Aerobic Biodegradation Analysis of Agricultural Mulch Films

Aerobic biodegradation analysis of agricultural mulch films determines how completely biodegradable plastic films decompose in oxygen-rich soil environments by measuring microbial oxygen consumption and carbon dioxide evolution. Conducted in accordance with international standards such as ISO 17556, ASTM D5988, and GB/T 35795, this testing is essential for manufacturers, researchers, and agricultural suppliers seeking to verify biodegradation performance, obtain certification, and ensure regulatory compliance for biodegradable mulch film products.

Technical Overview

Agricultural mulch films are widely used to suppress weeds, conserve soil moisture, regulate soil temperature, and improve crop yields. Conventional polyethylene (PE) mulch films, however, leave persistent plastic residues that accumulate in soil and degrade farmland productivity. Biodegradable mulch films based on polymers such as PBAT, PLA, PBS, PHA, and starch blends are designed to be plowed directly into the soil after the growing season, where soil microorganisms convert the polymer carbon into carbon dioxide, water, and microbial biomass under aerobic conditions.

Aerobic biodegradation analysis quantifies this conversion process. The test measures the biochemical oxygen demand (BOD) in a closed respirometer or the amount of carbon dioxide evolved from a soil inoculum containing the test film, and compares the result against the theoretical value (ThOD or ThCO2) calculated from the material's carbon content. The resulting biodegradation percentage indicates whether the film meets the acceptance thresholds defined in standards such as ISO 17556, ASTM D5988, EN 17033, and GB/T 35795. Accredited third-party laboratories perform these analyses to support product development, quality control, certification, and market access for biodegradable mulch film products.

Testing Samples

PBAT-Based Biodegradable Mulch Films

Films primarily composed of poly(butylene adipate-co-terephthalate) (PBAT), frequently blended with PLA or PBS, represent the dominant commercial category of biodegradable mulch films. Samples are typically supplied as film strips, granules, or cryogenically milled powder to increase surface area and ensure reproducible microbial attack during the incubation period.

PLA and Starch-Blend Films

Polylactic acid (PLA) films, starch-based films, and PLA/starch or PBS/starch composite films are tested to evaluate how formulation changes, plasticizers, and compatibilizers influence the rate and extent of aerobic biodegradation in soil.

Bio-Based and Reference Materials

Other test materials include PBS, PHA/PHBH films, cellulose-based films, and bio-based polyethylene. Cellulose powder or filter paper serves as the positive biodegradation reference, while inert reference materials such as polyethylene are sometimes included as negative controls to confirm the discriminating power of the test system.

Pre-Weathered and Field-Aged Films

Films recovered after field exposure, artificial weathering (xenon arc or UV aging), or simulated tillage can be submitted to correlate visible disintegration with actual mineralization of the polymer carbon, providing a complete picture of in-soil degradation behavior.

Testing Items

Aerobic degradation analysis of agricultural mulch films typically encompasses the following test items:

• Ultimate aerobic biodegradation rate in soil, expressed as the percentage of theoretical carbon dioxide (ThCO2) evolved or theoretical oxygen demand (ThOD) consumed
• Biochemical oxygen demand kinetics and biodegradation curves plotted over the incubation period
• Lag phase, biodegradation plateau, and calculated biodegradation half-time
• Disintegration degree in soil, determined by mass loss and fragment size distribution (typically requiring fragments smaller than 2 mm × 2 mm)
• Ecotoxicity assessment, including seed germination and plant growth tests and earthworm acute toxicity tests on soil containing degraded film residues
• Regulated heavy metals and toxic elements content, including lead, cadmium, chromium, mercury, arsenic, and others
• Total organic carbon (TOC) and carbon content determination of the test material for calculating theoretical values
• Mechanical property retention (tensile strength and elongation at break) before and after degradation exposure

Testing Methods

ISO 17556 / ASTM D5988 — Aerobic Biodegradation in Soil

ISO 17556 and ASTM D5988 are the primary methods for evaluating mulch film biodegradation in soil. Test material is mixed with an active, mature soil inoculum and incubated under controlled aerobic conditions, typically at 20–28 °C in the dark with controlled moisture content. Biodegradation is followed by continuous or periodic measurement of oxygen consumption in a closed respirometer or carbon dioxide evolution using infrared gas analysis or titration. The test is valid only if the cellulose reference achieves at least 70% biodegradation, and blank values are subtracted to correct for background soil respiration. Incubation commonly extends to 180 days or longer, and EN 17033 and GB/T 35795 specify that a minimum of 90% biodegradation (or 90% absolute, considering the reference) must be reached within a maximum of 24 months.

