Nutrient cycles, energy and ecosystems, fertilisers

Why is the nitrogen cycle important?

It is important for recycling nitrogen, and essential element to create biological molecule such as proteins, DNA and RNA which are needed by organisms to sustain life.

What human activities have disrupted the natural nitrogen cycle?

The use of fertilisers and and combustion of fossil fuels.

How much of the atmosphere is composed of nitrogen?

78%

Why is the nitrogen cycle useful? 

It converts inaccessible and unreactive nitrogen gas into different chemical forms so it can be used. 

What are the four key processes in the nitrogen cycle?

1. Nitrogen fixation 

2. Ammonification 

3. Nitrification 

4. Denitrification 

What role do detritivores play in the nitrogen cycle?

They speed up decay by feeding on waste and debris from decomposed organic matter, breaking it down into small pieces.

What is the role of saprobionts in the nitrogen cycle? 

They convert nitrogen containing molecules in dead organisms, faeces, and urine into ammonia.

What is assimilation by producers?

Producers absorbs useful substances and incorporate them into other compounds to aid survival. 

What is the process of nitrogen fixation?

• nitrogen gas is fixed into compounds such as ammonia or nitrates

• symbiotic bacteria in root nodules of leguminous plants fix nitrogen into ammonia, which is converted to amino acids for the plant, the bacteria is provided with carbohydrates from the plant

• free living bacteria in the soil fix nitrogen into ammonia and then into amino acids, when the bacteria die they are decomposed by sapribionts which releases nitrogen rich compounds which can be assimilated by plants

• lightning can fix nitrogen by breaking its triple bonds using energy, the nitrogen reacts with oxygen to create nitrogen oxides, it then forms nitric acid which dissolves in rain water to create nitrates which are carried down to the soil

What are saprobionts?

Organisms such as fungi and bacteria that obtain nutrients by decaying organic matter through extracellular digestion

What is the process of ammonification?

• saprobionts break down organic matter into ammonia

• proteins broken down into amino acids through extracellular enzymes

• broken down further to remove amino groups using deaminase enzymes

What is the process of nitrification?

• nitrifying bacteria convert ammonia to nitrate ions through oxidation

• nitrite used as an intermediate

What is the process of denitrification?

• nitrates are converted back into nitrogen using denitrifying bacteria under anaerobic conditions

• excessive denitrification can deplete the soil of nitrogen and negatively impact plant growth 

• occurs in waterlogged soils, can be prevented by soil being well drained and aerated 

What biological molecules does the phosphorous cycle form?

Phospholipids, DNA, RNA, ATP

How is phosphorous found? 

In sedimentary rock deposits and dissolved in water as phosphate ions 

What are the 7 key steps of the phosphorous cycle?

1. weathering - erosion of rocks to release phosphate ions into soil and water to be absorbed by producers

2. fertilisers - phosphorous rich fertilisers leach into water sources

3. symbiotic relationships - mycorrhizal fungi in roots

4. food chain - transferred when organisms consume one another

5. excretion and death - exit when organisms die and through waste products 

6. decomposition - saprobionts break down dead material and release phosphate ions to be reabsorbed 

7. sedimentation and deposition - phosphates in water sources, waste, and dead organic matter accumulate and reform sedimentary rocks 

What is a common mechanism that transfers phosphates from the ocean to land?

seabird guano which acts as a natural fertiliser for plant growth

What is the role of mycorrhizae in phosphate uptake?

• symbiotic relationship, provided with sugars and amino acids 

• increase root surface area through fungal networks 

• retain moisture and minerals in the roots to resist drought 

What are the implications of removing crops?

• mineral ions can’t be returned to the soil through decomposition 

• nutrients are removed from the nitrogen and phosphorous cycles 

• soil fertility is depleted over time as there is a net removal of nutrients 

• plant growth is limited 

Why are fertilisers used?

to reintroduce nutrients back into the soil to compensate for the nutrients lost during harvesting 

What are the two types of primary fertilisers? 

1. artificial fertilisers - chemically made for specific crops, have an appropriate balance of nutrients, powders and pellets 

2. natural fertilisers - made from organic matter like manure and compost, release nutrients gradually as the organic matter decomposes 

What are the environmental impacts of nitrogen rich fertilisers?

• reduces species diversity - nitrogen rich soils can favour the growth of rapidly growing grasses which outcompete other species

• leaching - nutrients like nitrates are washed out of the soil and can pollute water sources which can cause eutrophication

How can leaching effect humans?

nutrients may leach drinking water, this can increase the risk of oxygen transport issues in babies and stomach cancer

What is the process of eutrophication?

1. excess soluble nitrates and phosphates leach into water sources from soil

2. algae rapidly grow at the surface of the water sources

3. this blocks light from reaching plants and algae at lower depths

4. these plants and algae die as they can’t photosynthesise

5. levels of aerobic saprobiontic bacteria increase as they decompose the dead plant matter

6. this depletes oxygen levels in the water sources, causing aquatic life to die which disrupts the food webs

which type of primary fertiliser is more prone to leaching?

artificial fertilisers are more prone to leaching as they are more easily dissolved n water, so are the main cause of eutrophication

What is biomass?

the total mass of living material

How can biomass be measured?

1. dry the biomass sample in an oven until its mass remains constant

2. weigh this dry mass

3. burn the dry mass in a calorimeter

4. measure the volume and temperature change of the surrounding water

5. use to find heat energy released from the burnt biomass 

What is GPP?

gross primary production is the total amount of solar energy that plants convert into organic matter which is stored as chemical energy in their biomass 

What is NPP? 

net primary production is the chemical energy available to the next trophic level after accounting for respiratory losses, it is the energy converted into biomass

What is the formula for NPP?

NPP = GPP - R

How can energy be lost between trophic levels in a food chain?

• not all parts of the biomass are edible or digestible 

• energy is lost as heat through movement and respiration 

• energy is lost through urine and faeces 

• factors may limit photosynthesis 

What is net production of consumers?

the amount of ingested food converted into biomass