The relationship between gross and net primary production is:NPP = GPP − R where NPP is net primary production, GPP is gross primary production and R is respiratory losses to the environment. All three must be in the same units, usually kJ m−2 yr−1.The equation can be rearranged to find any one value:GPP = NPP + R R = GPP − NPP Worked example 1: A grassland has GPP = 22 000 kJ m−2 yr−1 and R = 13 200 kJ m−2 yr−1.
NPP = 22 000 − 13 200 = 8800 kJ m−2 yr−1.Worked example 2: A crop has NPP = 12 000 kJ m−2 yr−1, which is 40% of its GPP.
GPP = 12 000 / 0.40 = 30 000 kJ m−2 yr−1, so R = 30 000 − 12 000 = 18 000 kJ m−2 yr−1.The fraction NPP / GPP shows how much of the captured energy a plant keeps as biomass. Anything that increases plant respiration, such as warmer nights, raises R and lowers NPP even if GPP is unchanged.Common mistakes: using different units for GPP and R (convert first; 1 ha = 10 000 m2), and forgetting that R here means respiratory losses, not a rate of reaction.
NPP = 22 000 − 13 200 = 8800 kJ m−2 yr−1.Worked example 2: A crop has NPP = 12 000 kJ m−2 yr−1, which is 40% of its GPP.
GPP = 12 000 / 0.40 = 30 000 kJ m−2 yr−1, so R = 30 000 − 12 000 = 18 000 kJ m−2 yr−1.The fraction NPP / GPP shows how much of the captured energy a plant keeps as biomass. Anything that increases plant respiration, such as warmer nights, raises R and lowers NPP even if GPP is unchanged.Common mistakes: using different units for GPP and R (convert first; 1 ha = 10 000 m2), and forgetting that R here means respiratory losses, not a rate of reaction.