Nutrient Cycles

Introduction to Nutrient Cycles

Energy travels in an ecosystem in one direction. It goes from the sun and is transferred down the different trophic levels as we saw in Energy and Ecosystems

Unlike the one directional flow of energy through an ecosystem, nutrients are recycled. The nitrogen and phosphorus cycles are examples of nutrient cycles.

Human activity can interfere with this recycling of nutrients, disrupting the natural cycling of these nutrients. We look at this later with fertilisers & eutrophication.

Nitrogen Cycle

All living organisms need a source of nitrogen in order to synthesise nitrogen-containing compounds, such as DNA, RNA, proteins and chlorophyll.

The atmosphere contains 78% nitrogen, but it’s in a chemically inert (unreactive) form that is unavailable to plants or animals to use. They need bacteria to convert it into nitrogen-containing compounds first.

The nitrogen cycle is the continuous recycling of nitrogen between the atmosphere, soil and living organisms.

There are 4 main phases:

• Ammonification
• Nitrification
• Nitrogen fixation
• Denitrification

Nitrogen fixation

• Is the process of converting nitrogen gas (which is unusable for plants) into ammonia (NH₃), which forms ammonium ions (NH₄⁺)
• It is mainly carried out by microorganisms, although it can occur from lightning or in industrial processes
• Nitrogen-fixing bacteria are found in plants’ root nodules. They form a mutualistic relationship with plants. The bacteria provide the plant with nitrogen compounds, and the plant provides the bacteria with carbohydrates (required for respiration)
• Nitrogen-fixing bacteria can also be found ‘free living’ in the soil. These do not require a host plant to fix nitrogen

Ammonification

• The nitrogen-containing compounds (e.g. proteins, amino acids, DNA and urea) from dead organisms & animal waste are turned into ammonia by saprobiotic microorganisms
• This ammonia then forms ammonium ions

Nitrification

• This involves the conversion of ammonium ions into nitrate ions

• This is a two-stage oxidation reaction, which releases energy

  1. Nitrifying bacteria carry out the oxidation of ammonium ions (NH₄⁺) to nitrite ions (NO₂⁻)

  2. Nitrifying bacteria carry out the further oxidation of nitrites (NO₂⁻) to nitrates (NO₃⁻)

• Nitrifying bacteria require oxygen to carry out these reactions.

• These nitrates can then be used by the plants. They can assimilate the nitrates by absorbing them and converting them into organic nitrogen compounds like amino acids and proteins.

Denitrification

• The nitrates in the soil are converted into nitrogen gas by denitrifying bacteria
• These denitrifying bacteria work in anaerobic (low oxygen) conditions, often when the soil becomes waterlogged.

Phosphorus Cycle

Phosphorus is required for ATP, phospholipids and nucleic acids (DNA and RNA).

Unlike the nitrogen cycle, phosphorus doesn’t enter the atmosphere. Rocks and sediments are the largest reservoir of phosphorus, usually in the form of phosphate ions.

• Weathering & erosion - Phosphate ions become dissolved and available for absorption by plants which they can incorporate into their biomass

• Absorption (Mycorrhizae) - Fungi that form a mutualistic relationship with plant roots. The fungi have hyphae that can increase the surface area of the plant’s roots for the absorption of water and minerals (ions). The fungi (in return as it’s a mutualistic relationship) get glucose and other organic compounds from the plant, which they can use in respiration

• Consumers ingest producers - Phosphate ions are transferred to consumers as they ingest the plants

• Decomposition - Decomposers (saprobionts) break down dead animals and plants, releasing phosphate ions into the soil. These microorganisms also release phosphate from animal wastes.

• Sedimentation - When these organisms die, their remains sink and accumulate as sediments on the ocean floor. Over millions of years, these sediments form sedimentary rock.

Microorganisms’ Role

Fertilisers & Eutrophication

When crops are harvested they are removed from the soil. This means that the minerals (nitrates and phosphates) the crops absorbed from the soil are also removed. They are not recycled back into the soil by decomposers as part of the nitrogen and phosphorus cycle. The same happens when livestock are removed from the land. The minerals in their bodies, which came from the plants they ate, are lost too.

Farmers use fertilisers to replace these lost nutrients.

• Natural fertilisers - organic matter such as manure or compost. These release minerals slowly as they are decomposed by saprobiotic microorganisms
• Artificial fertilisers - manufactured chemicals containing specific ratios of minerals (e.g. NPK fertilisers). These dissolve quickly and are immediately available to plants

Leaching

• When excess fertiliser (particularly nitrates) dissolves in rainwater and is washed through the soil into waterways such as rivers and lakes
• Nitrates are very soluble, making them especially prone to leaching (more than phosphate ions)

Eutrophication

Eutrophication is the process by which excess minerals (from leached fertilisers) cause environmental damage.

1. Fertilisers leach into rivers/lakes, increasing the concentration of nitrates and phosphates in these rivers/lakes

2. This causes rapid growth of algae on the water surface (algal bloom). The algal bloom blocks light from reaching aquatic plants below the surface

3. These plants die as they can no longer photosynthesise

4. Saprobiotic microorganisms decompose the dead plant matter. These decomposers use oxygen for aerobic respiration

5. This causes the oxygen concentration in the water to decrease. Organisms that rely on aerobic respiration (e.g. fish) die due to lack of oxygen

Exam Question Practice