The nitrogen cycle and nitrate availability

Describe the distinct biological and physical pathways by which different materials continuously cycle through the abiotic and biotic components of an ecosystem.
Explain the ecological importance of the carbon cycle, detailing the main processes involved and highlighting the critical role of microorganisms acting as decomposers.
Explain the importance and key stages of the water cycle, including human technological interventions for producing potable water in drought-stricken areas via desalination.
Explain the various mechanisms by which essential nitrates are made available for plant cellular uptake, detailing the use of agricultural fertilisers, crop rotation practices, and the fundamental role of bacteria in the nitrogen cycle.

Why plants need a usable nitrogen supply

Plants need nitrogen to make amino acids, the building blocks of proteins. Proteins are needed for growth and include enzymes that control chemical reactions. Although about 78% of the atmosphere is nitrogen gas, plants cannot use this gas directly.

Instead, plant roots absorb nitrogen-containing compounds from the soil, including nitrate ions, $\mathrm{NO_3^-}$ , dissolved in soil water. Plants use this nitrogen to build amino acids and proteins. The nitrogen cycle changes nitrogen between different forms and moves it between the atmosphere, soil and living organisms.

Bacteria turn atmospheric nitrogen into usable compounds

Nitrogen-fixing bacteria convert nitrogen gas from the atmosphere into ammonia, which forms ammonium compounds in the soil. Some of these bacteria live freely in soil; others live in root nodules, small swellings on the roots of legumes such as peas, beans and clover.

The partnership in a root nodule benefits both organisms. The plant supplies nutrients to the bacteria, while the bacteria supply nitrogen compounds to the plant. The legume still cannot use nitrogen gas itself: the bacteria carry out the conversion.

Another group, nitrifying bacteria, converts ammonium compounds into nitrites and then nitrates. This process is called nitrification. It makes nitrogen available in a form that plant roots can absorb.

Recycling nitrogen from organisms

When an animal eats a plant, nitrogen in the plant's proteins passes into the animal. The animal uses it to make its own proteins, so nitrogen moves through food chains.

Dead plants, dead animals and animal waste contain nitrogen compounds. Decomposers, including bacteria and fungi, break down this material and release ammonia or ammonium compounds into the soil. Nitrifying bacteria can then convert these compounds into nitrates, allowing plants to take up the nitrogen again.

Not all bacterial activity increases nitrate availability. Denitrifying bacteria convert nitrates into nitrogen gas, which returns to the atmosphere. They act particularly in oxygen-poor conditions, such as waterlogged soil. This removes nitrates that plants could otherwise absorb and can reduce soil fertility.

Atmospheric nitrogen is converted by nitrogen-fixing bacteria into ammonia and ammonium compounds. Some bacteria live freely in soil and others in legume root nodules, where they can supply nitrogen compounds directly to their host. Nitrifying bacteria convert ammonium into nitrites and then nitrates. Roots take up nitrates to make plant proteins, and feeding transfers nitrogen to animals. Dead organisms and waste are decomposed by bacteria and fungi, returning ammonium to the soil. Denitrifying bacteria in oxygen-poor soil convert nitrates back into atmospheric nitrogen gas. — Bacteria change nitrogen into different forms. Nitrification supplies nitrates for plant uptake, while denitrification returns nitrogen to the atmosphere.

Follow the arrows by asking two questions: where is the nitrogen moving, and which process changes its form? Fixation brings atmospheric nitrogen into usable compounds; decomposition recycles nitrogen from organisms; nitrification produces nitrates; denitrification removes them.

Main nitrogen pathways

Plants absorb nitrates through their roots and use nitrogen to make amino acids and proteins. They cannot use atmospheric nitrogen gas directly.
Nitrogen-fixing bacteria: nitrogen gas → ammonia/ammonium compounds. Found in soil and legume root nodules.
Decomposers: nitrogen compounds in dead organisms and waste → ammonia/ammonium compounds.
Nitrifying bacteria: ammonium compounds → nitrites → nitrates.
Denitrifying bacteria: nitrates → nitrogen gas, particularly in oxygen-poor, waterlogged soil.
Feeding transfers nitrogen from plants to animals.

Maintaining nitrate availability

Artificial fertilisers: supply nitrogen-containing mineral ions; nitrate can be absorbed directly.
Manure and compost: decomposition releases ammonium compounds; nitrification produces nitrates.
Crop rotation with legumes: root-nodule bacteria fix nitrogen; decomposition of crop residues and nitrification supply nitrates for the next crop.

Distinguish nitrogen-fixing bacteria, which use atmospheric nitrogen, from nitrifying bacteria, which convert ammonium compounds into nitrates.

When explaining crop rotation, link legumes to root-nodule bacteria, then explain how decomposition and nitrification make nitrogen available to the next crop.

Denitrifying bacteria reduce nitrate availability: they convert nitrates into nitrogen gas, especially in oxygen-poor, waterlogged soil.

A nitrogen-containing mineral ion, $\mathrm{NO_3^-}$ , absorbed by plant roots and used to make amino acids and proteins.

The conversion of atmospheric nitrogen gas into nitrogen compounds by nitrogen-fixing bacteria.

The conversion of ammonium compounds into nitrites and then nitrates by nitrifying bacteria in soil.

The conversion of nitrates into nitrogen gas by denitrifying bacteria, particularly in oxygen-poor soil.

An organism, such as a bacterium or fungus, that breaks down dead organisms and waste, returning nutrients to the environment.

A plant such as a pea, bean or clover that can have nitrogen-fixing bacteria living in nodules on its roots.

Growing different crops on the same land in successive growing seasons; including legumes can help replenish soil nitrogen.

A substance added to soil to supply mineral nutrients needed for plant growth.