3.2 Nitrogen cycle

The nitrogen cycle is a crucial biogeochemical process in aquatic ecosystems. It involves the transformation and movement of nitrogen through various forms, impacting the growth and productivity of organisms in freshwater environments.

Understanding the nitrogen cycle is essential for limnologists studying lake ecosystems. From nitrogen fixation to denitrification, each step plays a vital role in nutrient availability, ecosystem health, and the potential for issues like eutrophication.

Nitrogen cycle overview

• The nitrogen cycle is a biogeochemical cycle that describes the transformation and movement of nitrogen through the environment
• Nitrogen is an essential nutrient for all living organisms and plays a crucial role in the growth and productivity of aquatic ecosystems
• Understanding the nitrogen cycle is fundamental to the study of limnology as it influences the structure and function of freshwater ecosystems

Nitrogen fixation

Biological nitrogen fixation

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• Process by which atmospheric nitrogen (N2) is converted into biologically available forms, such as ammonia (NH3) or ammonium (NH4+)
• Carried out by certain bacteria and archaea, known as diazotrophs, that possess the enzyme nitrogenase
• Examples of nitrogen-fixing organisms include cyanobacteria (Anabaena, Nostoc) and symbiotic bacteria (Rhizobium) associated with the roots of legumes
• Provides a significant input of nitrogen to aquatic ecosystems, particularly in nitrogen-limited environments

Abiotic nitrogen fixation

• Nitrogen fixation can also occur through abiotic processes, such as lightning and industrial processes (Haber-Bosch process)
• Lightning converts atmospheric nitrogen and oxygen into nitric oxide (NO), which can then be oxidized to form nitrate (NO3-) and deposited on the Earth's surface through precipitation
• Industrial nitrogen fixation, such as the Haber-Bosch process, produces ammonia for fertilizers, which can enter aquatic ecosystems through runoff and atmospheric deposition

Nitrification

Ammonia oxidation

• First step of the nitrification process, where ammonia (NH3) or ammonium (NH4+) is oxidized to nitrite (NO2-) by ammonia-oxidizing bacteria (AOB) and archaea (AOA)
• Key genera involved in ammonia oxidation include Nitrosomonas, Nitrosospira, and Nitrosococcus
• Ammonia oxidation is an aerobic process and requires the presence of oxygen
• The rate of ammonia oxidation can be influenced by factors such as temperature, pH, and substrate availability

Nitrite oxidation

• Second step of the nitrification process, where nitrite (NO2-) is oxidized to nitrate (NO3-) by nitrite-oxidizing bacteria (NOB)
• Key genera involved in nitrite oxidation include Nitrobacter, Nitrospira, and Nitrospina
• Nitrite oxidation is also an aerobic process and requires the presence of oxygen
• The rate of nitrite oxidation is generally faster than ammonia oxidation, preventing the accumulation of nitrite in the environment

Denitrification

Anaerobic respiration

• Process by which nitrate (NO3-) is reduced to gaseous forms of nitrogen, such as nitrous oxide (N2O) and dinitrogen (N2), under anaerobic conditions
• Carried out by a diverse group of heterotrophic bacteria, such as Pseudomonas, Paracoccus, and Thiobacillus
• Denitrification serves as a respiratory pathway for these bacteria in the absence of oxygen, using nitrate as the terminal electron acceptor
• Occurs in anoxic sediments and water columns of lakes, wetlands, and coastal marine environments

Nitrous oxide production

• Nitrous oxide (N2O) is a potent greenhouse gas and an intermediate product of the denitrification process
• Incomplete denitrification, often due to low carbon availability or sudden changes in environmental conditions, can lead to the accumulation and emission of nitrous oxide
• Aquatic ecosystems, particularly those with high nitrogen loading and fluctuating oxygen levels, can be significant sources of nitrous oxide to the atmosphere
• Mitigating nitrous oxide emissions from aquatic ecosystems is a growing concern in the context of climate change

Anammox

Anaerobic ammonium oxidation

• Process by which ammonium (NH4+) is oxidized to dinitrogen (N2) gas using nitrite (NO2-) as an electron acceptor under anaerobic conditions
• Carried out by a specialized group of bacteria, such as Candidatus Brocadia and Candidatus Kuenenia, belonging to the phylum Planctomycetes
• Anammox is a chemolithoautotrophic process, meaning that these bacteria use inorganic compounds (ammonium and nitrite) as energy sources and carbon dioxide (CO2) as a carbon source
• Discovered in the 1990s, anammox has since been recognized as a significant pathway for nitrogen removal in aquatic environments

