10.2 Nitrogen cycle and its importance in ecosystems

The nitrogen cycle transforms nitrogen between different chemical forms as it moves through the atmosphere, soil, water, and living organisms. Because nitrogen is a building block of amino acids, proteins, and DNA, every living thing depends on this cycle. Yet nitrogen gas ($N_2$), which makes up 78% of the atmosphere, is unusable by most organisms until it's converted into reactive forms.

Nitrogen Cycle Processes

Nitrogen Transformation Stages

The cycle has four major stages. Each one converts nitrogen into a different chemical form, and together they keep nitrogen moving between the living and non-living parts of ecosystems.

1. Nitrogen fixation converts atmospheric $N_2$ into biologically usable forms like ammonia ($NH_3$) or ammonium ($NH_4^+$). The triple bond in $N_2$ is extremely strong, so this conversion requires a lot of energy. It happens three ways:

   • Biological fixation by bacteria and cyanobacteria (the largest natural source)
   • Lightning, which provides enough energy to split $N_2$ and combine it with oxygen, forming nitrogen oxides that dissolve in rain
   • Industrial fixation via the Haber-Bosch process, which produces synthetic fertilizer and now rivals biological fixation in scale

2. Nitrification is a two-step oxidation carried out by specialized soil bacteria under aerobic (oxygen-rich) conditions:

   • Nitrosomonas bacteria oxidize $NH_3$ / $NH_4^+$ into nitrite ($NO_2^-$)
   • Nitrobacter bacteria then oxidize $NO_2^-$ into nitrate ($NO_3^-$)
   • The end product, $NO_3^-$, is the form most plants prefer to absorb

3. Denitrification reduces $NO_3^-$ back into $N_2$ gas (and some $N_2O$), returning nitrogen to the atmosphere. Denitrifying bacteria carry this out under anaerobic (low-oxygen) conditions found in waterlogged soils, wetlands, and marine sediments. This step is the main way reactive nitrogen is removed from ecosystems, so it acts as a critical counterbalance to fixation.

4. Ammonification (also called mineralization) breaks down organic nitrogen from dead organisms and waste products into $NH_3$ or $NH_4^+$. Decomposers like bacteria and fungi drive this process, recycling nitrogen back into the soil where it can re-enter the cycle through nitrification or direct plant uptake.

Importance of the Nitrogen Cycle

• It maintains a balance of reactive nitrogen in the environment, preventing both depletion and harmful accumulation.
• It supplies the usable nitrogen that plants need for growth, which in turn supports the productivity of entire food webs.
• It regulates nitrogen availability in soils and water, directly influencing which species thrive and how ecosystems are structured.
• Nitrogen is often the limiting nutrient in terrestrial and marine ecosystems, meaning even small changes in the cycle can have outsized effects on productivity.

Nitrogen-Fixing Organisms

Symbiotic Nitrogen Fixation

Legumes like soybeans, alfalfa, and clover form a mutualistic relationship with Rhizobium bacteria that live inside specialized root nodules. The bacteria convert $N_2$ into $NH_3$, which the plant uses to build proteins. In return, the plant supplies the bacteria with carbohydrates (energy) and a low-oxygen environment that the enzyme nitrogenase needs to function. Nitrogenase is irreversibly damaged by oxygen, which is why this sheltered environment matters so much.

This partnership is a major nitrogen input in both agricultural and natural ecosystems. Farmers often rotate crops with legumes specifically to replenish soil nitrogen without synthetic fertilizer. A single season of a legume cover crop can add roughly 50 to 200 kg of nitrogen per hectare to the soil, depending on the species and growing conditions.

Free-Living Nitrogen Fixation

Not all nitrogen fixers need a plant partner. Free-living bacteria (Azotobacter in aerobic soils, Clostridium in anaerobic soils) and cyanobacteria (Anabaena, Nostoc) fix $N_2$ independently. They're found across a wide range of environments, from temperate soils to aquatic systems to extreme habitats like hot springs.

Free-living fixers are especially important in ecosystems where legumes are scarce, such as early-successional landscapes after a disturbance or in nutrient-poor aquatic environments. In rice paddies, for example, cyanobacteria in the standing water provide a significant natural nitrogen supplement to the crop.

Nitrogen Compounds

Nitrates ($NO_3^-$)

$NO_3^-$ is the most common form of nitrogen that plants absorb from the soil. Because it's negatively charged, it doesn't bind well to negatively charged soil particles, making it highly soluble and mobile in water. That mobility cuts both ways:

• Plants can access it easily, but it also leaches readily into groundwater and surface water.
• Excess nitrates in water bodies fuel eutrophication and algal blooms (more on this below).
• High nitrate concentrations in drinking water can cause methemoglobinemia ("blue baby syndrome") in infants, where nitrite (converted from nitrate by gut bacteria) interferes with hemoglobin's ability to carry oxygen. The EPA drinking water standard for nitrate is 10 mg/L.

Ammonia ($NH_3$) and Ammonium ($NH_4^+$)

$NH_3$ is a gas produced during ammonification and nitrogen fixation. In water or moist soil, it quickly picks up a hydrogen ion to form $NH_4^+$. The distinction matters:

• $NH_4^+$ is positively charged, so it binds to negatively charged clay particles and organic matter through cation exchange capacity (CEC). This keeps it in the soil and available for plant and microbial uptake.
• $NH_3$ can volatilize (escape as gas) into the atmosphere, where it contributes to particulate matter formation and nitrogen deposition downwind.

The balance between $NH_3$ and $NH_4^+$ depends on soil pH. In alkaline (high pH) soils, more nitrogen exists as volatile $NH_3$, which is why ammonia loss is a bigger problem in limed or naturally basic soils.

Environmental Impact

Eutrophication

Eutrophication occurs when excess nutrients, primarily nitrogen and phosphorus, enter a water body and trigger explosive growth of algae and aquatic plants. The sequence of harm follows a predictable pattern:

1. Nutrient-rich runoff (agricultural fertilizer, sewage, atmospheric deposition) enters a lake, river, or coastal zone.
2. Algae populations boom at the surface, blocking light to submerged plants.
3. When the algae die, decomposing bacteria consume large amounts of dissolved oxygen.
4. Oxygen levels plummet, creating hypoxic zones (dead zones) where fish and other aquatic organisms suffocate.

A well-known example is the Gulf of Mexico dead zone, which in recent years has exceeded 15,000 $km^2$, fueled largely by nitrate-laden runoff from the Mississippi River watershed. In freshwater systems, phosphorus is typically the primary limiting nutrient driving eutrophication, while in coastal and marine systems, nitrogen is more often the culprit.

Nitrogen Deposition and Ecosystem Imbalances

Burning fossil fuels and volatilizing agricultural ammonia release reactive nitrogen compounds ($NO_x$, $NH_3$) into the atmosphere. These compounds eventually settle back onto land and water as nitrogen deposition, either dissolved in precipitation (wet deposition) or as dry particles and gases (dry deposition), often far from their source.

Chronic nitrogen deposition can push ecosystems past their capacity to absorb extra nitrogen, a condition called nitrogen saturation. The consequences include:

• Soil acidification, as nitrification produces hydrogen ions ($H^+$) that lower soil pH and can mobilize toxic aluminum
• Nitrate leaching into streams and groundwater, degrading water quality
• Loss of sensitive species such as lichens and mycorrhizal fungi that are adapted to low-nitrogen conditions
• Shifts in plant community composition, favoring fast-growing, nitrogen-loving species over native diversity