Productivity and Decomposition: Quick Revision

A brief note on Productivity and Decomposition. Read it before a test. Try each question at the end before you open its answer.

Productivity

• Primary production: the amount of organic matter made by plants in a unit area over a period of time.
• Productivity: the rate of production. The unit is $\text{g/m}^2/\text{yr}$ or $\text{kcal/m}^2/\text{yr}$.
• GPP: the rate of production of organic matter by photosynthesis.
• NPP: $\text{GPP} - R$. It is the biomass available to the herbivores and the decomposers.
• Secondary productivity: the rate of formation of new organic matter by consumers.
• Depends on: the plant species, the environmental factors, the nutrients and the photosynthetic capacity.
• Biosphere: about $170$ billion tons of dry matter a year. The oceans make about $55$ billion tons, with about $70\%$ of the surface.

Decomposition

• Detritus: dead leaves, bark, flowers, dead animals and faecal matter.
• Fragmentation: detritivores (earthworms) break detritus into smaller bits.
• Leaching: water takes soluble nutrients down, and some are precipitated as unavailable salts.
• Catabolism: enzymes of bacteria and fungi give simple inorganic substances.
• Humification: humus builds up. It is dark, amorphous, colloidal and very slow to decay. It is a reservoir of nutrients.
• Mineralisation: microbes break humus down and release inorganic nutrients.
• Rate: slower with lignin and chitin, faster with nitrogen and sugars. Warm, moist and oxygen rich is favourable. Cold and anaerobic is not.

A worked example

A field has a GPP of 2000 g and a respiration of 700 g for each square metre in a year. What is the NPP?

1. NPP is the GPP minus the respiration.
2. $2000 - 700 = 1300$ g for each square metre in a year.

Practice with new numbers

A program to try

The program works out the NPP of a place.

Mistakes to avoid

• Giving the GPP to the herbivores. They get the NPP.
• Adding the respiration. It is subtracted.
• Putting the steps in a strict order. The first three go on together.
• Saying that the oceans make most of the NPP. They make about a third.

Check yourself

1. What is the formula of the net primary productivity?

2. Which step releases inorganic nutrients from humus?

3. Name two things in the detritus that slow decomposition.

4. What does humification make?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy.