Microbial Biomass Carbon (MBC)

AUS-TSR-LVG-SMB General Moderate confidence

Benchmark Value

1200 mg/kg
Range: 750 to 2700 mg/kg
Thresholds: Lower: 200, Upper: —
Optimal Range: 750 to 2700
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 11 benchmarks together — the OptimalRange form drives the primary score, while 10 guard(s) constrain the result.

Contributing Benchmarks

OptimalRange Primary OptimalRange Guard MinimumOnly Guard Point Guard OptimalRange Guard MinimumOnly Guard Point Guard Point Guard MinimumOnly Guard OptimalRange Guard MinimumOnly Guard

Evidence & Context

This process yields a benchmark of 1200 mg/kg for MBC in the topsoil (0–10 cm).

Metric Definition:

Microbial Biomass Carbon (MBC) represents the total mass of carbon held within the living component of soil organic matter, excluding plant roots and fauna larger than approximately 5×10^3μm3.

Benchmark Definition:

Microbial Biomass Carbon (MBC) is the total mass of carbon in the living component of soil organic matter, excluding roots and larger fauna, indicating soil health in tropical and subtropical rainforest pastures.

Justification:

The benchmark is derived by applying a scientifically justified Microbial Biomass Carbon to Soil Organic Carbon ratio (Cmic​:Corg​) of 3.5% to a representative high-health Soil Organic Carbon (SOC) value of 3.5% for the top 10 cm of soil, supported by extensive literature.

Sources (1)

Preview of Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter - ResearchGate
Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter - ResearchGate Journal

Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter - ResearchGate

View Source

Supporting Sources (6)

Additional references from the underlying research that informed this benchmark.

Preview of Allen, D. E., Pringle, M. J., Bray, S. G., Hall, T. J., & Dalal, R. C. (2013). What determines soil organic carbon stocks in the grazing lands of north-eastern Australia? Soil Research, 51(8), 695-706.
Allen, D. E., Pringle, M. J., Bray, S. G., Hall, T. J., & Dalal, R. C. (2013). What determines soil organic carbon stocks in the grazing lands of north-eastern Australia? Soil Research, 51(8), 695-706.
Contextual Support Journal

Grazing management for soil carbon in Australia: A review

View Source
Preview of Ingram, J. S. I., & Fernandes, E. C. M. (2001). Managing carbon sequestration in soils: Concepts and terminology. Agriculture, Ecosystems & Environment, 87(1), 111-117.
Ingram, J. S. I., & Fernandes, E. C. M. (2001). Managing carbon sequestration in soils: Concepts and terminology. Agriculture, Ecosystems & Environment, 87(1), 111-117.
Direct Evidence Journal

Ingram, J. S. I., & Fernandes, E. C. M. (2001). Managing carbon sequestration in soils: Concepts and terminology. Agriculture, Ecosystems & Environment, 87(1), 111-117.

View Source
Preview of Microbial Biomass Carbon - NSW | Fact Sheets | soilquality.org.au, accessed August 5, 2025,
Microbial Biomass Carbon - NSW | Fact Sheets | soilquality.org.au, accessed August 5, 2025,
Contextual Support GreyLiterature

Microbial Biomass | Fact Sheets | soilquality.org.au, accessed July 18, 2025,

View Source
Preview of Soil organic matter and carbon sequestration in pastures - FutureBeef, accessed August 5, 2025,
Soil organic matter and carbon sequestration in pastures - FutureBeef, accessed August 5, 2025,
Contextual Support GreyLiterature

A farmer's guide to increasing Soil Organic Carbon under pastures - NSW Department of Primary Industries

View Source
Preview of Srivastava, S. C., & Singh, J. S. (1989). Effect of cultivation on microbial biomass C and N of a dry tropical forest soil. Biology and Fertility of Soils, 8(4), 343-348.
Srivastava, S. C., & Singh, J. S. (1989). Effect of cultivation on microbial biomass C and N of a dry tropical forest soil. Biology and Fertility of Soils, 8(4), 343-348.
Contextual Support Journal

Top 6 Sustainable Forest Management Techniques, accessed July 19, 2025,

View Source
Preview of Xu, X., Schimel, J. P., Thornton, P. E., Song, X., Yuan, F., & Goswami, S. (2017). Substrate and environmental controls on microbial assimilation of soil organic carbon. Soil Biology and Biochemistry, 112, 8-16.
Xu, X., Schimel, J. P., Thornton, P. E., Song, X., Yuan, F., & Goswami, S. (2017). Substrate and environmental controls on microbial assimilation of soil organic carbon. Soil Biology and Biochemistry, 112, 8-16.
Methodology Source Journal

Dalal, R. C. (1998). Soil microbial biomass—what do the numbers really mean? Australian Journal of Experimental Agriculture, 38(7), 649-655.

View Source

Context

  • Region Australia
  • Biome Tropical & Subtropical Rainforests
  • Land Use Livestock Grazing & Pasture
  • Assessment Pristine Reference
  • Evidence Type ReferenceCondition

Lifecycle

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

Notes

Lower Critical Threshold is 200 mg/kg below which soil ecological function is severely compromised. Optimal Range is 750 to 2700 mg/kg reflecting healthy SOC levels and microbial ratios. No detrimental upper threshold exists; higher values indicate better soil health.