Agricultural film microbial degradation testing is a laboratory-based evaluation that quantifies how mulch films, greenhouse films, and related biodegradable plastics are broken down by microorganisms into carbon dioxide, water, and biomass under controlled conditions. As residual plastic film pollution in farmland becomes a growing global concern, manufacturers, regulators, and researchers increasingly rely on biodegradability testing to verify material performance and compliance with international standards such as ISO 17556, ISO 14855, ASTM D5988, and GB/T 35795.
Technical Overview
Agricultural films—including mulch films, greenhouse covers, silage films, and drip irrigation tubing—play a vital role in modern farming by conserving moisture, suppressing weeds, and raising soil temperature. However, conventional polyethylene (PE) films fragment into persistent microplastics after use, contributing to serious "white pollution" in cultivated soil. This has driven the development and adoption of biodegradable films based on PBAT, PLA, PHA, PBS, and starch blends, which are designed to be consumed by soil and compost microorganisms.
Microbial degradation testing, also referred to as biodegradability testing, measures the extent to which microorganisms metabolize the organic carbon in a film material and convert it into carbon dioxide, water, and new microbial biomass. The most widely accepted approach is respirometry, in which the evolved CO2 (or consumed O2) is continuously monitored and compared against the theoretical carbon content of the test material. The result is expressed as a percentage of biodegradation relative to a reference material such as cellulose.
This type of testing is essential for distinguishing genuinely biodegradable films from oxo-degradable products that merely fragment into plastic debris without mineralization. Key standards governing agricultural film microbial degradation testing include ISO 17556, ISO 14855-1/-2, ISO 14852, ASTM D5988, ASTM D5338, ASTM D5988, EN 13432, and the Chinese national standard GB/T 35795 for biodegradable mulching film. Testing is typically performed by accredited third-party laboratories using validated inocula, controlled temperature and humidity, and strict validity criteria.
Testing Samples
Biodegradable Mulch Films
Films formulated from PBAT, PLA, PBS, PHA, PPC, and starch or cellulose blends intended for soil surface coverage. These are the primary subjects of microbial degradation testing and must demonstrate high mineralization rates within a defined period, typically 180 days or longer depending on the applicable standard.
Conventional Polyethylene and Composite Films
PE mulch films, LLDPE greenhouse films, and multi-layer composite films are tested to document their environmental persistence, evaluate additive effects, or support comparative studies between conventional and biodegradable products.
Raw Resins, Masterbatches, and Additives
Pellet-form base resins, starch fillers, plasticizers, pro-oxidant additives, and color masterbatches can be tested individually or in final formulation to screen biodegradability before film production and to verify that additives do not inhibit microbial activity.
Aged and Field-Recovered Films
Films exposed to natural weathering, UV radiation, or burial in real farmland for defined periods may be submitted to assess how field aging affects subsequent biodegradation kinetics and to support environmental fate studies.
Testing Items
Microbial degradation evaluation of agricultural films typically encompasses the following parameters:
- Ultimate aerobic biodegradation rate: the percentage of film carbon mineralized to CO2, calculated from cumulative CO2 evolution relative to theoretical CO2 (ThCO2), usually assessed over 90 to 180 days or extended up to 24 months for slow soil tests.
- Relative biodegradation: test material performance normalized against a cellulose positive control, which must typically exceed 70% degradation for the test to be valid.
- Disintegration degree: the mass loss and fragmentation behavior of film specimens in controlled compost or soil, expressed as the percentage of material passing through a 2 mm sieve.
- Microbial inhibition and ecotoxicity: verification that the film and its degradation intermediates do not suppress inoculum activity, seed germination, or plant growth, often assessed through barley or cress germination tests and biomass yield comparisons.
- Regulated elements and heavy metals: lead, cadmium, mercury, chromium, arsenic, and other restricted metals must remain below thresholds specified in EN 13432 and GB/T 35795.
