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Agronomy QUESTION #2089
Question 1
The soil microbial biomass carbon (MBC) is used as a soil health indicator because:
  • It directly measures total soil organic carbon content
  • It represents the living fraction of soil organic matter — the most active and responsive pool — and is the primary driver of nutrient cycling, decomposition, and soil aggregation; it responds rapidly to management changes making it an early and sensitive indicator of soil health improvement or degradation✔️
  • It measures the total weight of earthworms per unit soil volume
  • It is identical to the labile carbon fraction measured by permanganate oxidation
Correct Answer Explanation
Soil MBC (measured by chloroform fumigation-extraction) represents the microbial pool — bacteria, fungi, actinomycetes. It is 1–5% of total SOC but drives: N, P, S mineralization; aggregate formation through fungal hyphae and bacterial exopolysaccharides; and decomposition. MBC responds to management changes (tillage, organic inputs) within months — making it a sensitive early warning indicator unlike total SOC which changes slowly.