Microplastic effects on soil organic matter dynamics and bacterial communities under contrasting soil environments

The widespread use of plastic in agriculture and forestry includes several applications such as mulch, fertilizer bags, pesticide containers, seed germination tubes, greenhouse films, silage films, fruit protection bags, and irrigation pipes. Numerous studies have highlighted a substantial accumulation of plastic residues in soil, potentially leading to adverse effects on soil quality and agricultural productivity. A proposed solution for plastics that cannot be collected is to replace them with biodegradable plastics. However, the impact of biodegradable microplastics (MP) on the soil, especially in tropical regains, remains understudied. Climate and soil geochemical properties have an impact on microbial communities and their nutrient acquisition strategy. The minerals typical of highly weathered soils in tropical climates (kaolinite, gibbsite, goethite, and hematite) have functional groups that are reactive with soil organic matter (SOM) under acidic pH conditions, which enables the sorption and stabilization of SOM, making it physically inaccessible to microorganisms. Since MPs affect soil physicochemical characteristics, they indirectly affect SOM stability impacting soil carbon stock and fertility. In this work, we conducted an incubation experiment to evaluate MP biodegradation (pure polylactic acid - PLA and commercial polybutylene adipate terephthalate - PBAT, 1-2mm, 1 mgMP.g^-1dry soil), and their effects on SOM dynamics in contrasting soils typical from tropical climates (Ferralsol from Brazil) and temperate climates (Chernozem from Austria). The experiment was conducted under 60% WHC at distinct temperatures (22ºC and 27ºC) with four replicates. The control treatment involved MP-free soil, and empty jars as blanks. The CO₂ concentration and isotopic signature were measured by cavity ring-down spectroscopy. The geochemical soil properties were evaluated by EA-IRMS (C, N, δ¹³C and δ¹⁵N) and XRF (Ca, Na, K, Mg, P, Si, Al, Fe, and Mn) and the chemical index of alteration (CIA) was calculated as a proxy for its potential effects on microbial properties. The analysis of phospholipid fatty acids (PLFA) allowed the distinction of microbial groups and correlations between soil properties and microbial activity. CIA value was 39.7 for Chernozem and 96.6 for Ferralsol, which reflects the high degree of weathering of Ferralsol. C/N ratio was 14.0 for Ferralsol and 13.3 for Chernozem. The preliminary results (42 days) show that PLA inhibited microbial activity in both soils. CO₂ emissions in PBAT treatments was more than 200% higher in Chernozems than in Ferralsols. The higher biodegradation rate in Chernozem may be associated with the greater availability of nutrients, as they are eutrophic soils, while Ferralsols are dystrophic. Microbial communities adapt their nutrient acquisition strategies to changes in the soil geochemical properties and may shift, with increasing CIA, from the predominant demand for carbon to phosphorus. In high CIA soils, changes in land use might favor fungi, which tend to adopt a conservative nutrient allocation strategy in acidic and nutrient-poor conditions, minimizing C loss through respiration. Conversely, low CIA soils predominantly harbor bacterial communities that prioritize C acquisition over biomass, leading to increased CO₂ emissions via respiration. These dynamics must be evaluated with PLFA results and isotopic data to assess microbial changes and impacts on SOM.