The Nitrogen and Phosphorus Cycles

The nitrogen cycle moves nitrogen between the atmosphere, soil and living things through nitrogen fixation, nitrification, assimilation, ammonification and denitrification, and the phosphorus cycle moves phosphorus mainly through rocks, soil, water and organisms, without a significant gas phase. In brief: although 78 percent of air is nitrogen gas (N₂), most organisms cannot use it. Bacteria convert it to ammonium (nitrogen fixation), other bacteria turn ammonium into nitrate (nitrification), plants take up nitrate, and denitrifying bacteria return nitrogen to the air. Phosphorus is released slowly from rocks by weathering and is often the limiting nutrient in freshwater.

Nitrogen and phosphorus are essential for proteins, DNA, RNA and ATP, and they limit the growth of plants in many ecosystems. Human use of fertilizers has changed both cycles on a massive scale. This guide describes each step of the cycles, the organisms involved, the effects of human activities, compares the cycles, works through a nitrogen budget and a fertilizer calculation, and corrects common mistakes.

How does the nitrogen cycle work?

Nitrogen is a component of amino acids, proteins, nucleotides and chlorophyll. The atmosphere is the main reservoir, with about 78 percent N₂. The triple bond in N₂ is so strong that very few organisms can break it. Plants take up nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺), and animals get it by eating plants or other animals. Bacteria carry out the key transformations, as shown in the steps below.

What are the steps of the nitrogen cycle?

1. Nitrogen fixation: N₂ is converted into ammonia (NH₃, which forms ammonium NH₄⁺ in water). It is done by nitrogen-fixing bacteria using the enzyme nitrogenase. Rhizobium lives in nodules on the roots of legumes (peas, beans, clover) in a mutualistic relationship: the plant provides sugars and the bacteria provide ammonium. Free-living bacteria such as Azotobacter and cyanobacteria also fix nitrogen. Lightning fixes a small amount (nitrogen oxides, in rain), and industry does so in the Haber-Bosch process (N₂ + 3H₂ → 2NH₃ at high temperature and pressure).
2. Nitrification: in aerobic soils, Nitrosomonas bacteria oxidize ammonium to nitrite (NO₂⁻), and Nitrobacter bacteria oxidize nitrite to nitrate (NO₃⁻). The bacteria obtain energy from these reactions (chemosynthesis).
3. Assimilation: plants absorb nitrate (or ammonium) through the roots and use it to make amino acids, proteins and nucleic acids. Animals obtain nitrogen compounds by eating.
4. Ammonification (decomposition): decomposers (bacteria and fungi) break down dead organisms, feces and urea (excreted by animals), and release ammonium into the soil.
5. Denitrification: in waterlogged or oxygen-poor soils, denitrifying bacteria such as Pseudomonas use nitrate in place of oxygen and convert it to N₂ (and some N₂O, nitrous oxide, a greenhouse gas), which returns to the air.

Nitrate is very soluble and is easily washed out of the soil (leaching) into streams and groundwater.

How does the phosphorus cycle work?

Phosphorus is a component of ATP, DNA, RNA, phospholipids in membranes and the calcium phosphate of bones and teeth. Unlike carbon and nitrogen, it has no significant atmospheric reservoir. The cycle is slow, and it is mainly a sedimentary cycle:

• Weathering: rocks containing phosphate minerals (apatite) are weathered by rain and acids, which releases phosphate ions (PO₄³⁻) into the soil and water.
• Uptake: plants absorb phosphate through their roots, often with the help of mycorrhizal fungi, because phosphate binds strongly to soil particles and moves very slowly. Animals obtain it by feeding.
• Return: decomposers release phosphate from dead matter and wastes.
• Runoff and sedimentation: some phosphate is carried by rivers to the sea, where it is incorporated into marine organisms and sediments. Over millions of years, the sediments form rock and are uplifted, which completes the cycle.
• Guano: in some places, seabird droppings return phosphorus from the sea to land.

Phosphorus is often the limiting nutrient in freshwater and is limiting in many tropical soils. It can be recycled within ecosystems but is lost to sediments, so it is not easily replaced.

Nitrogen and phosphorus cycles compared

How have humans changed these cycles?

• Fertilizers: the Haber-Bosch process now fixes more nitrogen (about 120 million tonnes per year) than all natural land processes together, and has allowed the food supply to feed billions. Estimates suggest that about half of the nitrogen in human bodies comes from this process. Phosphate rock is mined, and is a finite resource. Only part of fertilizer is taken up by crops.
• Runoff and eutrophication: surplus nitrate and phosphate reach rivers and lakes, which causes algal blooms, oxygen depletion and dead zones (see the next article).
• Burning of fossil fuels: releases nitrogen oxides, which contribute to acid rain and smog. Nitrous oxide from soils and agriculture is a powerful greenhouse gas.
• Deforestation and soil loss: removes nutrients and increases leaching and erosion.
• Sustainable practices: crop rotation with legumes, applying the right amount of fertilizer at the right time, buffer strips along streams, using manure, and recycling phosphorus from sewage.

Worked example: nitrogen in fertilizer and crops

A farmer applies 150 kg of nitrogen per hectare as ammonium nitrate (NH₄NO₃). The wheat crop takes up 90 kg of N per hectare. Ammonium nitrate contains 28 kg of N in each 80 kg of compound (molar mass 80 g/mol, with 2 × 14 = 28 g of N).

1. Mass of fertilizer needed for 150 kg of N: 150 × (80 ÷ 28) = 428.6 kg of ammonium nitrate per hectare.
2. Nitrogen uptake efficiency: 90 ÷ 150 = 0.60, or 60 percent.
3. Nitrogen not taken up: 150 − 90 = 60 kg per hectare, which may remain in the soil, be lost by leaching as nitrate to water, or denitrified to the air.
4. For a farm of 200 hectares: 60 × 200 = 12 000 kg (12 tonnes) of nitrogen is at risk of being lost to the environment each year.
5. Alternative with legumes: if a clover crop fixes 100 kg of N per hectare in a year, the next crop could use that nitrogen, and the farmer could reduce the fertilizer by a similar amount.

The calculation shows the economic and environmental interest in raising the uptake efficiency. Practices such as splitting the fertilizer into smaller amounts during the growing season, testing soil, and using precision agriculture can raise efficiency above 60 percent. The proportion lost depends on rainfall, soil type and timing.

How were the cycles discovered?

Daniel Rutherford discovered nitrogen in 1772. Hermann Hellriegel and Hermann Wilfarth showed in 1886 that legumes fix atmospheric nitrogen with the help of root bacteria, and Martinus Beijerinck isolated Rhizobium in 1888. Sergei Winogradsky discovered nitrifying bacteria in the 1880s and 1890s. Fritz Haber developed the ammonia synthesis in 1909, and Carl Bosch scaled it up industrially by 1913. They each received Nobel Prizes in Chemistry. The rapid growth of the world population in the twentieth century is partly attributed to synthetic nitrogen fertilizer.

Which mistakes are common?

Mistake 1: Saying that plants take nitrogen gas from the air

Most plants cannot. They absorb nitrate or ammonium. Only certain bacteria fix N₂.

Mistake 2: Mixing up nitrification and denitrification

Nitrification forms nitrate from ammonium. Denitrification turns nitrate back into N₂ gas.

Mistake 3: Putting phosphorus in the atmosphere

The phosphorus cycle has no major gas phase.

Mistake 4: Thinking that decomposers make nitrate directly

Decomposers release ammonium (ammonification). Nitrifying bacteria convert it to nitrate.