The Nitrogen cycle

what is decompisiton?

the breakdown of dead organic matter and waste products by microorganisms, releasing nutrients back into the ecosystem

how are bacteria and fungi involved in decomposition?

they feed saprotrophically

what is extracellular digestion?

Digestion outside the organism where enzymes are secreted onto food and the soluble products are absorbed.

why are decomposers important in ecosystems?

They recycle nutrients and release mineral ions back into the soil for plant uptake.

what is ammonification?

The conversion of organic nitrogen compounds from dead organisms and waste into ammonium ions by decomposers

which molecules are broken down during ammonification?

Proteins, nucleic acids and urea

what mineral ion is released during ammonification?

Ammonium ions (NH₄⁺)

what are mycorrhizae?

Mutualistic associations between fungi and plant roots

how do fungi benefit from mycorrhizae?

receive organic compounds such as glucose from the plant

why is Nitrogen important in living organisms?

needed to make:

• amino acids

• proteins

• nucleic acids

• ATP

why can’t plants use atmospheric nitrogen directly?

Nitrogen gas (N2) is very stable and inert

what is nitrogen fixation

The conversion of atmospheric nitrogen gas into ammonia or ammonium compounds

which bacteria carry out nitrogen fixation?

• rhizobium

• azotobacter

where are rhizobium bacteria found?

in root nodules of legume plants

where are azotobacter bacteria found

free living in the soil

what enzyme is used in nitrogen fixation?

Nitrogenase

why does nitrogen fixation require a lot of ATP?

Breaking the triple bond in nitrogen gas requires large amounts of energy

What is the role of leghaemoglobin in root nodules

It maintains a low oxygen concentration to protect nitrogenase while still supplying oxygen for respiration

why must oxygen conc be low in root nodules

oxygen inhibits or denatures nitrogenase

what is nitrification

the conversion of ammonium ions into nitrites and then nitrates by nitrifying bacteria

what are the 2 stages of nitrification?

• Ammonium ions → nitrite ions

• Nitrite ions → nitrate ions

which bacteria convert ammonium ions into nitrates?

Nitrosomonas

which bacteria converts nitrites into nitrates?

Nitrobacter

what conditions are needed for nitrification?

Aerobic conditions with oxygen present

why does waterlogging reduce nitrification?

waterlogged soils contain less oxygen

why are nitrates important to plants?

nitrates are absorbed by roots and used to make amino acids, proteins and nucleic acids

why are nitrifying bacteria described as chemoautotrophs?

They obtain energy from chemical reactions and use it to synthesise organic molecules

what is denitrification?

The conversion of nitrates into nitrogen gas by denitrifying bacteria.

what conditions favour denitrification?

anaerobic conditions such as waterlogged soil

why does denitrification increase in waterlogged soils?

Oxygen levels are low, favouring anaerobic bacteria.

what is a consequence of denitrification for plants?

soil nitrate levels decrease, reducing plant growth

why is denitrification important in the nitrogen cycle?

it returns nitrogen gas to atmosphere

how does nitrogen enter food chains?

Plants absorb nitrate ions and use them to make organic molecules which are then eaten by animals.

how is nitrogen returned to the soil from animals?

Through waste products, death and decomposition

why can nitrate ions be easily lost from soil?

they are soluble and can be leached away by water

what are the 4 main processes in the nitrogen cycle?

• nitrogen fixation

• ammonification

• nitrification

• denitrification

why is nutrient recycling important in ecosystems?

ensures essential elements are continuously available for living organisms

why are legumes important in agriculture?

They contain nitrogen-fixing bacteria which increase soil nitrogen levels

how do human activities affect the nutrient cycle?

Fertiliser use and farming can increase nitrate levels and alter nutrient cycling

why can excessive nitrate fertiliser use be harmful?

can lead to eutrophication in aquatic ecosystems