E16 Transport of Mass and Energy in the Ecosystem

Topic

Terrestrial ecosystems continuously exchange carbon, water, and energy with the atmosphere. The net balance of these exchanges at any given moment is the result of flows moving from the ecosystem to the atmosphere and from the atmosphere to the ecosystem.

Regarding carbon, gross primary productivity quantifies the amount of carbon fixed by vegetation through photosynthesis, while ecosystem respiration quantifies the amount of carbon released back into the atmosphere through the respiration of vegetation and soil organisms. The difference between these two processes determines the ecosystem's net carbon exchange. When gross primary productivity exceeds ecosystem respiration, the balance favors carbon sequestration, and the ecosystem acts as a sink; when respiration exceeds gross primary productivity, the balance favors carbon release, and the ecosystem acts as a source.

Regarding water, evapotranspiration quantifies the transfer of water from the terrestrial ecosystem to the atmosphere. It primarily includes transpiration from vegetation and evaporation from the soil and wet vegetation surfaces. Thus, evapotranspiration directly links the ecosystem to the hydrological cycle: water vapor transferred to the atmosphere becomes part of atmospheric water circulation and may later return to the surface as precipitation, either in the same area or in other regions.

These exchanges can be studied at the ecosystem scale using the eddy covariance technique employed at flux towers. A tower continuously measures rapid fluctuations in vertical air velocity alongside CO₂ and water vapor concentrations, enabling the determination of net turbulent fluxes of carbon and water between the surface and the atmosphere. It also allows for the quantification of energy exchange components, such as sensible and latent heat fluxes.

In the case of carbon, it is important to distinguish between what the tower measures directly and what is subsequently derived from those measurements. The tower determines the ecosystem's net CO₂ exchange, yet this exchange is the combined result of two opposing processes: carbon uptake via photosynthesis and carbon release via respiration. Consequently, gross primary productivity and ecosystem respiration are not measured directly and independently by the tower; instead, they are estimated by subsequently separating the observed net exchange using partitioning procedures. The major advantage of eddy covariance is that it integrates exchanges occurring over a large area surrounding the tower, allowing for the observation of the ecosystem's collective behavior without the need to individually measure every plant, soil patch, or specific process contributing to the total flux.

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