Nitrogen and Carbon Cycles: Essential Biogeochemical Processes

The Biogeochemical Cycle of Nitrogen

Nitrogen constitutes 78% of the troposphere. However, in this gaseous form (N2), it cannot be used directly as a nutrient by plants or animals. It must first be converted into usable forms through a process called nitrogen fixation.

Key Steps in Nitrogen Conversion

The conversion of nitrogen involves several specialized bacterial processes:

• Nitrogen Fixation: Some bacteria reduce atmospheric N2 to form ammonia (NH3).
• Nitrosation: Ammonia is transformed by a specialized group of bacteria (such as the genus Nitrosomonas) that oxidize it, forming nitrite (NO2-). This process is called nitrosation.
• Nitrification: Bacteria of the genus Nitrobacter oxidize the nitrite ion, converting it to nitrate (NO3-), which is the main source of nitrogen for higher plants.
• Denitrification: A number of bacteria, called denitrifying bacteria, perform the reverse process, liberating N2 back into the atmosphere.

Atmospheric Nitrogen Processes

Light also plays a role in N2 conversion, combining with atmospheric oxygen to form nitrogen dioxide (NO2). These gases react with water vapor in the atmosphere, transforming into the nitrate ion (NO3-), which returns to Earth as nitric acid dissolved in precipitation.

The Biogeochemical Cycle of Carbon

Carbon is the main constituent of organic matter. It can only be incorporated into this matter by producers (organisms that absorb CO2 in the atmosphere or dissolved in water) using light energy through photosynthesis, fixing inorganic carbon into organic compounds such as glucose.

Photosynthesis and Respiration Balance

These organic compounds are then degraded by organisms carrying out aerobic respiration, releasing CO2 back into the atmosphere or water. Thus, photosynthesis and cellular respiration act as antagonistic processes in their end terms, complementing each other and maintaining the balance of the carbon cycle.

Long-Term Carbon Storage

Some carbon is retained in the Earth's crust over long periods in the form of fossil fuels until it is released into the atmosphere as CO2 when they are burned. Volcanic eruptions also release some of this stored carbon into the atmosphere.

Marine Carbon Sequestration

Some marine organisms use dissolved CO2 in water to form their shells and skeletons. When these organisms die, they fall to the bottom, re-entering the carbon cycle very slowly as the sediments are dissolved or exposed to the elements by geological or biological phenomena.