A-Level Environmental Science: Biogeochemical Cycles

Comprehensive practice questions covering the carbon, nitrogen, and phosphorous cycles, including human impacts and sustainability methods as per A-Level Environmental Science Topic 2.

Which three biogeochemical cycles are covered in Paper 1 of the Physical Environment module?

The carbon cycle, nitrogen cycle, and phosphorous cycle.

Why are biogeochemical cycles considered recycling processes?

Because there have been no new carbon or nitrogen deliveries from outer space; the elements that exist here have always been here and are continuously recycled.

What roles do decomposers and detritivores play in nutrient cycling?

They break down dead organisms and release nutrients back into the soil or atmosphere, making them available for use by other organisms.

What are the four main reservoirs for the carbon cycle?

The atmosphere, hydrosphere, lithosphere, and biosphere.

In what forms is carbon found in the atmosphere?

Carbon dioxide ($CO_2$), carbon monoxide ($CO$), and methane ($CH_4$).

In what form is carbon usually found in the lithosphere?

Carbonate rocks, such as limestone ($CaCO_3$).

What is carbon sequestration in the context of photosynthesis?

The process where plants absorb $CO_2$ from the atmosphere and convert it into glucose stored in their biomass.

How do anaerobic environments like landfill sites and rice padi fields affect the carbon cycle?

They release methane ($CH_4$) instead of carbon dioxide ($CO_2$).

What chemical reaction occurs when $CO_2$ dissolves in water?

It forms carbonic acid, which breaks down into a bicarbonate ion and a hydrogen ion, increasing water acidity.

How are fossil fuels formed?

By the incomplete decomposition of dead organic matter under anaerobic conditions over many years (e.g., coal from dead plants, oil from marine plankton).

What is the impact of deforestation on the carbon cycle?

It reduces the amount of photosynthesis, slowing the rate of carbon sequestration from the atmosphere into biomass.

How does human activity like shipping affect ocean carbon uptake?

Release of oil and heavy metals can kill photosynthesising phytoplankton and algae, reducing the volume of $CO_2$ they can absorb.

What are SSSI and NNR, and how do they help the carbon cycle?

Sites of Special Scientific Interest and National Nature Reserves; they protect carbon sinks like peat bogs and forests from damaging activities.

What is Carbon Capture and Storage (CCS)?

A manmade process where $CO_2$ is captured from the atmosphere and pumped underground into disused mining structures in the lithosphere.

Why is atmospheric nitrogen ($N_2$) unavailable to most organisms?

The two nitrogen atoms are joined by a triple bond which most organisms lack the enzyme to break.

What is the definition of Nitrification?

A series of oxidation reactions where bacteria convert ammonium ions into nitrites ($NO_2$) and then into nitrates ($NO_3$).

What is the Haber process?

An artificial agricultural process that uses high temperatures and pressures (from fossil fuel combustion) to turn nitrogen and hydrogen into ammonia for inorganic fertilisers.

How does ionisation contribute to the nitrogen cycle?

Lightning provides the high energy needed for oxygen and nitrogen in the atmosphere to fuse and create oxides of nitrogen ($NO_x$).

What is the symbiotic relationship found in leguminous crops?

Legumes (peas/beans) provide sugars to nitrogen-fixing bacteria in their root nodules; the bacteria use the enzyme nitrogenase to provide fixed nitrogen to the plant.

What conditions encourage Denitrification?

Anaerobic conditions, such as waterlogged or compacted soil, where bacteria convert nitrates back into nitrogen gas and nitrous oxide.

What is Eutrophication?

A process where nutrient runoff causes algal blooms that block sunlight, leading to plant death and oxygen depletion by aerobic decomposing bacteria.

How do catalytic converters improve the sustainability of the nitrogen cycle?

They use a metal catalyst (commonly platinum) to reduce nitrous oxide gases back into nitrogen ($N_2$) and oxygen ($O_2$).

What is a 'buffer strip' in agricultural management?

An area of land closest to rivers left free from crops and fertiliser use to stop fertiliser from leaching into the water.

Why is the phosphorous cycle slower than the carbon or nitrogen cycles?

Phosphorous has no gaseous form (not found in the atmosphere) and has low solubility in water.

How does soil pH affect phosphate availability?

In acidic conditions, solubility increases (leading to leaching); in alkaline conditions, phosphates become insoluble and unavailable for root absorption.

What is guano?

An accumulation of seabird droppings that is rich in phosphorous and often mined for inorganic fertilisers.

How do mycorrhizal fungi assist in the phosphorous cycle?

They form a symbiotic relationship with crop roots to efficiently absorb phosphorous from the soil in exchange for sugars.