The carbon cycle, a fundamental process for life on Earth, describes the movement of carbon among the planet’s major spheres: the atmosphere, the biosphere, the hydrosphere, and the lithosphere. The primary biological mechanisms that govern the flow of carbon are profoundly affected by the presence or absence of sunlight. This diurnal variation in energy input means the carbon cycle’s function shifts dramatically between day and night.
Carbon Exchange Driven by Daylight
The terrestrial carbon cycle during the day is dominated by the process of photosynthesis, which occurs in plants and other autotrophs. This process utilizes solar energy to convert atmospheric carbon dioxide (\(\text{CO}_2\)) and water into glucose, a stored form of chemical energy, and oxygen. Photosynthesis is the primary engine for drawing carbon out of the atmosphere and sequestering it in the biosphere.
To facilitate carbon uptake, plants open tiny pores on their leaves called stomata, allowing \(\text{CO}_2\) to enter the internal leaf structure. The rate of \(\text{CO}_2\) absorption during the day significantly exceeds the rate of \(\text{CO}_2\) release from respiration, resulting in a net carbon gain for the ecosystem. This difference between the carbon fixed by photosynthesis and the carbon lost by plant respiration is known as Net Primary Production (NPP).
Terrestrial ecosystems, such as forests and grasslands, function as a net carbon sink due to solar-driven carbon fixation. This activity maximizes the storage of energy as biomass, which is then made available to the rest of the food web. This period of net carbon uptake is characterized by a negative flux, meaning the flow of \(\text{CO}_2\) is from the atmosphere into the ecosystem.
Carbon Exchange During Darkness
The carbon exchange reverses direction during the night because the energy source for photosynthesis ceases to exist. While the light-dependent reactions of photosynthesis stop entirely without sunlight, the process of cellular respiration continues without interruption. Respiration is the metabolic pathway used by all living organisms—plants, animals, and microbes—to break down stored sugars for energy to fuel growth and maintenance.
In the darkness, the continuous breakdown of organic molecules releases carbon dioxide back into the atmosphere. Plant respiration at night is a significant component of the total carbon lost from an ecosystem, often accounting for a substantial portion of the carbon fixed during the previous day. Since photosynthesis is zero, respiration becomes the sole biological process driving the flux, causing a net release of \(\text{CO}_2\) from the biosphere.
This switch means that the terrestrial ecosystem temporarily becomes a net carbon source, releasing carbon back into the atmosphere. The rate of respiration itself is sensitive to temperature, so a warmer night can lead to a greater loss of stored carbon. Carbon is constantly cycled, even when the ecosystem is not actively growing.
The point where the rate of photosynthetic carbon uptake exactly balances the rate of respiratory carbon release is called the compensation point. During the transition from day to night, the light intensity drops below this point, and net carbon assimilation becomes zero before switching to a net carbon loss. For a plant to achieve net growth and accumulate biomass, the integrated daytime photosynthesis must significantly surpass the total carbon lost through 24 hours of respiration.
Understanding the Net Diurnal Carbon Balance
During daylight hours, the ecosystem is a net sink, actively drawing \(\text{CO}_2\) out of the atmosphere. During the night, however, it becomes a net source, releasing \(\text{CO}_2\) back into the air. This swing is the defining characteristic of the diurnal carbon balance.
Scientists use the term Net Ecosystem Exchange (NEE) to quantify the overall balance of carbon fluxes between the ecosystem and the atmosphere over a 24-hour period. NEE integrates the gross carbon uptake (photosynthesis) and the total ecosystem respiration, which includes respiration from plants, animals, and microbes in the soil. A negative NEE value indicates a net carbon sink, typically seen during the day, while a positive value indicates a net carbon source, characteristic of the night.
The overall health and function of an ecosystem are determined by its total daily NEE. If the amount of carbon sequestered during the day consistently outweighs the carbon released over a full 24-hour cycle, the ecosystem serves as a long-term carbon sink. Measuring this complex diurnal pattern using techniques like eddy covariance flux towers provides the necessary data to model the terrestrial carbon cycle’s response to environmental changes.