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Productivity and decomposition are the two ends of the Ecosystem chapter: how organic matter is made and how it is broken down again. They look like definitions to memorise, but NEET questions often add a number or a twist, such as which step produces humus or why the oceans are less productive than land. This post covers both topics with worked calculations, the mistakes that cost marks and a short practice routine.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus from the National Medical Commission lists "Ecosystem: Patterns, components; productivity and decomposition; Energy flow; Pyramids of number, biomass, energy". The CBSE Biology curriculum for 2026-27 uses the same wording for Chapter 12, Ecosystem. Everything below follows sections 12.2 and 12.3 of the NCERT Class 12 Ecosystem chapter. Energy flow and pyramids are covered in our post on ecology for NEET.
What is the difference between GPP, NPP and secondary productivity?
NCERT defines primary production as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. Productivity is the rate of that production, in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis.
Plants use a large part of GPP in their own respiration (R).
Net primary productivity (NPP) is what remains: GPP − R = NPP. It is the biomass available to heterotrophs, meaning herbivores and decomposers.
Secondary productivity is the rate of formation of new organic matter by consumers.
Worked example 1: from GPP to NPP
These are sample numbers. A grassland has GPP = 1,800 g m⁻² yr⁻¹, and the plants respire 700 g m⁻² yr⁻¹.
NPP = GPP − R = 1,800 − 700 = 1,100 g m⁻² yr⁻¹
The fraction used in respiration is 700/1,800 = 0.39, or 39 per cent.
Worked example 2: working backwards
Again with sample numbers: a crop field has NPP = 900 g m⁻² yr⁻¹, and respiration uses 40 per cent of GPP. Then NPP is 60 per cent of GPP, so:
GPP = 900/0.60 = 1,500 g m⁻² yr⁻¹, and R = 1,500 − 900 = 600 g m⁻² yr⁻¹.
Check: 600 is 40 per cent of 1,500.
Why are the oceans less productive than land?
NCERT gives the annual NPP of the whole biosphere as about 170 billion tons of organic matter (dry weight). The oceans cover about 70 per cent of the surface but produce only 55 billion tons, which leaves 115 billion tons on land.
Per unit of area, land makes about 115/30 ≈ 3.8 billion tons for each 1 per cent of Earth's surface it covers, while the oceans make 55/70 ≈ 0.79 billion tons. Land is roughly five times as productive per unit area. NCERT asks you to discuss the reason with your teacher. A widely cited satellite-based study, Field and colleagues (1998), found ocean NPP limited mainly by light, nutrients and temperature. Note that the same study estimated global NPP at 104.9 petagrams of carbon a year, with roughly equal shares from land and ocean. It measured carbon, not dry weight, and used different methods. For NEET, answer with NCERT's figures.
What are the steps of decomposition?
NCERT defines decomposition as the breakdown of complex organic matter into inorganic substances such as carbon dioxide, water and nutrients. The raw material is detritus: dead leaves, bark, flowers, dead animals and faecal matter. The steps are:
Fragmentation. Detritivores such as earthworms break detritus into smaller particles.
Leaching. Water-soluble inorganic nutrients move down into the soil and precipitate as unavailable salts.
Catabolism. Bacterial and fungal enzymes break detritus into simpler inorganic substances.
Humification. This produces humus, a dark, amorphous substance that resists microbes and decomposes extremely slowly. Being colloidal, it acts as a reservoir of nutrients.
Mineralisation. Some microbes degrade humus further and release inorganic nutrients.
NCERT stresses that the first three steps operate on detritus at the same time, while humification and mineralisation happen in the soil.
What controls how fast decomposition happens?
According to NCERT, decomposition is largely an oxygen-requiring process. Its rate depends on the chemistry of the detritus and on climate. Litter rich in lignin and chitin decomposes slowly, and litter rich in nitrogen and water-soluble sugars decomposes quickly. Temperature and soil moisture are the most important climatic factors, because they control soil microbes. Warm, moist conditions speed decomposition. Low temperature and anaerobic conditions slow it, so organic matter builds up.
Research data supports this. Aerts (1997) analysed first-year leaf litter decomposition at 44 sites, from cool temperate to humid tropical. At a global scale, climate was the strongest predictor of decomposition rate. Mean actual evapotranspiration in the humid tropics was three times that of temperate and Mediterranean regions, and mean decomposition rates were more than six times higher. Within a single climate region, litter chemistry, especially the lignin-to-nitrogen ratio, was the better predictor.
Common mistakes and how to avoid them
Saying leaching makes nutrients available. NCERT says the leached nutrients precipitate as unavailable salts.
Mixing up humification and mineralisation. Humification makes humus. Mineralisation releases nutrients from it.
Calling NPP the energy for herbivores alone. NCERT says it is available to herbivores and decomposers.
Calling secondary productivity a property of plants. It belongs to consumers.
Forgetting the conditions. Low temperature and lack of oxygen slow decomposition, which is why organic matter builds up in cold or waterlogged soils.
A practice routine
Daily, ten minutes: write the five decomposition steps in order, with one line on what each does.
Twice a week: make up two GPP, R and NPP problems with your own numbers and solve them both forwards and backwards.
Weekly: read sections 12.2 and 12.3 of NCERT again and underline every comparison, such as faster or slower and available or unavailable. NEET statement questions are built from these lines.
If the link between photosynthesis and productivity is unclear, Fermi's Concept Library covers every concept in biology, each with its own lesson and its place in the map. For the next chapter, see our post on biodiversity and conservation.
Today, close the book and write the five decomposition steps from memory, then check them against NCERT.
