Unseen invaders: Microplastics reshape Earth's carbon cycle and threaten plant health

The terrestrial environment, a vast and complex reservoir, is experiencing an alarming influx of microplastic pollution, accumulating at rates significantly exceeding marine environments. New research, published in Carbon Research, synthesizes a wealth of existing literature to meticulously examine how these pervasive plastic fragments interact with soil, altering its fundamental properties, influencing the soil carbon pool, and affecting the performance of terrestrial plants. This extensive review underscores the urgent need to understand and mitigate the subtle yet profound ecological transformations driven by microplastics.

This comprehensive investigation draws upon an exhaustive literature review, integrating findings from numerous studies across diverse global contexts. The researchers synthesized data covering a broad spectrum of microplastic types, sizes, concentrations, and their varying interactions with different soil textures and microbial communities. This method allowed for a nuanced understanding of microplastic influence on soil physicochemical properties, the intricate dynamics of soil organic matter (SOM) and dissolved organic matter (DOM), and the multifaceted responses of terrestrial plants, providing a holistic perspective on this emerging environmental challenge.

Microplastics significantly modify crucial soil physicochemical properties, including bulk density, porosity, aggregation, and pH levels. Intriguingly, microplastics can masquerade as soil carbon storage, introducing non-photosynthetic carbon and related leachates that confound conventional carbon assessments. These synthetic fragments also influence SOM turnover through complex priming effects and alter the distribution of carbon within particulate and mineral-associated organic matter, impacting its accumulation and stability. Furthermore, microplastics modify the chemodiversity of soil DOM, increasing its aromaticity and molecular weight while deepening its humification, with these changes stemming from microplastic-derived inputs and intricate organo-organic, organo-mineral interactions, alongside microbial degradation.

The presence of microplastics profoundly affects soil microbial communities, key mediators of the terrestrial carbon cycle. Microplastics serve as unique ecological niches, fostering distinct microbial assemblages compared to surrounding soil. This review details how microplastics induce shifts in microbial community structure and enzyme activities, influencing critical processes like SOM decomposition and nutrient cycling. A significant finding distinguishes between conventional microplastics, which can reduce microbial necromass carbon contributions to stable soil carbon, and bio-microplastics, which may conversely increase it, highlighting divergent ecological outcomes based on polymer type.

Beyond their impact on soil structure and microbiology, microplastics exert a wide array of effects on plant performance. These include impaired seed germination, altered vegetative and reproductive growth, and induced ecotoxicity and genotoxicity. These adverse outcomes can arise from external factors, such as microplastics modifying the soil environment's water transport and nutrient availability, or acting as physical barriers at root surfaces. Internal factors also play a role, as plants may take up nanoplastics, and the leaching of plastic additives and weathering products can introduce toxic compounds into plant tissues, posing potential threats through the food chain.

Unpacking the Complexities of Microplastic-Soil Interactions

Despite considerable research, the precise mechanisms and long-term consequences of microplastic-soil interactions remain intricate and often contradictory. Studies frequently report divergent effects depending on polymer type, size, shape, concentration, soil characteristics, and exposure duration. The challenge of distinguishing CO₂ emissions derived from native soil organic carbon versus microplastic carbon further complicates assessments of the soil carbon cycle, necessitating advanced techniques for accurate carbon source attribution. These complexities underscore the inherent difficulty in drawing universal conclusions and highlight the need for standardized methodologies.

Charting Future Research Pathways

Addressing these knowledge gaps requires a concerted, multidisciplinary research effort. Future investigations must precisely elucidate the correlations between microplastic-induced physicochemical alterations and soil carbon cycling, with a specific focus on microbial stabilization and mineral protection of carbon, particularly concerning microbial necromass accumulation and mineral-associated fractions. Implementing advanced ¹³C isotope technology is paramount for accurately tracing the sources of carbon emissions from both soil and microplastics. Further research should also delve into rhizosphere dynamics, including microbial activity, function, and the stabilization mechanisms for rhizodeposits. Moreover, a critical examination of so-called "eco-friendly" bioplastics is essential, as their biodegradation characteristics may introduce unique and pronounced effects on soil biophysical properties within agricultural systems.

A Call for Holistic Ecosystem Understanding

The escalating production of plastics and inadequate waste management strategies ensure the persistent increase of microplastic contamination in soil ecosystems. The findings presented in this review make a compelling case for a holistic understanding of these ecosystem feedbacks. Integrating advanced analytical tools with ecological studies will be crucial for developing effective countermeasures and ensuring the sustainable management of our invaluable terrestrial resources in the face of this enduring environmental challenge.

Corresponding Author: Ke Sun

Original Source: https://doi.org/10.1007/s44246-024-00124-1

Contributions: All authors contributed to the study conception and design. Yalan Chen: Conceptualization; Data curation; Validation; Visualization; Writing—original draft. Ke Sun: Conceptualization, Supervision, Funding acquisition, Project administration, Writing—review & editing. Yang Li, Xinru Liang, Siyuan Lu, Jiaqi Ren, Yuqin Zhang, Zichen Han: Conceptualization, Writing—review & editing. Bo Gao: Supervision, Writing—review & editing. All authors read and approved the final manuscript.