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

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

Primary productivity is the rate at which plants make organic matter by photosynthesis. Gross primary productivity (GPP) is all the organic matter made. Plants use a large part of it in their own respiration. What is left after these losses is the net primary productivity (NPP): GPP – R = NPP. NPP is the biomass available to herbivores and decomposers.

Option (1) NPP is the organic matter synthesised by photosynthesis plus the utilisation in respiration and other losses. — Incorrect. Respiration uses up organic matter, so it must be subtracted, not added.

Option (2) NPP is the organic matter synthesised by photosynthesis minus the utilisation in respiration and other losses. — Correct. NPP is the organic matter made in photosynthesis minus what is used in respiration and other losses.

Option (3) NPP is the rate of increase in body weight of producers plus the loss suffered through respiration and damages. — Incorrect. Adding the respiration losses to the producers’ gain in weight gives back the total amount made, which is GPP, not NPP.

Option (4) NPP is the rate of resynthesis of organic matter by the consumers. — Incorrect. The rate of formation of new organic matter by consumers is secondary productivity, not primary productivity.

NCERT Extract:
Productivity – “… Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP). GPP – R = NPP … Secondary productivity is defined as the rate of formation of new organic matter by consumers.”