The rhizosphere effect can be quantitatively represented by

Correct option is D

✅ (d) R : S ratio [Root : Soil ratio] • The R:S ratio indicates the microbial population density in the rhizosphere (root zone) relative to non-rhizosphere soil. • It is a widely accepted method to quantify the rhizosphere effect. • A higher R:S ratio implies a greater influence of roots on microbial biomass. • It helps in understanding plant-microbe interactions for soil health assessments.
❌ (a) C : P ratio [Carbon to Phosphorus] • Represents nutrient balance but not specific to rhizosphere activity. • Mostly used in ecological stoichiometry, not rhizosphere studies.
❌ (b) O : C ratio [Oxygen to Carbon] • Used in soil organic matter characterization. • Irrelevant for quantifying microbial activity in rhizosphere.
❌ (c) C : N ratio [Carbon to Nitrogen] • Important for decomposition and mineralization processes. • Does not directly reflect the rhizosphere effect.