Clarify plant respiration bi-products and net carbon effects. [From Alan Journet]

When plants photosynthesize, they consume water and carbon dioxide from their environment.  In the presence of sunlight, they extract the H from water (H₂O) and insert it into CO₂ to produce carbohydrates (C₆H₁₂O₆)n where n refers to multiples of the product.  The carbohydrate produced by this is termed Gross Photosynthetic production (Gross Productivity). Oxygen is released as a bi-product of this process.  In fact, it was the initial development of photosynthesis that resulted in the build-up of oxygen in our atmosphere –  for which respiring animals are eternally grateful (or should be).

Plants (and animals) respire, by which the energy in the carbohydrate (food) is essentially burned and the energy released to allow growth, movement, neuronal action etc.  During this process oxygen is consumed (just as in a fire in the fireplace) and the product is the water and carbon dioxide. Gross productivity minus Respiration is Net Primary Productivity and results in the biomass that is accumulated (i.e. the plant growth).

The standard way to assess the relative rates of photosynthesis and respiration is to take advantage of the fact that plants both photosynthesize and respire in the light, but only respire in the dark. Thus, it is possible to measure the oxygen consumption during the dark to assess respiratory rate. Since both processes are temperature dependent, we can then measure oxygen production in the light at the same temperature and (recognizing that the apparent production of oxygen in the light is a function of its production by photosynthesis plus its consumption by respiration in the dark at the same temperature).  In summary, we add the respiratory (dark) oxygen consumption to the apparent (light) production to assess the Gross Primary Production rate.

Since photosynthesis rate is dependent on the intensity (brightness) of light, this will vary during the day, peaking around noon to early afternoon when light intensity and temperature are optimal.

Overall, during the northern summer (where most terrestrial plants ae found), the rate of photosynthetic consumption of carbon dioxide far exceeds the rate of respiratory production, so atmospheric carbon dioxide concentration globally cycles down. Then, during winter, respiration exceeds photosynthesis so the atmospheric concentration of carbon dioxide builds up.

Since different plant species have different photosynthetic responses to increasing light intensity, it is difficult to generalize about the relative rates of these processes in a given community, much less globally.  Spec ices that are found naturally on the forest floor, are adapted to low light conditions (most of our house plants fall into this category).  Such plants generally are more effective photosynthesizers under low light conditions than species that generally are found in the open.   This is the basic reason why some plants we buy for our gardens are listed as needing shaded conditions while others require exposed conditions.  The problem that species that are effective under high light conditions have in the shade is that their photosynthetic rate is never as a high as the respiration rate, and thus they never build up biomass (i.e. never grow).

An estimate of global gross primary productivity is 5.83 * 10⁶ calories per square meter per year.  Of this, the net primary productivity is 4.95 x 10⁶ calories per square meter per year.  This means respiration accounts for the difference, so must be 0.86 calories per square meter per year.  Thus, globally, plants are 85% efficient at turning gross primary productivity into biomass.  (https://globalchange.umich.edu/globalchange1/current/lectures/kling/energyflow/energyflow.html).