Priming effects induced by degradable microplastics in agricultural soils

Microplastics (MPs) in farmland soils have the potential to cause environmental and human health risks due to their ubiquitous and persistent nature. Degradable MPs may significantly alter the decomposition of soil organic carbon by the so-called priming effect (PE). However, the amplitude and direction of the PE induced by degradable MPs remain uncertain in farmland soils. In this study, a 56-d laboratory incubation experiment was conducted by adding three types of degradable MPs, i.e., poly-hydroxyalkanoates (PHA), polybutylene succinate (PBS), and polylactic acid (PLA) to three farmland soils planted with strawberry, blueberry, and paddy-rape rotation, respectively. The differences in isotopic values (δ 13C) between soils (−25.30−-27.46‰) and MPs (−11.94−-9.99‰) were used to assess the PE and net carbon balance by the natural 13C isotopic tracing method. We found that the stronger the degradability of MPs, the greater the PE induced. Subsequently, PHA-induced PE was the highest, followed by PBS- and PLA-induced PE corresponding to their biodegradability. Moreover, PHA-induced PE reached an impressive 552−1744%, while PBS- and PLA-induced PE achieved 44−179%, and −29−43% across the above three soils, respectively. Overall, PHA resulted in the highest cumulative PE and the lowest net carbon balance relative to PBS and PLA in all three soils. This study, for the first time, demonstrates that the amplitude of PE is dependent on the degradability of MPs, emphasizing that microbial nitrogen mining may play an important role in driving the PE in the long term due to the nitrogen deficiency of MPs.