Microbial Biomass Carbon (MBC)

AUS-TMI-CON-SMB General Moderate confidence

Benchmark Value

1000 mg/kg
Range: 400 to 1200 mg/kg
Thresholds: Lower: 200, Upper: —
Optimal Range: 400 to 1200
Direction: Higher is desirable ↑
Form: Point

Scoring Curve

This curve shows how a field measurement for this indicator would score across all available benchmark forms in this context. The scoring engine uses 3 benchmarks together — the OptimalRange form drives the primary score, while 2 guard(s) constrain the result.

Contributing Benchmarks

OptimalRange Primary Point Guard Point Guard

Evidence & Context

A benchmark value of 1000 mg⋅kg−1 is proposed. This value is consistent with direct measurements in analogous high-health natural forests and falls within the range calculated from expected high SOC levels.

Metric Definition:

Microbial Biomass Carbon (MBC) is a measure of the carbon contained within the living component of soil organic matter, primarily bacteria and fungi.

Benchmark Definition:

This benchmark represents the Microbial Biomass Carbon level in a high-health, mature, protected subtropical rainforest ecosystem on fine-textured volcanic soils, indicating soil microbial carbon content essential for ecosystem function.

Justification:

It is derived via a proxy-based triangulation due to a lack of direct field data. The 'Moderate' confidence reflects the strength of the methodology, which combines data from analogous forest ecosystems with established soil science principles (MBC/SOC ratios).

Sources (1)

Preview of Derived benchmark based on a triangulated analysis of: 1) "Soil microbial biomass, C, N, and P in Chinese subtropical and temperate forests" (Zhang et al., 2009); 2) "Microbial biomass C and N stocks across land uses and soil types in the Brazilian tropical dry forest region" (Menezes et al., 2023); and 3) Established MBC/SOC ratios from "Interpreting Microbial Biomass Carbon" (soilquality.org.au).
Derived benchmark based on a triangulated analysis of: 1) "Soil microbial biomass, C, N, and P in Chinese subtropical and temperate forests" (Zhang et al., 2009); 2) "Microbial biomass C and N stocks across land uses and soil types in the Brazilian tropical dry forest region" (Menezes et al., 2023); and 3) Established MBC/SOC ratios from "Interpreting Microbial Biomass Carbon" (soilquality.org.au). Journal

Derived benchmark based on a triangulated analysis of: 1) "Soil microbial biomass, C, N, and P in Chinese subtropical and temperate forests" (Zhang et al., 2009); 2) "Microbial biomass C and N stocks across land uses and soil types in the Brazilian tropical dry forest region" (Menezes et al., 2023); and 3) Established MBC/SOC ratios from "Interpreting Microbial Biomass Carbon" (soilquality.org.au).

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Context

  • Region Australia
  • Biome Tropical & Subtropical Maritime Islands
  • Land Use Conservation / Protected Natural Areas
  • Assessment Pristine Reference
  • Vegetation Forest
  • Evidence Type ReferenceCondition

Lifecycle

  • Status Active
  • Version 1
  • Effective From 20 Mar 2026

Notes

Lower Critical Threshold: 200 mg/kg. Optimal functional state range: 400–1200 mg/kg. No upper detrimental threshold — higher values are generally better unless associated with invasive species dominance indicating degradation.