ISO 14852 / ISO 14851 — Aerobic Biodegradation in Aqueous Medium

ISO 14852 determines biodegradation by measuring evolved CO2 in an aerated aqueous medium inoculated with activated sludge or compost extract, while ISO 14851 measures oxygen demand in a closed respirometer. These methods are faster screening tools that complement soil tests and provide insight into the inherent biodegradability of the polymer.

ISO 14855-1 / ASTM D5338 — Controlled Composting Conditions

These methods evaluate ultimate aerobic biodegradation under optimized composting conditions at approximately 58 °C using mature compost as the inoculum. Although mulch films are destined for soil rather than compost, compost testing is frequently requested for materials that may also enter organic waste streams or for comparative biodegradability screening.

Disintegration and Ecotoxicity Methods

Disintegration in soil is assessed according to ISO 16929 or field-relevant protocols, followed by sieving and weighing of recovered fragments. Ecotoxicity of degradation residues is evaluated using plant germination and growth tests per OECD 208 guidelines and earthworm acute toxicity tests per OECD 207, ensuring that the degraded film does not harm soil ecosystems. Heavy metals content is quantified by ICP-MS or ICP-OES after acid digestion.

Chinese National Standard GB/T 35795

GB/T 35795 specifies the technical requirements for fully biodegradable agricultural mulch films in China, including thickness, mechanical properties, biodegradation performance tested according to ISO 17556 with a minimum 90% biodegradation requirement, heavy metals limits, and disintegration and ecotoxicity provisions. Testing against GB/T 35795 is essential for domestic market access and government agricultural procurement programs.

Applications

Aerobic biodegradation analysis of agricultural mulch films serves a broad range of practical purposes across the agricultural plastics value chain:

• Verification of biodegradation claims for biodegradable mulch film products prior to market launch and labeling
• Compliance demonstration against GB/T 35795, EN 17033, and related regulatory or certification requirements
• Research and development support for optimizing polymer formulations, blend ratios, thickness design, and pro-oxidant or nucleating additives
• Evaluation of raw materials such as PBAT, PLA, PBS, and PHA resins and masterbatches before film extrusion
• Assessment of the influence of pigments (such as carbon black), UV stabilizers, and processing aids on biodegradation kinetics
• Quality control for mulch film manufacturers supplying demonstration projects, government tender programs, and export markets
• Comparison of competing products and verification of supplier declarations by agricultural distributors and large-scale farming operations
• Supporting documentation for scientific publications, patent applications, and life cycle assessments

FAQ

What biodegradation level is required for a mulch film to pass?

Under EN 17033 and GB/T 35795, a biodegradable mulch film must achieve at least 90% ultimate aerobic biodegradation within a maximum of 24 months when tested according to ISO 17556 or an equivalent soil biodegradation method. The reference cellulose must reach at least 70% biodegradation within the test period for the results to be considered valid.

How long does the aerobic biodegradation test take?

Soil biodegradation tests according to ISO 17556 typically run for 180 days in the first instance and may be extended up to 24 months depending on the standard invoked and the biodegradation progress. Aqueous screening tests (ISO 14851/14852) usually conclude within 60 to 90 days, while composting tests (ISO 14855, ASTM D5338) run for approximately 45 to 180 days.

What sample form and quantity should be submitted?

Film samples are generally accepted as strips, granules, or milled powder with known composition. Laboratories typically require 50 to 500 grams of material depending on the selected method, along with formulation information and carbon content data if available, so that theoretical CO2 and oxygen demand values can be calculated accurately.

Can conventional PE mulch films be tested?

Yes. Conventional polyethylene films can be tested as negative controls or to document their lack of biodegradability. PE is expected to show negligible mineralization in the test time frame, which highlights the environmental rationale for certified biodegradable alternatives.

Why is ecotoxicity testing required in addition to biodegradation?

Biodegradation percentage alone does not guarantee environmental safety. Ecotoxicity testing verifies that degradation residues, additives, and degradation intermediates do not adversely affect seed germination, plant growth, earthworms, or other soil organisms, which is a mandatory component of EN 17033 and GB/T 35795 compliance.