ammonia monooxygenase
• Produces energy for chemolithoautotrophic growth of nitrifying microorganisms

Nitrite oxidation

• Second step of nitrification, performed by nitrite-oxidizing bacteria
• Oxidizes nitrite (NO₂⁻) to nitrate (NO₃⁻)
• Employs the enzyme nitrite oxidoreductase
• Completes the transformation of reduced nitrogen to its most oxidized form

Denitrification

Nitrate reduction

• Anaerobic process carried out by facultative anaerobic bacteria
• Reduces nitrate (NO₃⁻) to nitrite (NO₂⁻), then to nitric oxide (NO) and nitrous oxide (N₂O)
• Occurs in oxygen-limited environments (waterlogged soils, sediments)
• Serves as an alternative electron acceptor for microbial respiration

Nitrogen gas production

• Final step in denitrification, converting nitrous oxide (N₂O) to dinitrogen gas (N₂)
• Completes the nitrogen cycle by returning fixed nitrogen to the atmosphere
• Catalyzed by the enzyme nitrous oxide reductase
• Important process for removing excess nitrogen from ecosystems

Anammox

Anaerobic ammonium oxidation

• Microbial process that oxidizes ammonium (NH₄⁺) using nitrite (NO₂⁻) as an electron acceptor
• Produces dinitrogen gas (N₂) without the intermediate formation of nitrate
• Carried out by specialized bacteria in the order Planctomycetales
• Occurs in anoxic environments such as marine sediments and wastewater treatment plants

Environmental significance

• Contributes significantly to nitrogen loss in marine ecosystems
• Provides an alternative pathway for nitrogen removal in wastewater treatment
• Competes with denitrification for nitrite in some environments
• Discovered relatively recently (1990s), altering our understanding of the nitrogen cycle

Nitrogen assimilation

Plant uptake

• Absorption of inorganic nitrogen (NH₄⁺, NO₃⁻) through plant roots
• Nitrate reduction to ammonium within plants using nitrate reductase
• Incorporation of ammonium into amino acids via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway
• Translocation of organic nitrogen compounds throughout the plant

Microbial assimilation

• Uptake of inorganic and organic nitrogen forms by soil and aquatic microorganisms
• Incorporation into microbial biomass (proteins, nucleic acids)
• Immobilization of nitrogen in microbial cells, temporarily reducing plant-available nitrogen
• Release of assimilated nitrogen upon microbial death and decomposition

Nitrogen cycle in ecosystems

Terrestrial nitrogen cycling

• Complex interactions between plants, soil microorganisms, and abiotic factors
• Nitrogen fixation by legumes and free-living bacteria in soil
• Mineralization and immobilization processes affecting nitrogen availability
• Leaching and gaseous losses influencing ecosystem nitrogen balance

Aquatic nitrogen cycling

• Nitrogen inputs from atmospheric deposition, terrestrial runoff, and in situ fixation
• Phytoplankton uptake and incorporation into the aquatic food web
• Remineralization of organic nitrogen in the water column and sediments
• Denitrification and anammox in anoxic zones and sediments

Human impacts

Agricultural fertilizers

• Increased use of synthetic nitrogen fertilizers since the Green Revolution
• Enhanced crop yields but also led to nitrogen pollution in water bodies
• Altered natural nitrogen cycling in agricultural ecosystems
• Contributed to increased nitrous oxide emissions, a potent greenhouse gas

Industrial emissions

• Release of nitrogen oxides (NOx) from fossil fuel combustion
• Contribution to acid rain formation and atmospheric nitrogen deposition
• Impacts on terrestrial and aquatic ecosystem functioning
• Interactions with other air pollutants (ozone formation)

Wastewater discharge

• Release of excess nitrogen from human and animal waste into aquatic systems
• Contribution to eutrophication in freshwater and coastal ecosystems
• Alteration of aquatic nitrogen cycling and ecosystem structure
• Challenges in wastewater treatment to remove nitrogen effectively

Biogeochemical feedbacks

Climate change effects

• Increased temperatures affecting rates of nitrogen cycling processes
• Changes in precipitation patterns altering soil moisture and nitrogen availability
• Potential for increased nitrogen fixation due to elevated CO₂ levels
• Feedbacks between nitrogen cycle and carbon cycle under changing climate conditions

Ecosystem responses

• Shifts in plant community composition due to altered nitrogen availability
• Changes in microbial community structure and function
• Potential for increased nitrogen losses through leaching and gaseous emissions
• Impacts on ecosystem productivity and biodiversity

Nitrogen cycle modeling

Mass balance approaches

• Quantification of nitrogen inputs, outputs, and internal transformations
• Use of stoichiometric relationships to estimate fluxes between ecosystem compartments
• Application of differential equations to describe temporal dynamics
• Integration of multiple nitrogen cycle processes in ecosystem-scale models

Isotope tracing techniques

• Use of stable isotopes (¹⁵N) to track nitrogen transformations
• Natural abundance methods to infer sources and sinks of nitrogen
• Enrichment experiments to quantify specific process rates
• Combination with molecular techniques to link microbial identity and function

Global nitrogen budgets

Natural vs anthropogenic fluxes

• Comparison of pre-industrial and current global nitrogen cycles
• Quantification of human-induced changes in nitrogen fixation and mobilization
• Assessment of altered nitrogen fluxes between atmospheric, terrestrial, and aquatic reservoirs
• Identification of major anthropogenic sources (fertilizers, fossil fuel combustion, legume cultivation)

Future projections

• Scenarios of future nitrogen use in agriculture and industry
• Predictions of nitrogen cycle responses to climate change and land-use changes
• Potential feedbacks between nitrogen cycle and other biogeochemical cycles
• Implications for ecosystem functioning and global environmental change

Environmental implications

Eutrophication

• Excess nitrogen input leading to algal blooms in aquatic ecosystems
• Depletion of dissolved oxygen, creating hypoxic or anoxic conditions
• Impacts on aquatic biodiversity and ecosystem services
• Economic consequences for fisheries and recreational water use

Acid rain

• Formation of nitric acid from nitrogen oxides in the atmosphere
• Acidification of soils and water bodies upon deposition
• Impacts on forest health and aquatic ecosystems
• Interactions with other acidifying pollutants (sulfur dioxide)

Greenhouse gas emissions

• Nitrous oxide (N₂O) as a potent greenhouse gas with long atmospheric lifetime
• Contributions from agricultural soils, industrial processes, and wastewater treatment
• Interactions with stratospheric ozone depletion
• Challenges in mitigating N₂O emissions while maintaining food production