7.3: Carbon and Nitrogen

The Natural Carbon Cycle

Due to the importance of carbon dioxide (CO₂) as a greenhouse gas, the carbon cycle is a crucial part of the climate system. Since carbon exchanges with the biosphere, life processes need to be considered in climate science. The carbon cycle is part of broader cycles, which include other biologically important chemical elements such as nitrogen and oxygen.

Carbon exchanges relatively rapidly between three large reservoirs: the ocean, the land, and the atmosphere. Of those, the ocean contains the most carbon, about 40 times more than the atmosphere and 10 times more than land. A key feature of the ocean is it takes up more carbon from the atmosphere than it gives up.

Carbon dioxide from the atmosphere dissolves in ocean water, especially the surface where it is in contact with the atmosphere. Some of the carbon dioxide remains as dissolved gas in the water. A lot of it is converted into other things. Photosynthesis by phytoplankton (tiny plants in the ocean) in the sunlit surface waters turns the carbon into organic matter. Many marine organisms use the carbon to make a mineral called calcite as they build shells. Chemical processes use the carbon to create a rock made of calcite called limestone. All these processes use up the carbon dissolved in the water. The carbon tied up in shells or rock can remain there for millions or even hundreds of millions of years. This allows more carbon dioxide to enter the water from the atmosphere. It is important to note that some organisms introduce carbon back into the water as they respire (breathe).

The second biggest of the three rapidly exchanging carbon reservoirs is the land. On land carbon is stored in living vegetation, in soils, and in permafrost (ground that remains frozen for two or more years).

Photosynthesis by land plants removes carbon from the atmosphere and turns it into organic matter. Organic matter cycles through the land food web as animals eat plants ane one another. The carbon eventually gets into the soil as animal and plant materials die and decompose. Carbon gets released back into the atmosphere as animals (including humans) respire (breathe). Land carbon uptake and release does not depend strongly on atmospheric CO₂ concentrations. They depend more on water availability which promotes plant growth and temperature which affects respiration rates.

The atmospheric carbon reservoir is small (40 times smaller than the ocean and 4 times smaller than the land). However, the atmosphere is crucial in linking land and ocean through rapid exchanges as described above.

The Natural Nitrogen Cycle

The nitrogen cycle tracks how nitrogen is converted into various forms as it circulates within Earth's oceans, land, and atmosphere. Since almost 80% of Earth's atmosphere is composed of nitrogen, the atmosphere is the largest source. The problem is it is not always accessible to biological processes which need it for survival. The conversion of nitrogen is carried out through biological and physical processes include fixation, ammonification, nitrification, and denitrification.

Nitrogen Fixation

The conversion of nitrogen gas (N2) into nitrates and nitrites (forms of nitrogen that can be used by plants) through atmospheric, industrial, and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. One way nitrogen is introduced into the soil is through lightning strikes which create nitrogen dioxide which dissolves in water and converts to a useable form of nitrogen by plants.

Most fixation is done by bacteria known as diazotrophs. These bacteria combine nitrogen gas with hydrogen gas to produce ammonia (a gas), which contains nitrogen that can eventually be used by plants as described below. Most biological nitrogen fixation occurs by the activity of bacteria as well. In one example nitrogen-fixing bacteria such as Rhizobium live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutually beneficial relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. Today, about 30% of the total fixed nitrogen is produced industrially using a process which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.

While the process of creating ammonia is important, it must be converted to nitrates or nitrites (describe below) because ammonia gas is toxic to plants.

Ammonification

When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (nitrogen containing ion in solution), using a process called ammonification. The nitrogen will eventually be available for plants to use as described below.

Nitrification

Soil-living bacteria and other bacteria convert ammonia (gas) or ammonium (ion in solution) to nitrate (a form of nitrogen that can be used by plants) in a process called nitrification. First, one set of bacteria converts it to nitrites. Then another set converts the nitrites to nitrates.

Denitrification

Denitrification is the reduction of nitrates back into nitrogen gas, completing the nitrogen cycle. This process is performed by bacterial species under anaerobic (lacking available oxygen) conditions experienced in waterlogged soils. When oxygen is not available during respiration, the nitrate fills its normal role, and in the process, produces nitrogen gas as a byproduct. This nitrogen is no longer available to plants unless it goes through the nitrogen fixation process again.