Biodegradable microplastic type and wheat-straw addition shape soil CO₂ emissions: links to dissolved organic matter turnover and microbial carbon allocation

<title>Abstract</title> <p>Biodegradable microplastics (BMPs) are increasingly recognized as important factors affecting soil carbon cycling. However, how BMP-induced changes in respiration relate to dissolved organic matter (DOM) transformation and microbial carbon use under crop-residue addition remains poorly resolved. Here, we combined a 120-d poly(butylene adipate-co-terephthalate) (PBAT) × wheat-straw incubation with a 64-d comparison of BMP type and dose and quantified CO₂ emission, DOM optical and molecular properties, hydrolytic enzymes, functional genes, microbial biomass and ¹³C-DOC allocation. PBAT increased cumulative CO₂ emission by 214% without wheat straw and by 161% with wheat straw. Although the proportional response was smaller with wheat straw, the absolute PBAT-associated increment was larger. PBAT also caused an early DOC increase and a transient humic-like fluorescence response, whereas day-10 DOM contained more assigned formulae but had a higher intensity-weighted nominal oxidation state of carbon and a lower molecular-lability boundary. In addition, PBAT increased the absolute abundance of several C-degradation genes, but most differences weakened after normalization to bacterial 16S rRNA gene abundance, suggesting that bacterial growth accounted for much of the increase. Among BMP treatments, 1% PHA produced the highest CO₂ emission, microbial biomass and 24-h ¹³C-DOC-derived apparent carbon use efficiency. Total ¹³C incorporation into PLFAs did not differ significantly among treatments. Under 1% PHA, however, relative allocation shifted away from Gram-positive bacteria and fungi toward actinomycetes. Collectively, these findings show that increased CO₂ emission was not consistently accompanied by higher DOC concentration, greater gene abundance per bacterial 16S rRNA gene or greater total short-term tracer assimilation. Evaluating carbon flux together with DOM composition and microbial allocation therefore provides a more complete assessment of BMP effects on soil carbon processing under controlled incubation conditions.</p>