Nutrient Cycle and Carbon Cycle

Lesson Objectives:
- Construct a carbon cycle diagram
- Explain the conditions of an ecosystem as a carbon sink and carbon source
- Explain how recycling all chemical elements is required by living organisms in an ecosystem

The rates of photosynthesis and respiration are not necessarily equal.
If photosynthesis exceeds respiration, there is net uptake, and the ecosystem is acting as a carbon sink.
If respiration exceeds photosynthesis, there is net release, and the ecosystem is acting as a carbon source.

The natural flows of carbon dioxide ( $CO_2$ ) through the biosphere and ocean are much larger than humanity’s contribution of industrial and land use emissions.
These emissions gradually increase atmospheric concentration of $CO_2$ and other greenhouse gases (GHGs) that are warming the earth.
Carbon emissions from land use and deforestation account for 17% of total greenhouse gas emissions, or 22% of total $CO_2$ emissions into the earth’s atmosphere and oceans.

Carbon is found in pools, which are inorganic or organic supplies of carbon.
Carbon is one of the main elements found in all organic molecules including carbohydrate, protein, and lipids.
Four main pools:
- Biosphere
- Hydrosphere
- Atmosphere
- Sediments
Carbon is moved between these four pools by biological, geochemical, or industrial processes called fluxes.

Carbon sources include emissions from burning fossil fuels, forest fires, and respiration. Carbon sinks include the oceans, the plants, and soil.
A carbon sink is growing in size and storing more carbon compared to a carbon source, which is shrinking in size and releasing more carbon.
Typically, sources and sinks balance one another. For example, the carbon emitted during respiration is offset by photosynthesis.
The burning of fossil fuels is an important carbon source.

Pool: A reserve of the element; it can be organic or inorganic. For example, carbon dioxide in the atmosphere is an inorganic pool of carbon, and the biomass of producers in an ecosystem is an organic pool.
Flux: The transfer of the element from one pool to another.
Three main types of carbon flux due to living organisms in ecosystems:
- Photosynthesis: Absorption of carbon dioxide from air or water and its conversion to carbon compounds.
- Feeding: Gaining carbon compounds from other organisms.
- Respiration: Release to the atmosphere of carbon dioxide produced by respiring cells.

What is a pool?
- A pool is a reservoir of carbon. It can be organic or inorganic. Example: The atmosphere contains $CO_2$ in an organic form.
What is a flux and what do they show?
- A flux is the transfer of an element from one pool to another. Example: $CO_2$ is absorbed by the autotrophs through photosynthesis.
Which ones are the main pools of the Earth?
- Atmosphere, Biosphere, hydrosphere, sediments
Explain why fossil fuels are classified as non-renewable resources when the carbon cycle indicates they are renewed (refer to the pictorial carbon cycle).
- It takes millions of years to form fossil fuels
Diffusion is a flux that moves $CO_2$ from the atmosphere to the hydrosphere and back again. Taken together, these fluxes are largest in the cycle. Suggest why.
- The surface area of oceans, lakes, and the entire atmosphere is very large, so the amount of $CO_2$ that can be moved between these pools is very large.

$Fuel + O<em>2 \rightarrow CO</em>2 + H_2O$
Biomass, peat, coal, oil and natural gas form big carbon sinks.
When these carbon supplies burn as a fuel in complete combustion (with sufficient oxygen) carbon dioxide and water is released.

Oil and gas are formed over a large period of time from dead marine organisms containing calcium carbonate ( $CaCO_3$ ).
When buried under the sediment layer, heat and compression act on transforming these organisms into oil and gas, which are stored away in pockets of rocks or sand.
While oil and natural gas are formed in oceans by $CaCO_3$ containing organisms under the exclusion of oxygen, coal has been formed from plant material.
Peat is the first stage in coal formation and consists of incompletely decomposed organic matter.
Decaying plant material, usually in wet areas, is prevented from decaying fully by acidic and anaerobic conditions.
When sediments accumulate over partially decomposed peat, coal is formed by further transformation through weight and pressure.

Although wild forest fires caused by lightning or spontaneous ignition of coal supplies sometimes can lead naturally to the combustion of large fuel supplies, the majority of carbon dioxide emission from combustion originates from the burning of fossil fuels by humans.

Autotrophs obtain their inorganic nutrients from the abiotic environment (air, soil, rock), including carbon and nitrogen.
Living organisms need a supply of chemical elements:
- Carbon, hydrogen, and oxygen are needed to make carbohydrates, lipids, and other carbon-containing compounds.
- Nitrogen and phosphorus are also needed to make many of these compounds.
- Ca. 15 other elements are needed by living organisms – some in traces only (Mg, Ca, Mn, Cu, Na, K,…), e.g. hemoglobin, chlorophyll, structural materials…
Heterotrophs obtain these elements from the food they eat and also from the abiotic environment (Sodium, Potassium, Calcium…)
Recycling of chemical elements required by living organisms in ecosystems happens in the form of nutrient cycles, such as the carbon or nitrogen cycle.

Decomposers (saprotrophs and detritivores) break down the body parts of dead organisms, or organic matter left over in faeces.
The digestive enzymes of decomposers convert the nutrients that were ‘locked-up’ in complex organic matter into a more usable form for themselves and for other organisms.
Nitrosomonas bacteria living in the soil break down toxic ammonium into nitrites.
The nitrogen cycle is an important nutrient cycle which uses bacteria in the soil to trap nitrogen ( $N_2$ ) from the air or in the form of proteins, enzymes, DNA etc from dead animals & plants.
Other bacteria then convert nitrogen into ammonia ( $NH<em>3$ ) and ammonium ( $NH</em>4^+$ ), and finally into useful nitrates ( $NO_3^{2-}$ ).
Nitrates can be assimilated by plants and incorporated back into proteins or chlorophyll, and essentially eaten by animals which further use the nitrogen.