Mineralization and Transfer of Polymer-Derived Carbon from Biodegradable Mulch into the Soil Microbial Biomass and Organic Matter Pool

The use of biodegradable mulch (BDM) instead of a conventional plastic mulch film has the potential to reduce the accumulation of legacy plastic in agroecosystems. The fate of BDM polymer carbon (C) in soil, however, remains poorly understood, especially the fraction of polymer-C that enters microbial catabolic (mineralization) versus anabolic (immobilization) pathways. We present a novel approach that allows tracking of polymer-C into CO₂, macro- and microplastic residues, living microbial biomass, and soil organic matter (SOM) through the combination of CO₂ emission, ¹³C- and ¹⁴C-phospholipid fatty acid (PLFA) analysis, and plastic polymer analysis. After exposing a clear BDM piece (2 cm × 2 cm) in an agricultural soil for up to 1 year, we found that 22 ± 9% (mean and standard deviation) of the polymer-C remained as macroplastic residues (>1 mm), 19 ± 3% was present in microplastic particles (<1 mm), 22 ± 1% was emitted as CO₂, 0.9 ± 0.1% was present in living microbial biomass, and 37 ± 9% was present in microbial necromass or SOM. Similar values were observed for black BDM (21 ± 3%, 10 ± 2%, 21 ± 4%, 0.8 ± 0.0%, and 47 ± 6%, respectively). Our findings indicate that, within 1 year of soil incubation, a fraction of the macroscopic BDM pieces fragmented into microplastics, while a fraction of polymer-C was mineralized and emitted as CO₂, and another substantial fraction transferred into SOM. Our research advances knowledge on reducing reliance on polyethylene-based plastics and offers practical implications for improving agroecosystem sustainability.