Priming effect as determined by adding 14C-glucose to modified controlled composting test

The development of new biodegradable packaging materials, especially biodegradable plastics, has created a need for biodegradability testing.
The European standard for controlled composting test was used in this study for assessing if the addition of a test material results in excess CO2 production in compost. This effect, designated as the priming effect, would give an erroneous result for biodegradation, which is based on CO2 formation from the test material.
Glucose was selected as a test substrate because it is the degradation product of starch and cellulose, which are major compounds of many packaging materials. Both 14C-glucose and non-labelled glucose was applied to nine compost samples of variable stability and agefrom two weeks to 1.5 years. CO2 and 14CO2 evolution were measured during the incubation.
Biodegradation of glucose in unstable composts (age leq6 months) was negative and 14CO2 evolution was poor, although the respective composts without glucose produced relatively high amounts of CO2.
It was concluded that a negative priming effect was observed in unstable composts, in which glucose remained mostly non-degraded and apparently inhibited the mineralization of native organic matter in the compost. In stable composts (age ≥6 months), biodegradation of glucose was high and approximately equal to 14C-glucose mineralization, i.e., the composts showed no priming effect.
Young composts were unsuitable for controlled composting test due to lack of stability. It is important to ensure that the compost inoculum used for the test is sufficiently stable.