Plant biomass

Plants take in carbon dioxide from the atmosphere or, for aquatic plants, from the surrounding water. They use it to synthesise sugars and other organic compounds. These compounds contain carbon and form much of the material in plant tissues, known as plant biomass.

Most of the sugars produced are used as respiratory substrates: they are broken down in respiration to release energy for the plant’s activities. The remaining sugars can be used to make other biological molecules, such as cellulose and lipids, which contribute to biomass.

Measuring plant biomass

Plant biomass can be expressed as the mass of carbon in the tissue per unit area, or as the dry mass of tissue per unit area. Expressing mass per unit area allows biomass from areas of different sizes to be compared.

Dry mass is measured after water has been removed from the tissue. This avoids differences in water content affecting the measurement. Dry mass includes the plant’s organic material, as well as any mineral substances remaining in the tissue.

Estimating energy in dry biomass

The organic compounds in dry biomass store chemical energy. This energy can be estimated by calorimetry: a dried sample is burned, and the heat released is used to warm a known amount of water. The rise in the water’s temperature is used to estimate the energy released by the sample.

The estimate may be too low because some heat escapes to the surroundings rather than warming the water.

Primary production

Plants capture energy and incorporate it into chemical substances that make up their biomass. Some of this energy is lost to the environment through plant respiration, so not all of it remains available for growth.

Gross primary production ($$GPP$$) is the total chemical energy incorporated into plant biomass in a given area or volume, before respiratory losses are subtracted. Net primary production ($$NPP$$) is the energy remaining after those losses:

Here, $$R$$ is the energy lost to the environment through respiration. NPP is available for plant growth and reproduction and to other trophic levels, including herbivores that consume plant material and decomposers that break down dead plant material.

Primary productivity

Primary productivity is the rate of primary production: it expresses production per unit time as well as per unit area or volume. For example, energy-based productivity can be measured in $$\mathrm{kJ,ha^{-1},year^{-1}}$$: kilojoules of energy produced per hectare per year.

Consumer net production

A consumer obtains chemical energy by eating food. Not all of this energy becomes part of the consumer’s biomass: some is lost in faeces and urine, and some is lost to the environment through respiration.

The chemical energy remaining after these losses is called net production. It is available for the consumer to produce new biomass.

In this equation:

• $$N$$ is net production.
• $$I$$ is the chemical energy in ingested food.
• $$F$$ is the chemical energy lost in faeces and urine.
• $$R$$ is the energy lost through respiration.

Subtracting both losses from the energy in ingested food gives the energy available for new biomass.

Secondary productivity

Secondary productivity is the rate of production of consumer biomass, expressed per unit time and per unit area or volume. Energy-based secondary productivity can be reported in units such as $$\mathrm{kJ,ha^{-1},year^{-1}}$$: kilojoules per hectare per year.

Farming and energy transfer efficiency

Food contains energy stored in biomass, such as plant and animal tissue. Farming can increase the amount of this production available to humans by directing more of it into the human food chain and reducing energy losses within that chain.

Simplifying the food web

In a food web, the same crop may be eaten by humans, farm animals or organisms that humans do not use for food. Production eaten by those other organisms is diverted into food chains that do not lead to humans.

Farmers can simplify the food web by reducing these competing food chains. For example, controlling crop pests leaves more of the crop available for human food or animal feed. This reduces the diversion of production away from the human food chain; it does not necessarily reduce respiratory losses within that chain.

Reducing respiratory losses

Organisms in the human food chain use some of the energy in their food for respiration. Respiration releases energy for life processes, and much of that energy is eventually transferred to the environment as heat. It is therefore no longer available to be stored in new biomass.

Reducing the energy used in respiration leaves a greater proportion available for production. For example, if farm animals expend less energy moving, less food energy is used for that activity and more can be stored as biomass available to humans.

The two approaches act on different losses: simplifying the food web reduces diversion into other food chains, while reducing respiratory losses increases the proportion available for production within the human food chain.