Rhizosphere accelerates breakdown of large biodegradable microplastics in soil

Area of Science:

• Environmental Science
• Soil Science
• Polymer Science

Background:

• Biodegradable mulch films (BMFs) are increasingly used in agriculture to reduce plastic pollution.
• The degradation of BMFs, particularly biodegradable microplastics (BMPs), in soil ecosystems is not well understood.
• Plant roots significantly influence soil biogeochemistry, potentially affecting BMP degradation rates and byproduct accumulation.

Purpose of the Study:

• To investigate the impact of the soybean rhizosphere on the degradation of poly(butylene adipate-co-terephthalate) microplastic (PBAT-MP).
• To determine how particle size influences PBAT-MP degradation in bulk soil versus the rhizosphere.
• To characterize the accumulation of degradation byproducts and associated microbial community shifts.

Main Methods:

• Incubation of PBAT-MPs of different sizes (large and small) in soybean rhizospheres and bulk soil for 70 days.
• Quantitative proton nuclear magnetic resonance (qHNMR) spectroscopy to analyze hydrolysis products.
• Microbial community profiling using 16S rRNA sequencing and measurements of microbial biomass and soil carbon.

Main Results:

• The soybean rhizosphere showed size-selective degradation: large PBAT-MPs degraded faster, while small PBAT-MPs were protected within soil aggregates.
• Preferential hydrolysis of aliphatic adipate units led to higher monomer accumulation in the rhizosphere compared to bulk soil.
• Microbial communities in the rhizosphere were enriched with Proteobacteria (e.g., Bradyrhizobium, Ramlibacter), linked to PBAT hydrolysis, alongside increased microbial biomass and altered soil carbon.

Conclusions:

• The degradation of biodegradable mulch films is not uniform and is significantly influenced by the rhizosphere environment.
• Rhizosphere-specific assessments are crucial for evaluating the environmental safety of biodegradable plastics in agriculture.
• Findings necessitate revised strategies for sustainable soil management and plastic pollution mitigation.