Contrasting responses of loamy and sandy soils to biodegradable plastic residues: microbial biomass, aggregation, and aggregate-associated carbon and nitrogen

Soil-biodegradable plastic (BDP) mulches are increasingly used as alternatives to conventional plastic films because they can be incorporated into soil after use. However, their degradation introduces an exogenous carbon source that may affect microbial activity, soil aggregation, and carbon (C) and nitrogen (N) stabilization. This study investigated the effects of increasing BDP doses on microbial biomass carbon (MBC), bacterial 16S rRNA gene-copy abundance, and fungal target-gene abundance, aggregate-size distribution, and aggregate-associated C, N, δ¹³C, and δ¹⁵N in a loamy and a sandy soil. The soils were incubated for one year with BDP fragments at 100, 1,000, and 10,000 mg kg⁻¹ soil, corresponding to 0.01, 0.1, and 1% w/w, respectively. Microbial biomass C and bacterial and fungal target-gene abundance were assessed during incubation, while macroaggregates, microaggregates, and the silt–clay fraction were separated at the end of the experiment. The highest BDP dose produced the clearest responses in both soils, increasing microbial biomass C by 69% in the loamy soil and 25% in the sandy soil relative to their respective controls after one year. This treatment also increased macroaggregate abundance by 32% and 11% in the loamy and sandy soils, respectively, while decreasing the free silt–clay fraction. These changes were accompanied by higher C and N contents in macroaggregates, and also in sandy-soil microaggregates. Lower BDP doses produced weaker and more soil-specific effects, including increased microaggregate formation in the loamy soil. δ¹⁵N progressively increased from macroaggregates to finer fractions in both soils, but was not affected by the presence of BDP. In contrast, δ¹³C enrichment toward finer fractions occurred only in the loamy soil, demonstrating that soil texture strongly regulated C transformation and potential stabilization. Overall, BDP effects were dose-dependent and modulated by soil properties. Although the strongest responses occurred at a concentration exceeding realistic annual inputs, detectable effects at lower doses highlight the need for long-term field studies evaluating repeated BDP incorporation.