Microplastic-induced soil organic carbon transformation: Evaluating the risks of conventional and biodegradable polymers during wet-dry cycles

Organic carbon formation analysis POC was extracted using a sodium hexametaphosphate dispersion method (Koorneef et al., 2023). MBC was measured using the chloroform fumigation-extraction method (Hu et al., 2024). DOC samples extraction methods followed Chen et al. (2024). The organic carbon content was determined using a total organic carbon (TOC) analyzer (TOC-LCPH/CPN, Shimadzu, Japan). The details are provided in Supporting Information S1 The UV-Visible spectrum of the DOC samples was analyzed by a UV–Visible photometer (UV-2600, Shimadzu, Japan). Reportedly, the specific UV-Visible absorbance at 254 nm (SUVA254) can indicate the aromatic content of DOC. The fluorescence characteristics of DOC were analyzed using three-dimensional fluorescence excitation-emissions matrices (EEMs) in conjunction with parallel factor analysis (PARAFAC) modeling (Fan et al., 2023). CO2 emissions analysis The gas samples were collected using the methods of Yan et al.(2024) at designed time. Prior to gas collection, the glass columns were left open for 20 minutes to balance with the ambient air before being sealed with a silicone stopper. The stopper was fitted with two glass tubes: one connected to a three-way valve for gas sampling and exchange, and the other attached to a balloon to balance internal pressure during sampling. The headspace gas was immediately homogenized by repeatedly flushing with a gas-tight syringe. A 20 mL gas sample was then withdrawn as the initial (T0) sample and transferred to a pre-evacuated gas bag. This sampling was repeated after one hour (T1). The CO2 emission rate was calculated from the concentration difference between T1 and T0, as quantified by gas chromatography (GC 9790 plus, China). 2.5 16S rDNA Extraction and High-Throughput Sequencing The collected soil samples were immediately flash-frozen on dry ice and stored at -80°C until DNA extraction. Bacterial community analysis was conducted via high-throughput sequencing of the 16S rRNA gene V4 region using primers 515F (5′-GTGCCAGCMGCCGCGG-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). The resulting PCR products were purified and sequenced on an Illumina MiSeq platform (Illumina, San Diego, USA). Raw sequence data were processed and analyzed using QIIME 1.9.1 and Mothur 1.30.2. Data analysis In this study, Parallel factor analysis (PARAFAC) was performed using the DOM Fluor v.1.7 Toolbox in MATLAB R2024b (Math Works, USA), then run it according to the tutorial of Stedmon and Bro, (2008). Statistical analysis was performed using ANOVA to compare the effects of different microplastic treatments on soil properties and microbial communities. Significant differences were determined at the P ≤ 0.05 level. The “plspm” package in R language to build a partial least squares path model (PLS-PM), and then use Adobe Illustrator for plotting. Data were analyzed using excel and SPSS 27 software, all the curves were drawn via origin 2024, MATLAB 2024b and RStudio.