The same kind of leaf litter is left on two plots. Plot P has warm, moist soil and plot Q has cold, waterlogged soil. On which plot will organic matter build up, and why?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Plot Q, because low temperature and anaerobiosis inhibit decomposition
Decomposition is carried out by soil microbes, so anything that slows the microbes slows decomposition. Temperature and soil moisture are the most important climatic factors. A warm and moist place favours decomposition, while low temperature and anaerobiosis (lack of oxygen, as in waterlogged soil) inhibit it, and organic matter then builds up.
Option (1) Plot P, because warmth and moisture slow down the activities of soil microbes — Incorrect. Warmth and moisture do the opposite: they speed up the activity of soil microbes, so the litter on plot P is broken down quickly and little organic matter is left.
Option (2) Plot Q, because low temperature and anaerobiosis inhibit decomposition — Correct. Plot Q is cold and waterlogged. Low temperature slows the microbes and waterlogging cuts off oxygen; decomposition is largely an oxygen-requiring process. With decomposition held back, organic matter builds up on plot Q.
Option (3) Plot P, because humification is quicker than mineralisation in warm soil — Incorrect. NCERT does not compare the speed of humification and mineralisation; both go on during decomposition in the soil. Plot P is warm and moist, so decomposition there is fast and organic matter does not build up.
Option (4) Plot Q, because waterlogging speeds up the catabolism of detritus — Incorrect. Waterlogging brings anaerobic conditions, which inhibit decomposition. Catabolism by bacterial and fungal enzymes is slowed, not speeded up.
NCERT Extract:
Decomposition – “… Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. … Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials.”