Impact of moisture on the degradation and priming effects of poly(lactic acid) microplastic

The degradation of biodegradable microplastics (MPs) can either stimulate or inhibit the decomposition of soil organic carbon (SOC), but the factors influencing these phenomena remain unclear. In this study, we used the C-13 natural abundance to differentiate between carbon dioxide (CO2) arising from the mineralization of SOC and poly(lactic acid) (PLA) MP under varying soil water holding capacity (WHC) in alkaline and acidic soils. We also quantified the incorporation of soil- or PLA-derived carbon (C) into phospholipid fatty acid (PLFA)-distinguishable microbial groups. An increase in soil moisture did not significantly affect PLA MP degradation in alkaline soil, but significantly increased PLA degradability in acidic soil. In particular, the percentages of PLA-derived C incorporated into PLFA-distinguishable gram-negative and gram-positive bacteria were 14%-63% and 5%-33%, respectively, in all the treatments. The presence of PLA MP induced positive priming effects from 0 to 20 d in all the treatments but subsequently induced negative priming effects under some conditions. The total priming effects induced by PLA MP were significantly greater in alkaline soil with >= 70% WHC (37-43 mg C kg(-1) soil) than in this soil with 50% WHC (8.6 mg C kg(-1) soil). The total priming effect was 72-78 mg C kg(-1) soil in acidic soil with <= 70% WHC, but a negative priming effect was observed in this soil with 90% WHC (-56 mg C kg(-1) soil). In alkaline soil, the dissolved organic carbon content was positively correlated with the priming effect, but a negative relationship was observed between the priming effect and the amount of soil-derived C incorporated into gram-negative bacteria and fungi. In acidic soil, a positive correlation was found between the priming effect and the soil nitrate content. In summary, our findings indicate that 0.4%-2.8% of PLA MP was degraded in soils after 2 months, and that the intensity and direction of the priming effect induced by PLA MP are regulated by soil moisture and pH, but further exploration is needed to elucidate the microbial mechanisms underlying these effects.