Question

Given below are two statements:
Statement (I): Nitrogen is lost from waterlogged soils only by leaching
Statement (II): Waterlogged soils may develop iron toxicity to plants
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

In waterlogged/submerged soils:
Nitrogen is lost mainly via denitrification (gas loss).
Iron is reduced to its soluble ferrous (\(\text{Fe}^{2+}\)) form, often causing toxicity (bronzing) in rice.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Waterlogging refers to the saturated condition of soil where all pores are filled with water, restricting air exchange with the atmosphere.
This results in rapid oxygen depletion (anoxia), forcing soil microorganisms to utilize alternative electron acceptors for respiration.
These anaerobic conditions trigger a sequence of biochemical reduction reactions that profoundly alter nutrient availability, chemical solubility, and plant health.

Step 2: Detailed Explanation:
Let us evaluate both statements individually:
Statement (I) is incorrect:
In waterlogged soils, the highly anaerobic (reducing) environment promotes the biological process of denitrification.
Facultative anaerobic bacteria utilize nitrate (\(\text{NO}_3^-\)) as an alternative electron acceptor, reducing it to gaseous nitrogen oxides (\(\text{N}_2\text{O}\)) and dinitrogen gas (\(\text{N}_2\)), which escape into the atmosphere.
Additionally, under alkaline or warm waterlogged conditions, nitrogen is lost through ammonia volatilization.
While nitrate leaching does occur in some saturated soils, saying that nitrogen is lost *only* by leaching is completely incorrect, as denitrification is often the dominant pathway of nitrogen loss in waterlogged environments.
Statement (II) is correct:
Under prolonged waterlogging, the redox potential of the soil drops significantly.
Once oxygen, nitrate, and manganese are reduced, soil microbes reduce insoluble ferric iron (\(\text{Fe}^{3+}\)) into highly soluble ferrous iron (\(\text{Fe}^{2+}\)):
\[ \text{Fe}^{3+} + e^- \rightarrow \text{Fe}^{2+} \]
The concentration of soluble \(\text{Fe}^{2+}\) in the soil solution can rise to exceptionally high, toxic levels.
Plants (especially lowland rice) growing in these environments absorb excessive amounts of ferrous iron, leading to “iron toxicity” (characterized by leaf bronzing, brown spots, and root damage).
Therefore, Statement (I) is incorrect, but Statement (II) is correct.

Step 3: Final Answer:
Thus, the correct choice is that Statement (I) is incorrect but Statement (II) is correct.