Microplastic pollution in terrestrial ecosystems threatens to destabilize large soil carbon stocks that help to mitigate climate change. Carbon-based substrates can release from microplastics and contribute to terrestrial carbon pools, but how these emerging organic compounds influence carbon mineralization and sequestration remains unknown. Here, microcosm experiments are conducted to determine the bioavailability of microplastic-derived dissolved organic matter (MP-DOM) in soils and its contribution to mineral-associated carbon pool. The underlying mechanisms are identified by estimating its spectroscopic and molecular signatures and comparing its sorption properties on model minerals with natural organic matter (NOM). The results show that MP-DOM leads to 21–576% higher CO2 emissions and 34–83% lower mineral-associated organic carbon in soils than NOM, depending on the type of plastic polymer. DOM from biodegradable microplastics induces higher CO2 emissions than conventional microplastics. It is found that MP-DOM is 7.96 times more labile than NOM, making it more accessible for microbial utilization. The lower degree of humification, fewer polar functional groups, and higher H/C ratios in MP-DOM also led to 3.96 times less sorption with mineral particles. The findings provide insights into the effects of microplastics on soil carbon storage and highlight their consequences for wider terrestrial carbon cycling and climate warming.
