Aileen Jung

Microplastics (MPs) can alter soil physical structure, chemistry, and microbial communities; ultimately affecting plant production. However, soil heterogeneity and high variability in plastic properties pose a challenge for evaluating the direction of MPs effects on soil functions. We hypothesized that MPs can influence soil functions depending on polymer type, size, concentration, degradability, and soil type by affecting soil aggregate stability that is key to carbon storage, water holding capacity (WHC), nutrient retention, and soil microbial communities. To address this, Zea mays was planted in a greenhouse with silty loam and sandy loam soil and spiked with polyethylene (PE), polyethylene terephthalate (PET), or biodegradable polybutylene adipate terephthalate (PBAT), varied by concentration (0%, 0.1%, 1% w/w) and size (200–400, 75–200, <75 µm). Soil aggregate fractions and stability, WHC, 15N-ammonium-nitrate and 13C-yeast as models for soil N and C cycling, plant growth, and microbiota within aggregate fractions were analyzed. Overall, soil type dominated the response of soil functions to MP addition, largely independent of polymer size or type. However, biodegradable PBAT tended to increase soil microaggregate formation, stability, and WHC in silty loam; it immobilized 15N, resulting in increased plant uptake especially in sandy loam, and significantly altered soil prokaryotic communities in aggregate fractions, with stronger shifts in sandy loam. Here, microbial taxa involved in reductive nitrogen cycling, depletion of N-fixing Azospirillum spp., and enrichment of the potentially plant-pathogenic Xylophiluss spp. was observed. For both soils and across all MPs, retention of C was reduced about 50%. We reveal that MPs exert fundamentally bidirectional effects on soils. Biodegradable MPs enhance beneficial properties such as aggregation and water retention yet concurrently disrupt key microbiome functions. Across all polymer types, MPs accelerate soil carbon turnover, underscoring the need to resolve their net impact on critical soil functions in a soil- and polymer-specific context.

Lower‐order streams in agricultural landscapes receive major anthropogenic nitrogen inputs. Streambed sediments host diverse microbial communities that can influence nitrogen (N) fluxes and water chemistry. Both bacterial and archaeal ammonia oxidizers inhabit streambeds, but their respective contributions to nitrification are often unresolved. We investigated a first‐order stream in southern Germany to assess the contribution of distinct ammonia‐oxidising populations to streambed nitrification. We combined in situ geochemical data, 16S rRNA and functional‐gene amplicon sequencing, quantitative PCR and microcosm incubations with selective chemical inhibitors. A process‐based reaction model quantified total nitrification rates and inferred contributions of ammonia‐oxidising archaea (AOA) and bacteria (AOB), while population‐specific kinetic parameters were estimated using Bayesian inference. We found that AOB dominated nitrification and responded more strongly to ammonium inputs than AOA despite being less abundant. Among them, populations of Nitrosomonas and Nitrosospira spp. were most important. Differences in ammonia‐oxidation rates and ammonia‐oxidising communities between sediment depths and successive stream segments suggest a hydrological influence on streambed nitrification. Our study demonstrates the strength of combining field data, microcosm incubations and modelling to better understand microbial N‐cycling in the environment. It also mandates caution when interpreting functional‐gene abundance as a proxy for in situ reactive potential.

Microplastics (MPs) have shown different effects on soil physical structure, chemistry, and microbial communities. As soils are known to differ in their potential to develop structure, the observed variability in MP effects may be strongly attributed to MPs impact on soil aggregation. Hence, we here address the impact of different MPs on soil aggregation and associated soil functions such as water, carbon, and nutrient storage, and microbial habitat in soils differing in texture and soil organic matter (SOM). We hypothesized that conventional MPs hinder soil aggregation, speeding up the turnover of soil inherent OM, and reducing soil water holding capacity (WHC), while degradable MPs represent substrates altering soil microbial community composition, and directly promoting microbial activity and nutrient turnover. To address this, Zea mays was planted, as stimulator of soil aggregation and responder to changes in soil quality, in a greenhouse with silty loam and sandy loam soil and spiked with polyethylene (PE), polyethylene terephthalate (PET), or biodegradable polybutylene adipate terephthalate (PBAT), varied by concentration (0%, 0.1%, 1% w/w) and size (200–400, 75–200, <75 µm). Soil aggregate fractions and stability, WHC, 15N-ammonium-nitrate and 13C-yeast as models for soil N and C cycling, available P and K, as well as plant growth, and microbiota within aggregate fractions were analyzed. Overall, MP addition led to a decrease in more stable, occluded soil microaggregates, and a respective increase in free aggregates, while macroaggregate fractions remained unaffected. For both soils and across all MPs, soil C retention was reduced about 50%. Soil texture dominated the response of soil to MP addition: finer-textured soils had increased loss of microaggregates, SOM turnover, and WHC. As this was independent of polymer size and type, we relate this to physical MP effects. An MP-specific biochemical effect was observed for biodegradable PBAT tending to increase soil microaggregate formation, stability, and WHC. It led to 15N immobilization in soil and increased plant uptake and significantly altered soil prokaryotic communities in aggregate fractions. Here, microbial taxa involved in reductive nitrogen cycling, depletion of N-fixing Azospirillum spp., and enrichment of the potentially plant-pathogenic Xylophilus spp. was observed. We reveal that MPs exert fundamentally bidirectional effects on soils. Biodegradable MPs enhance beneficial properties such as aggregation and water retention yet concurrently disrupt key microbiome functions. Across all polymer types, MPs accelerate soil C turnover, underscoring the need to resolve their net impact on critical soil functions in a soil- and polymer-specific context.

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.