Overview
ISO 14240-1:1997 - "Soil quality - Determination of soil microbial biomass - Part 1: Substrate‑induced respiration method" describes a standardized laboratory method to estimate the active aerobic heterotrophic microbial biomass in aerated agricultural and mineral soils. The method uses substrate‑induced respiration (SIR): adding D‑glucose to soil, measuring the short‑term increase in CO2 production, and converting that respiration rate into microbial carbon content. The standard is intended for routine soil quality and soil microbiology testing where the focus is on living microbial biomass.
Key topics and requirements
- Scope: Applies to aerated agricultural and mineral soils; excludes testing of chemical effects on biomass.
- Principle: Add increasing concentrations of glucose until a maximal initial respiration rate is reached; estimate biomass from the maximum initial respiration.
- Temperature and moisture control: Tests are performed at a constant temperature of 22 ± 1 °C and at the same soil water content as when sampled.
- Reagents and materials:
- D‑glucose (finely powdered)
- Fine quartz sand (0.1–0.5 mm) or talc (used to mix with glucose)
- Soil samples with documented physical and chemical properties (particle size, water content, pH, organic matter)
- Apparatus: Mortars for mixing, mechanical stirrer, and a CO2 measurement system (infrared gas analyzer, gas chromatography, or equivalent).
- Procedure highlights:
- Determine the optimal glucose concentration using a range of additions (eg, 500–6 000 mg/kg for arable soils).
- Mix glucose with sand/talc (ratio ~1:5) and add to soil.
- Measure CO2 release at regular intervals (hourly for at least 6 hours); the maximum initial rate often occurs within the first hour.
- For biomass determination, perform at least three replicates at the optimal glucose level.
- Calculation: Microbial carbon (X, mg C/kg) is calculated from respiration rate (R, ml CO2/kg/h) using the equation:
- X = 40 R + 0.37
- (The factor 40 is derived from correlation with fumigation/incubation methods.)
Applications and users
- Practical applications:
- Monitoring soil biological fertility and health
- Assessing potential for organic matter turnover and nutrient cycling
- Comparing management practices (tillage, crop rotations, organic amendments)
- Baseline assessments for land restoration and environmental monitoring
- Typical users:
- Soil scientists, microbiologists and agronomists
- Environmental and agricultural testing laboratories
- Research institutions studying soil processes
- Consultants and agencies performing soil quality assessments
Related standards
- ISO 14240-2 - Soil microbial biomass: fumigation‑extraction method (companion method)
- ISO 10381-6 - Sampling guidelines for soils intended for laboratory studies of aerobic microbial processes
- ISO 10390 - Soil pH determination
- ISO 11277, ISO 11465, ISO 10694 - Methods for particle size, water content, and organic carbon determination
Keywords: ISO 14240-1, soil microbial biomass, substrate-induced respiration, soil quality, soil testing, soil respiration, soil microbiology.