Eva Lehndorff

Microplastics (MP) are relevant contaminants in agroecosystems, influencing soil nutrient dynamics and soil–plant–microbial interactions. As agriculture shifts from conventional to biodegradable plastics, their impacts on different crop rhizosphere microbiomes considering both total (DNA-derived) and transcriptionally active (rRNA-derived) communities have not been clearly elaborated. We hypothesized that microbiome impacts would be distinct across different plants and polymer types. Maize and strawberry plants were cultivated with 1% MP by soil weight, including two conventional polymers (low-density polyethylene—LDPE and polyethylene terephthalate—PET) and one biodegradable polymer (poly(butylene adipate-co-terephthalate)—PBAT). Strawberry plants increased biomass across all MP treatments, accompanied by greater soil nitrate depletion. MP-induced impacts on soil prokaryotic communities were mostly additive to plant effects, as determined by 16S rRNA amplicon sequence profiling. PBAT stimulated Cupriavidus spp. and members of Saccharimonadales, suggesting a selection of potential polymer-degraders and microbial interactions, independent of plant species and root proximity. In contrast, conventional MPs induced a less selective response with compositional shifts across a greater number of taxa. MP-induced changes were more apparent in rRNA- than DNA-derived profiles, suggesting a profound response of putative active taxa. Together, we demonstrate that plant species and MP type jointly modulate rhizosphere microbial community response to MP pollution, with direct implications for soil biogeochemistry, rhizosphere functioning, and crop performance.

Background and aims: Shrubs like Vaccinium vitis-idaea substantially contribute to forest carbon (C) sequestration. Therefore, understanding their allocation patterns under climate change is crucial. We conducted a translocation experiment with ¹³ CO ₂ pulse-labelling to test if (I) belowground pools accumulate total and recently assimilated C, (II) within fine roots, assimilated C is preferentially allocated to root tips and mycorrhized cells and (III), whether warming alters C allocation patterns. Methods: We translocated soil cores with V. vitis-idaea from North- to South-Finland (+ 3.7 °C mean temperature). After 2.5 years, we excavated and pulse-labelled them with ¹³ CO ₂ , tracing the ¹³ C in plant, soil, and respiration. We used laser-ablation isotope ratio mass spectrometry to examine ¹³ C distribution in fine roots. Results: Roots represented the largest plant C pool (north: 49.69 ± 9.73%, translocated: 43.92 ± 6.59% of plant C stock). A substantial amount of recently assimilated ¹³ C was recovered belowground (roots + soil, north: 15.59 ± 5.77%, translocated: 39.21 ± 29.32%) 8 days after labelling. Most assimilated C was respired (north: 63.3 ± 5.9%, translocated: 44.1 ± 3.2%) or recovered in leaves (north: 17.9 ± 5.92%, translocated: 19.2 ± 3.19%). Within fine roots, ¹³ C content tended to be higher at root tips, while mycorrhizal infection had no effect. Warming tended to increase C allocation to the shoot and significantly decreased ¹³ C in the microbial biomass in the mineral soil. Conclusion: V. vitis-idaea allocates a large portion of assimilates belowground, but 2.5 years of warming only marginally change C allocation patterns. Accelerated soil microbial processes after translocation may increase plant N availability, potentially affecting C allocation over longer time spans.

In soil studies, the current definition of microplastics as particles <5 mm was adopted directly from marine research. To our opinion, a more precise and differentiated size definition is needed to focus studies on specific challenges plastics cause for soil ecosystems. As relevant soil functions such as water, carbon, and nutrient retention and provision are mainly controlled by soil structure, biota, and chemical processes dominantly appearing in the micro‐ to nanoscale, we suggest adapting size ranges of plastics to the respective process scales in soil ecosystem studies. Even more, we expect that larger particles will not be incorporated into soil until they reach a size threshold compatible to soil structure (<1000 µm, depending on soil properties). Redefining plastic sizes in accordance with soil processes and the International System of Units (SI) should be implemented to focus research. A unified definition of microplastics (1–1000 µm) and nanoplastics (1–1000 nm) will set a standard to further allow relating plastic sizes across research disciplines.

Quantification of microplastics in soil is needed to understand their impact and fate in agricultural areas. Often, low sample volume and removal of organic matter (OM) limit representative quantification. We present a method which allows simultaneous quantification of microplastics in homogenized, large environmental samples (>1 g) and tested polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS) (200-400 μm) overestimation by fresh and diagenetically altered OM in agricultural soils using a new combination of large-volume pyrolysis adsorption with thermal desorption-gas chromatography-tandem mass spectrometry (TD-GC-MS/MS). Characteristic MS/MS profiles for PE, PET, and PS were derived from plastic pyrolysis and allowed for a new mass separation of PET. Volume-defined standard particles (125 × 125 × 20 μm³) were developed with the respective weight (PE: 0.48 ± 0.12, PET: 0.50 ± 0.10, PS: 0.31 ± 0.08 μg), which can be spiked into solid samples. Diagenetically altered OM contained compounds that could be incorrectly identified as PE and suggest a mathematical correction to account for OM contribution. With a standard addition method, we quantified PS, PET, and PE_corrected in two agricultural soils. This provides a base to simultaneously quantify a variety of microplastics in many environmental matrices and agricultural soil.