Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In a particular climatic condition, the decomposition rate is slower if detritus is rich in lignin and chitin.
Reason (R): Temperature and soil moisture regulate decomposition through their effects on the activities of soil microbes.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
The rate of decomposition is controlled by two separate things: the chemical composition of the detritus and the climatic factors. Under the same climate, detritus rich in nitrogen and sugars breaks down quickly, while detritus rich in lignin and chitin breaks down slowly.
Assertion (A): In a particular climatic condition, the decomposition rate is slower if detritus is rich in lignin and chitin. — True. Lignin and chitin are tough compounds that resist breakdown, so such detritus decays slowly.
Reason (R): Temperature and soil moisture regulate decomposition through their effects on the activities of soil microbes. — True. Warm and moist conditions speed up the microbes and so speed up decomposition.
Why R does not explain A? A keeps the climate the same and talks about what the detritus is made of. R is about the climate itself, that is temperature and moisture. A change in climate cannot explain a difference seen when the climate is held constant. The slow decay in A comes from the chemical nature of lignin and chitin. So R is not the correct explanation of A.
NCERT Extract:
Decomposition – “… The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars. 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.”