What is the correct definition of net primary productivity (NPP)?

NPP is the organic matter synthesised by photosynthesis minus the utilisation in respiration and other losses.

The correct answer is Option (2) -NPP is the organic matter synthesised by photosynthesis minus the utilisation in respiration and other losses.

Primary production refers to the amount of biomass or organic matter produced per unit area over a given time period by plants during photosynthesis. It is commonly measured in terms of weight (grams per square meter, g m^–2) or energy (kilocalories per square meter, kcal m^–2). Productivity, on the other hand, represents the rate of biomass production and is expressed as grams per square meter per year (g m^–2 yr^–1) or kilocalories per square meter per year (kcal m^–2 yr^–1) to facilitate comparison between different ecosystems.

Primary production can be further divided into two components: gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity represents the total rate of organic matter production during photosynthesis in an ecosystem. However, a significant portion of this energy is utilized by plants for their own respiration needs.

Net primary productivity, on the other hand, is the amount of organic matter available for consumption by heterotrophs, such as herbivores and decomposers. It is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP):

GPP - R = NPP.