Mineralisation and priming effects of a biodegradable plastic mulch film in soils: Influence of soil type, temperature and plastic particle size

There is increasing use of biodegradable mulch films (BDMs) as an environmentally sustainable alternative to traditional mulch films in agriculture. However, their degradation fate and priming effect (PE) on native soil organic matter (SOM), including in the form of microplastics (MPs) after disintegration, are not well known. In this study, we investigated mineralisation of a BDM (0–2000 μm MPs) and its PEs in three soils, i.e., a Ferralsol, a Vertisol and a Solonetz. The MPs were incubated at a concentration of 0.8% (w/w) separately in the three soils at three temperatures i.e., 20, 30 and 40 °C for 150 days. We also evaluated the effect of BDM MP particle size on its mineralisation and PEs in two soils (a Ferralsol and a Vertisol). Six size fractions (i.e., <100, 100–250, 250–500, 500–1000, 1000–2000, 2000–4750 μm) were incubated in the soils at 30 °C for 150 days. A δ13C technique was employed to determine the proportion of C mineralised from MPs and SOM owing to their distinct δ13C values. Temperature, soil type, particle size and their interactions were all key factors in the mineralisation of MPs and PEs. The mineralisation of MPs increased with the increasing temperatures from 20 to 40 °C in the three soils over time with few exceptions. At 20 and 40 °C, the Ferralsol with greater carbon and clay content showed greater MP mineralisation than the Vertisol and Solonetz. Opposite trends were observed for the MP mineralisation and PEs in the Vertisol at 20 and 30 °C and in the Ferralsol at 20 °C. However, at 40 °C, both MP mineralisation and PEs exhibited similar trends in the three soils suggesting that a ‘co-metabolism’ may be dominant at high temperatures. Furthermore, we observed that smaller MPs tended to induce greater mineralisation, whereas the larger MPs caused greater PEs than small particles.