Significance in aquatic ecosystems

• Anammox can contribute to a substantial portion of nitrogen removal in oxygen-limited environments, such as anoxic water columns and sediments
• In some marine and freshwater systems, anammox has been found to account for up to 50% of the total nitrogen removal
• Anammox can help mitigate the effects of eutrophication by removing excess nitrogen from the system without the need for external carbon sources
• Understanding the role of anammox in nitrogen cycling is crucial for predicting the response of aquatic ecosystems to changing environmental conditions and nutrient loading

Assimilation

Uptake by phytoplankton

• Process by which inorganic nitrogen compounds, such as ammonium (NH4+) and nitrate (NO3-), are incorporated into the biomass of phytoplankton
• Phytoplankton, the primary producers in aquatic ecosystems, require nitrogen for the synthesis of essential biomolecules, such as proteins, nucleic acids, and chlorophyll
• Nitrogen uptake by phytoplankton is influenced by factors such as light availability, water temperature, and the concentration and form of available nitrogen
• Phytoplankton can exhibit a preference for ammonium over nitrate, as the assimilation of ammonium requires less energy compared to the reduction of nitrate

Uptake by macrophytes

• Macrophytes, or aquatic plants, also assimilate inorganic nitrogen compounds from the water column and sediments
• Rooted macrophytes can access nitrogen from both the water column and the sediments, while floating macrophytes rely primarily on water column sources
• Macrophytes play a significant role in nitrogen cycling in shallow lakes and wetlands, where they can compete with phytoplankton for available nitrogen resources
• The uptake of nitrogen by macrophytes can help reduce the risk of eutrophication by temporarily storing nitrogen in their biomass and promoting denitrification in the rhizosphere

Mineralization

Ammonification

• Process by which organic nitrogen compounds, such as proteins and amino acids, are decomposed to release ammonium (NH4+)
• Carried out by a wide range of heterotrophic bacteria and fungi that possess extracellular enzymes capable of breaking down complex organic matter
• Ammonification is a key step in the nitrogen cycle, as it converts organically bound nitrogen into a form that is readily available for uptake by plants and microorganisms
• The rate of ammonification is influenced by factors such as temperature, moisture, and the quality (C:N ratio) of the organic matter being decomposed

Organic nitrogen decomposition

• Organic nitrogen, in the form of dead organisms, fecal matter, and other detritus, undergoes decomposition through the action of microorganisms
• Decomposition of organic nitrogen is a gradual process that involves the breakdown of complex organic molecules into simpler compounds, ultimately releasing ammonium through ammonification
• The rate of organic nitrogen decomposition is influenced by environmental factors, such as temperature, oxygen availability, and the presence of decomposer organisms
• In aquatic ecosystems, the decomposition of organic nitrogen in sediments can play a significant role in the regeneration of inorganic nitrogen for primary production

Nitrogen budget

Sources of nitrogen

• Nitrogen can enter aquatic ecosystems through various sources, including atmospheric deposition, biological nitrogen fixation, surface runoff, and groundwater discharge
• Atmospheric deposition of nitrogen occurs through the wet and dry deposition of nitrogenous compounds, such as nitric acid (HNO3), ammonium (NH4+), and nitrate (NO3-)
• Biological nitrogen fixation by cyanobacteria and other diazotrophs can provide a significant input of new nitrogen to aquatic ecosystems, particularly in nitrogen-limited environments
• Surface runoff and groundwater discharge can transport nitrogen from terrestrial sources, such as agricultural lands and urban areas, into nearby water bodies

Sinks of nitrogen

• Nitrogen can be removed from aquatic ecosystems through various processes, including denitrification, anammox, burial in sediments, and uptake by organisms
• Denitrification and anammox convert inorganic nitrogen compounds into gaseous forms (N2 and N2O), which can then diffuse out of the system into the atmosphere
• Burial of organic nitrogen in sediments can serve as a long-term sink for nitrogen, removing it from the active cycling pool
• Uptake of nitrogen by phytoplankton, macrophytes, and other organisms incorporates nitrogen into biomass, temporarily storing it within the ecosystem

