Biomass as the mass of living material, measured as dry mass or as the chemical energy stored in dry biomass using calorimetry; gross primary production (GPP) as the chemical energy store in plant biomass; net primary production (NPP) as GPP minus respiratory losses; the calculation and units of GPP, NPP and net production of consumers; the ways in which farming practices increase the efficiency of energy transfer in food production.

What this dot point is asking

AQA wants you to define biomass and explain why it is measured as dry mass or energy, describe calorimetry, define and calculate GPP, NPP and net production of consumers with correct units, and explain how farming raises efficiency.

The answer

Biomass is the mass of living material in an organism or area. It represents the chemical energy stored in that material.

Measuring biomass

• Dry mass. A sample is heated in an oven (around 80 degrees Celsius) to evaporate water without burning the organic material, then reweighed repeatedly until its mass is constant. Because the organism is killed, only a representative sample is used and the result scaled up.
• Energy content by calorimetry. A known dry mass of sample is burned completely in a calorimeter in pure oxygen. The heat released raises the temperature of a known mass of water. From the temperature rise the chemical energy stored per gram is calculated, giving units of energy per unit mass (for example kJ per gram).

Gross and net primary production

Plants fix energy by photosynthesis but also respire, so we distinguish two measures.

• Gross primary production (GPP). The total chemical energy store in plant biomass made by photosynthesis in a given area or volume in a given time.
• Net primary production (NPP). GPP minus the energy lost in plant respiration:

NPP is the energy left as biomass that is available to the next trophic level (the primary consumers). The usual units are kJ per m squared per year (energy per area per time) or kJ per hectare per year.

Net production of consumers

Consumers also gain and lose energy, so for a consumer level:

where N is net production, I is the chemical energy of ingested food, F is the energy lost in faeces and urine, and R is the energy lost in respiration. The units are again kJ per m squared per year.

Farming practices that increase efficiency

Farmers try to channel more of the available energy into the human food chain and less into competitors, pests and wasted respiration.

To raise plant (primary) productivity:

• Use of fertilisers to supply mineral ions (nitrate, phosphate) that limit growth.
• Pesticides and herbicides to reduce energy lost to pests and to weeds competing for light, water and minerals.
• Greenhouse and polytunnel cropping to control light, temperature and carbon dioxide.

To raise animal (consumer) productivity:

• Restricting movement (for example keeping animals indoors), which reduces energy lost in respiration so more is stored as biomass.
• Keeping the temperature warm so less energy is used to maintain body temperature.
• High-energy or selectively formulated feed and selective breeding for fast growth.

These methods raise the efficiency of energy transfer to humans, but raise ethical and environmental questions, including animal welfare and the eutrophication that fertilisers can cause.

Examples in context

Example 1. Intensive poultry farming. Broiler chickens are kept warm, in restricted space, and fed a high-energy formulated diet. Restricting movement and providing warmth cut the energy lost in respiration and temperature regulation, so a larger share of the feed energy is stored as muscle (biomass), raising the efficiency of energy transfer to humans, though at a recognised welfare cost.

Example 2. Fertiliser use on UK wheat. Adding nitrate fertiliser removes nitrogen as the limiting factor for protein and chlorophyll synthesis, raising GPP and therefore NPP. The extra biomass is the harvested grain. The trade-off is that nitrate leaching into rivers can cause eutrophication, linking this topic to nutrient cycling.

Try this

Q1. Define gross primary production and net primary production, and write the equation linking them. [3 marks]

• Cue. GPP: total chemical energy fixed by photosynthesis per area per time. NPP: energy remaining as plant biomass after respiration. NPP = GPP minus R (respiratory losses).

Q2. A crop has a GPP of 30 000 kJ per m squared per year and loses 11 000 kJ in respiration. Calculate the NPP. [1 mark]

• Cue. 30000 minus 11000 = 19 000 kJ per m squared per year.

Q3. Explain how restricting the movement of farmed animals increases the efficiency of energy transfer to humans. [3 marks]

• Cue. Movement requires muscle contraction powered by respiration, which loses energy as heat; restricting movement reduces respiratory losses, so more of the ingested energy is stored as biomass, increasing the proportion of feed energy that reaches humans.