Net primary productivity can be defined as:

The correct answer is Option (1) → Gross primary productivity minus respiration losses

Primary productivity is the rate at which plants make organic matter, measured as biomass per unit area per unit time. Gross primary productivity (GPP) is the total organic matter made in photosynthesis. Plants use a part of it in their own respiration (R). What remains is the net primary productivity (NPP), and it is this biomass that is available to the heterotrophs.

    \( \displaystyle GPP - R = NPP \)

(1) Gross primary productivity minus respiration losses — Correct. Respiration uses up part of the GPP, so it is taken away.

(2) Gross primary productivity plus respiration losses — Incorrect. Respiration is a loss of organic matter, so it cannot be added. The result would be larger than GPP.

(3) Respiration losses minus gross primary productivity — Incorrect. The subtraction is the wrong way round and would give a negative value.

(4) Respiration losses plus gross primary productivity — Incorrect. This is the same wrong addition as in Option (2).

Note: A similar question was also asked in CUET (UG) 2023 on 22-06-2023 (Slot 3).

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).”