Microbial Biomass Carbon (MBC)

AUS-TSR-FOR-SMB General High confidence

Benchmark Value

167 mg/kg

Thresholds: Lower: 167, Upper: —

Direction: Higher is desirable ↑

Form: LowerThreshold

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 5 benchmarks together — the OptimalRange form drives the primary score, while 4 guard(s) constrain the result.

Contributing Benchmarks

OptimalRange Primary Point Guard Point Guard LowerThreshold Guard OptimalRange Guard

Evidence & Context

A defensible lower boundary can be inferred by examining the lowest MBC values observed in forest systems that are heavily disturbed but still functioning. The study by Chen et al. (2003) on newly established second-rotation hoop pine plantations provides such a data point. Under certain intensive site preparation treatments, MBC in the 0–10 cm layer dropped to as low as 167 mg/kg. This value represents an 82% decline from the native forest baseline and a 75% decline from the best-practice 1R plantation benchmark. A level this low indicates a severely compromised soil biological system and can therefore be considered a plausible Lower Critical Threshold.

Metric Definition:

Lower critical threshold of Microbial Biomass Carbon (MBC) in the 0–10 cm soil layer below which ecosystem function is compromised

Benchmark Definition:

Lower critical threshold below which soil biological function is severely compromised.

Justification:

Inferred from lowest MBC values observed in heavily disturbed but still functioning forest systems, specifically from intensive site preparation treatments in second-rotation hoop pine plantations.

Sources (1)

Preview of Variation in soil microbial biomass in the dry tropics: impact of land-use change, accessed on June 7, 2025,

Variation in soil microbial biomass in the dry tropics: impact of land-use change, accessed on June 7, 2025, Journal

Soil microbial biomass during the early establishment of hoop pine ...

View Source

Supporting Sources (4)

Additional references from the underlying research that informed this benchmark.

A meta-analysis of soil microbial biomass responses to forest disturbances - PMC, accessed August 28, 2025,

Contextual Support Journal

Selective logging impacts on soil microbial communities and functioning in Bornean tropical forest - Frontiers

View Source

ORGANIC CARBON IN FORESTED SANDY SOILS: PROPERTIES, PROCESSES, AND THE IMPACT OF FOREST MANAGEMENT - Scion

Contextual Support Journal

FOREST CARBON MARKETS - Forestry Australia, accessed July 21, 2025

View Source

ResearchOnline@JCU - James Cook University, accessed on June 7, 2025

Contextual Support Journal

Sugarcane cultivation altered soil nitrogen cycling microbial processes and decreased nitrogen bioavailability in tropical Australia - ResearchOnline@JCU, accessed July 29, 2025

View Source

Preview of Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

Direct Evidence Journal

Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

View Source

Context

Region Australia
Biome Tropical & Subtropical Rainforests
Land Use Production Forestry
Assessment Conservation Target
Evidence Type DegradationThreshold

Lifecycle

Status Active
Version 1
Effective From 20 Mar 2026

Notes

Represents a level associated with significant disturbance and functional impairment. No upper detrimental threshold — higher values are always better up to natural saturation.

Related Benchmarks

Other benchmarks in the AUS-TSR-FOR-SMB family.