Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : In the initial phase of flooding (phase 1), there is decrease in hydraulic conductivity.
Reason (R) : In the initial phase of submergence, the rate of reduction of hydraulic conductivity due to microbial sealing exceeds the rate of increase of hydraulic conductivity caused by the removal of entrapped air.
In light of the above statements, choose the correct answer from the options given below.
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Initial soil flooding causes clay dispersion and rapid anaerobic microbial activity. The resulting microbial bypass and pore sealing are the main reasons why hydraulic conductivity drops initially in flooded rice soils.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
(A) is true but (R) is false.
(A) is false but (R) is true.
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The Correct Option isA
Solution and Explanation
Step 1: Understanding the Concept:
When dry soil is submerged or flooded (as in rice cultivation), its physical, chemical, and biological properties undergo rapid changes. These changes directly affect the soil's hydraulic conductivity (the ease with which water moves through soil pores). Step 2: Detailed Explanation:
Let us evaluate both statements:
Assertion (A): When a dry soil is first flooded, water displaces air from the macro- and micro-pores.
During this initial phase of flooding, there is a rapid, temporary decrease in the soil's hydraulic conductivity. This occurs due to soil structural collapse, aggregate slaking, clay dispersion, and biological activity. Thus, Assertion (A) istrue.
Reason (R): Submergence triggers anaerobic microbial growth, which leads to the production of microbial slime, polysaccharides, and gases.
These biological products seal and clog the soil pores, a process known as "microbial sealing."
Although the gradual dissolving and removal of entrapped air tends to increase hydraulic conductivity over time, in this initial phase, the rate of pore clogging due to microbial sealing is much faster and completely dominates.
This causes the net hydraulic conductivity to decrease. Thus, Reason (R) istrue.
Relationship: The dominance of microbial sealing over air removal (Reason) directly explains the physical mechanism behind the decrease in hydraulic conductivity observed during the initial phase of soil flooding (Assertion).
Therefore, both are true and R is the correct explanation of A. Step 2: Final Answer:
The correct option is (A).
