Gross Primary Productivity (GPP) involves:

Gross Primary Productivity (GPP) refers to the total amount of carbon dioxide that is captured and converted into organic matter by primary producers, primarily through the process of photosynthesis. This measure indicates the overall uptake of carbon by photosynthetic organisms, such as plants, algae, and some bacteria, in a given area over a specific time period.

In the context of GPP, it encompasses all forms of carbon assimilation by these producers, which plays a crucial role in the ecosystem by driving energy flow and supporting various trophic levels. By focusing on the total carbon uptake, GPP serves as a foundation for understanding energy dynamics in ecosystems, including how much energy is available for growth and reproduction in higher trophic levels after accounting for respiration and other losses.

The other options presented do not capture the complete definition of GPP. For example, nutrient uptake pertains specifically to the assimilation of minerals and does not account for the carbon fixation element central to GPP. Similarly, while the growth of terrestrial plants involves primary productivity, it does not encompass all primary producers, such as those in aquatic systems. Lastly, decomposition rates relate to the breakdown of organic matter and its return to the soil, which is separate from the processes involved in capturing carbon for GPP.