The rate of biomass production is expressed in terms of ____________…

The correct answer is Option (4) → gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹

A rate of production has to show three things:

• the amount: weight in grams (g) or energy in kilocalories (kcal);

• per unit area: per square metre, m⁻²;

• per unit time: per year, yr⁻¹.

Option (1) kg⁻¹ yr⁻² or (kcal m⁻¹) yr⁻² — Incorrect. Area is m⁻², not m⁻¹, and time appears once (yr⁻¹), not squared.

Option (2) gm⁻² yr⁻² or (kcal m⁻²) yr⁻² — Incorrect. The area unit is right, but yr⁻² is not a rate per year.

Option (3) kg⁻¹ yr⁻¹ or (kcal m⁻¹) yr⁻¹ — Incorrect. Productivity is measured per square metre of area, not per kilogram or per metre.

Option (4) gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ — Correct. Weight or energy, per square metre, per year.

Note: A similar question was also asked in CUET (UG) 2025 on 02-06-2025 (Slot 2), on 20-05-2025 (Slot 1), on 15-05-2025 (Slot 2) and on 15-05-2025 (Slot 1).

NCERT Extract:
Productivity – “A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m^–2) or energy (kcal m^–2). 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.”