Anthropogenic impacts

Eutrophication

• Eutrophication is the excessive growth of algae and aquatic plants due to increased nutrient inputs, particularly nitrogen and phosphorus
• Human activities, such as agricultural runoff, wastewater discharge, and fossil fuel combustion, have greatly increased the amount of nitrogen entering aquatic ecosystems
• Excess nitrogen can stimulate the growth of phytoplankton and macrophytes, leading to algal blooms, reduced water clarity, and oxygen depletion (hypoxia) in the water column
• Eutrophication can have severe ecological consequences, including the loss of biodiversity, fish kills, and the formation of dead zones in coastal areas

Nitrate contamination

• Nitrate (NO3-) is a highly soluble form of nitrogen that can easily leach from soils and enter groundwater and surface water resources
• Elevated nitrate concentrations in drinking water can pose a human health risk, particularly for infants, causing a condition known as methemoglobinemia or "blue baby syndrome"
• Nitrate contamination of groundwater is a growing concern in areas with intensive agricultural activities, where the application of nitrogen fertilizers and manure can lead to significant nitrate leaching
• Mitigating nitrate contamination requires the implementation of best management practices, such as precision fertilizer application, cover cropping, and riparian buffer zones

Nitrogen limitation

N:P ratios

• The relative availability of nitrogen (N) and phosphorus (P) in aquatic ecosystems can influence the growth and composition of phytoplankton communities
• The Redfield ratio, which describes the average atomic ratio of C:N:P in marine phytoplankton as 106:16:1, is often used as a benchmark for assessing nutrient limitation
• Aquatic ecosystems with N:P ratios lower than the Redfield ratio are considered to be nitrogen-limited, while those with higher ratios are considered to be phosphorus-limited
• N:P ratios can vary widely across different aquatic ecosystems and can be influenced by factors such as nutrient loading, water residence time, and the presence of nitrogen-fixing organisms

Liebig's law of the minimum

• Liebig's law of the minimum states that the growth of an organism is limited by the nutrient that is in the shortest supply relative to the organism's needs
• In the context of aquatic ecosystems, phytoplankton growth can be limited by the availability of either nitrogen or phosphorus, depending on which nutrient is in the shortest supply relative to the phytoplankton's stoichiometric requirements
• The concept of nutrient limitation has important implications for the management of aquatic ecosystems, as reducing the input of the limiting nutrient can be an effective strategy for controlling eutrophication
• However, the application of Liebig's law in aquatic ecosystems is complicated by the fact that different phytoplankton species have different nutrient requirements and can exhibit variable responses to nutrient limitation

Nitrogen cycling in lakes

Epilimnion vs hypolimnion

• In stratified lakes, the nitrogen cycle can exhibit distinct patterns in the epilimnion (upper, well-mixed layer) and hypolimnion (lower, isolated layer)
• The epilimnion is characterized by higher light availability, warmer temperatures, and greater exposure to atmospheric nitrogen inputs, favoring processes such as nitrogen fixation and assimilation by phytoplankton
• The hypolimnion, in contrast, is characterized by lower light availability, cooler temperatures, and limited mixing with the epilimnion, creating conditions that favor processes such as denitrification and anammox
• The extent of nitrogen cycling in the epilimnion and hypolimnion can be influenced by factors such as lake morphometry, water residence time, and the strength and duration of thermal stratification

Seasonal patterns

• Nitrogen cycling in lakes can exhibit distinct seasonal patterns, driven by changes in temperature, light availability, and mixing regimes
• During the spring, increased light availability and nutrient inputs from snowmelt and runoff can stimulate phytoplankton growth and nitrogen uptake in the epilimnion
• In the summer, thermal stratification can lead to the development of a nutrient-depleted epilimnion and a nutrient-rich hypolimnion, with nitrogen cycling processes such as denitrification and anammox becoming more prominent in the hypolimnion
• Fall turnover, or the breakdown of thermal stratification, can lead to the redistribution of nutrients throughout the water column, stimulating phytoplankton growth and nitrogen uptake
• Winter conditions, characterized by low temperatures and limited light availability, can lead to reduced nitrogen cycling rates and the accumulation of ammonium in the water column