Biogeochemical Cycles

Page 1

The Biogeochemical Cycles

The Carbon Cycle

The Nitrogen Cycle

The Phosphorus Cycle

Page 2

Human Impact on the Carbon Cycle

Photosynthesis:

Deforestation - less carbon from the atmosphere into biomass.

Afforestation - more carbon from the atmosphere into biomass.

Marine pollution with toxic materials - reduced numbers of phytoplankton = reduced absorption of dissolved carbon dioxide.

Climate change reduces the area of sea ice - less algae (and therefore krill) = less carbon dioxide removed from the sea.

Aerobic Respiration:

Ploughing - increased rate of decomposition = more carbon dioxide released back to the atmosphere.

Anaerobic Respiration:

Produced by human activities - rice padi fields, landfill sites, livestock intestines, anaerobic sediments in rivers.

Destroyed by drainage - waterlogged fields, marshlands, peat bogs.

Biomass movements:

Adding compost/mulch to improve soil fertility.

Deforestation/crop harvesting = reduced soil organic matter and soil erosion.

Sewage discharge - moves biomass into the sea.

Fishing/aquaculture moves marine biomass into the terrestrial system.

Methane releases from fossil fuel extraction.

Carbon dioxide releases from the combustion of fossil fuels.

Human activities that affect the concentration of carbon dioxide in the atmosphere also affect the concentrations of carbonic acid in the sea.

Sustainable Management of the Carbon Cycle

Carbon Capture and Storage:

Pre-combustion technology (e.g. gasification/oxy-fuel combustion systems).

Post-combustion technology - removing carbon dioxide from exhaust gases. (e.g. dissolving it in a solvent, high pressure membrane filtration, adsorption/desorption processes, cryogenic separation, graphene adsorption).

Storage (e.g. underground, in depleted aquifers/oil fields/gas fields, used for secondary oil recovery).

Conversion of Biomass Carbon Stores:

Protect habitats such as peat bogs/forests (which contain large amounts of carbon).

Carbon Sequestration:

Large-scale tree planting/replanting.

Use alternatives to fossil fuels, like renewables and nuclear.

Page 3

Human Impact on the Nitrogen Cycle

Haber Process:

Producing ammonia from nitrogen and hydrogen using an iron catalyst at high temperature and pressure.

Uses large amounts of energy.

Ammonia can then be converted to nitrates for use in agricultural fertilisers.

Agriculture:

Nitrate fertilisers may increase problems caused by leaching, including eutrophication(especially if application is followed by heavy rain/if fertilisers are applied close to rivers.

Field drainage makes them more aerobic, increasing numbers of nitrifying bacteria and reducing numbers of denitrifying bacteria.

Ploughing increases rate of decomposition, releasing more nitrous oxides into the atmosphere.

Leguminous plants (peas/beans/clover) may be grown to increase levels of nitrogen compounds in the soil which crops can then absorb.

Pollution:

Oxides of nitrogen are released by combustion processes.

They may subsequently increase the quantity of nitrates washed into the soil by rain.

Sustainable Management of the Nitrogen Cycle

Control Combustion Processes:

Reducing the use of fossil fuels would reduce nitrous oxide releases.

Adopting circular economy would result in more low-temperature manufacturing processes.

Control Nitrous Oxide Releases:

Use post-combustion technology (catalytic converters/urea sprays).

Organic Fertilisers:

Use manure, sewage and food production wastes to increase nutrient and humus levels.

Management of Soil Processes:

Cultivate leguminous crops.

Crop rotation.

Minimal use of pesticides that harm soil biota.

Control of nitrate leaching.

Not applying fertilisers during rain/when rain is likely.

Use low-solubility fertilisers (e.g. urea).

Use low-tillage techniques to reduce soil disturbance.

Uncultivated 'buffer strips' along rivers.

Minimal use of nitrate fertiliser where leaching into aquifers is likely.

Manage Biological Wastes (releases of ammonia/eutrophication).

Page 4

Human Impacts on the Phosphorus Cycle

Mining of phosphate rocks:

Most is mined as calcium phosphate, then treated to produce ammonium phosphate (more soluble).

In the past, large accumulations of bird droppings were mined for the ammonium phosphates they contained (dry areas - Peru/tropical islands like Nauru in the Pacific). Most large guano deposits have been fully exploited.

Fertiliser use:

The use of fertilisers mobilises phosphorus compounds into the environment in more soluble forms.

This increases crop production, it can also contribute to eutrophication.

Sustainable Management of the Phosphorus Cycle

Use biological wastes as fertilisers to maintain phosphate nutrient availability.

Crop breeding programmes can increase the efficiency of phosphate absorption by crops.