- Material characterization: thickness, tensile strength, elongation at break, melt flow index, carbon content, and thermal properties (DSC, TGA) to support biodegradation calculations and quality documentation.
- Inoculum viability: plate counts or respiration checks confirming active microbial populations in the soil or compost inoculum used for the test.
Testing Methods
Aerobic Biodegradation in Soil
Following ISO 17556 or ASTM D5988, film specimens are mixed with a defined natural or synthetic soil inoculated with active microorganisms and incubated at 20–28°C with controlled moisture and oxygen supply. CO2 evolution is measured by infrared analysis, titration, or gas chromatography, and biodegradation percentage is calculated over a test period that may extend to six months or longer. This method best simulates the real fate of mulch films left in fields.
Controlled Composting Conditions
Under ISO 14855-1 or ASTM D5338, specimens are introduced into mature, well-aerated compost at 58°C with high humidity, representing industrial composting environments. The intensive thermophilic microbial activity accelerates degradation, and the method forms the biodegradation basis of compostability schemes such as EN 13432. ISO 14855-2 offers a smaller-scale variant using closed vessels with CO2 trapping.
Aqueous Medium Respirometry
ISO 14852 and OECD 301 series methods assess biodegradation in an aerated mineral medium inoculated with activated sludge or soil suspension. These liquid-phase tests are useful for screening resin formulations and comparing the inherent biodegradability of polymers under optimized microbial access conditions.
Disintegration and Ecotoxicity Assessment
Pilot-scale or laboratory composting trials (ISO 16929, ISO 20200) evaluate physical breakdown, while plant germination and growth tests on the resulting compost confirm the absence of phytotoxic degradation products. Weight-loss soil burial studies may supplement respirometric data, though mass loss alone cannot distinguish mineralization from mere fragmentation.
Applications
Microbial degradation testing serves multiple purposes across the agricultural plastics value chain:
- Product certification: demonstrating compliance with compostability and biodegradability schemes required for environmental labels and market access.
- Regulatory compliance: meeting national agricultural film standards and soil protection policies, including the biodegradation requirements in GB/T 35795 and comparable international regulations.
- Research and development: screening new polymer formulations, optimizing starch or PBAT blend ratios, and evaluating the effect of additives on degradation kinetics.
- Manufacturing quality control: verifying batch-to-batch consistency of biodegradable resin feedstock and finished film products before shipment.
- Environmental impact assessment: generating mineralization data for life cycle assessments, carbon accounting, and farmland residue pollution studies.
- Procurement verification: enabling government agricultural programs, distributors, and large farms to confirm supplier claims regarding biodegradable film performance.
FAQ
What biodegradation rate is required for a film to be considered biodegradable? Most standards, including EN 13432 and GB/T 35795, require at least 90% biodegradation within 180 days, either in absolute terms or relative to the cellulose reference material, together with heavy metal limits, disintegration, and ecotoxicity requirements.
How long does a microbial degradation test take? Controlled composting tests typically run 45 to 180 days, while soil burial tests under ISO 17556 or ASTM D5988 may extend from six months to two years, depending on the degradation kinetics of the material and the requirements of the applicable standard.
Can conventional polyethylene mulch pass microbial degradation testing? No. PE is a carbon backbone polymer that microorganisms cannot efficiently metabolize; it achieves negligible mineralization in these tests. Oxo-degradable PE only fragments into small particles and does not meet biodegradation criteria.
What sample quantity and form are needed for testing? Laboratories generally require several hundred grams of film or resin in powder, granule, or cut-film form with a known carbon content, along with material safety and formulation information so that theoretical CO2 production can be calculated accurately.
How is the test validated? Each test includes a cellulose positive control (which must exceed 70% degradation), an inoculum-only blank, and sometimes an inert reference. Replicate vessels, controlled temperature and moisture, and continuous aeration ensure the reliability of the biodegradation curves reported in the final test report.