Step 1: Define C:N Ratio.
The Carbon to Nitrogen (C:N) ratio quantifies the proportion of organic carbon to total nitrogen in soils or organic matter.
It significantly impacts decomposition rates, nutrient availability, and overall soil fertility.
Step 2: C:N Ratio in Soils.
- Microorganisms in arable soils require a balanced C:N ratio for effective decomposition.
- A C:N ratio exceeding 30:1 results in nitrogen immobilization.
- A C:N ratio below 8:1 leads to rapid decomposition and nitrogen release.
- The optimal soil C:N ratio for cultivated land is between 8:1 and 15:1.
Step 3: Evaluate Options.
- (A) 8:1 to 15:1: Correct. This range is typical for the top 15 cm of cultivated land.
- (B) 20:1 to 30:1: Excessive. This ratio is characteristic of undecomposed organic residues.
- (C) 30:1 to 40:1: Very wide. This ratio causes nitrogen immobilization.
- (D) 50:1 to 60:1: Extremely high. This ratio is found only in fresh plant materials such as straw.
Step 4: Conclusion.
The C:N ratio in the top 15 cm of arable lands typically ranges from 8:1 to 15:1.