27.9
The Nitrogen Cycle
Nitrogen atoms, present in all proteins and DNA, are recycled between abiotic and biotic components of the ecosystem. However, the primary form of nit…
The nitrogen cycle involves the movement of nitrogen atoms between biotic and abiotic components in the ecosystem.
Most of the ecosystem's nitrogen is stored in the atmosphere as a gas, N2. But only bacteria posses the necessary enzymes to convert nitrogen gas into a usable form by other organisms, a term called nitrogen fixation. Therefore nitrogen is a very limited nutrient in the ecosystem.
Once bacteria in the soil convert the gas into ammonia, plants can the convert the nitrogen into proteins, DNA, and other organic molecules, which are consumed by herbivores and broken down into amino acids.
These can be reassembled into usable proteins for growth and development and for nutrition for consumers. Simultaneously, herbivores and consumers are returning some nitrogen back to the soil as waste products.
Both dead organisms and waste products contain nitrogen in the form of protein, urea, and uric acid. Decomposers like bacteria and fungi can break down these molecules releasing ammonia back into soil, which can be absorbed by plants or converted into nitrates by nitrifying bacteria.
These nitrates can also be either absorbed by plants or in the absence of oxygen be converted into nitrogen gas by denitrifying bacteria.
Human also input nitrogen into the ecosystem through the use of agricultural fertilizers. However, when used at the wrong time or in large quantities, excess nitrogen can flow into aquatic ecosystems and increase growth of bacteria, algae, and aquatic plants, a process called eutrophication.
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Q1: Why is nitrogen fixation essential in the nitrogen cycle?
Nitrogen fixation is essential because most atmospheric nitrogen exists as N2 gas, which most organisms cannot use directly. Only specialized bacteria possess the enzymes to convert N2 into ammonia, making nitrogen available for plants and other organisms. Without nitrogen fixation, nitrogen would remain locked in the atmosphere and unavailable to the biosphere.
Q2: How do plants obtain usable nitrogen from the soil?
Plants absorb nitrogen in two forms: ammonia produced by nitrogen-fixing bacteria, and nitrates created by nitrifying bacteria. These bacteria convert ammonia into nitrites and nitrates, which plants can directly absorb through their roots. This process makes nitrogen accessible for plant growth and protein synthesis.
Q3: What role do decomposers play in returning nitrogen to soil?
Decomposers like bacteria and fungi break down dead organisms and waste products containing nitrogen compounds such as proteins, urea, and uric acid. This decomposition releases ammonia back into the soil, which can be reabsorbed by plants or converted into nitrates by nitrifying bacteria, continuing the nitrogen cycle.
Q4: How does denitrification complete the nitrogen cycle?
Denitrifying bacteria convert nitrates back into nitrogen gas in anaerobic environments lacking oxygen. This process releases N2 back into the atmosphere, completing the cycle and returning nitrogen to its primary atmospheric reservoir where it remains until nitrogen-fixing bacteria convert it again.
Q5: What is eutrophication and how does agricultural fertilizer cause it?
Eutrophication occurs when excess nitrogen from agricultural fertilizers flows into aquatic ecosystems, stimulating rapid growth of bacteria, algae, and aquatic plants. This nutrient overload disrupts aquatic ecosystems and can create toxic algal blooms. Improper fertilizer timing or excessive quantities are primary causes of this environmental problem.
Q6: How do herbivores and consumers contribute to the nitrogen cycle?
Herbivores and consumers obtain nitrogen by eating plants, converting plant proteins into amino acids. They reassemble these amino acids into usable proteins for growth and development. Simultaneously, they return nitrogen to soil through waste products like urine and feces, which decomposers then break down to release ammonia.
Q7: Where is nitrogen stored long-term in ecosystems?
The primary long-term nitrogen reservoir is nitrogen gas in the atmosphere, comprising about 78% of air. Secondary sinks include swamps, marine sediments, and sedimentary rock, where nitrogen compounds can be stored for extended periods. Weathering of sedimentary rock can release stored nitrogen back into the ecosystem.