Net primary productivity can be expressed as:

The correct answer is Option (1) → GPP-R

Gross primary productivity (GPP) is the rate at which an ecosystem makes organic matter in photosynthesis. Plants use a good part of it in respiration (R). What remains is the net primary productivity (NPP), the biomass available to herbivores and decomposers.

\( \displaystyle NPP = GPP - R \)

(1) GPP-R — Correct. This is the formula above.

(2) GPP-NPP — Incorrect. This equals R, the respiratory loss, and not NPP.

(3) R-GPP — Incorrect. Respiration is less than GPP, so this would be negative. Productivity cannot be negative.

(4) GPP+R — Incorrect. Respiration is a loss. Adding it to GPP would give a value larger than GPP.

NCERT Extract:
Productivity – “The rate of biomass production is called productivity. It is expressed in terms of gm–2 yr –1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). 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).”