Microplastics Have Shape- and Polymer-Dependent Effects on Soil Processes
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bioRxiv preprint doi: https://doi.org/10.1101/2020.06.02.130054; this version posted June 3, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Titel: Microplastics have shape- and polymer-dependent effects on soil processes 2 3 Authors: Anika Lehmann1,2,†, Eva F. Leifheit1,2, Maurice Gerdawischke1, Matthias C. Rillig1,2 4 1 Freie Universität Berlin, Institut für Biologie, 14195 Berlin, Germany; 5 2 Berlin-Brandenburg Institute of Advanced Biodiversity Research, 14195 Berlin, Germany 6 † Correspondence: [email protected]; Tel.: +49-30-838-53145 7 8 Keywords: microplastic; soil aggregation; organic matter; shape 9 10 11 Abstract: Microplastics are a diverse and ubiquitous contaminant, a global change driver with 12 potential to alter ecosystem properties and processes. Microplastic-induced effects in soils are 13 manifold as microplastics differ in a variety of properties among which the shape is of special 14 interest. Microplastic shapes can resemble natural forms or be dissimilar from natural objects. 15 Our knowledge is limited regarding the impact of various microplastic shapes on soil processes. 16 Therefore, we conducted this two-part research comprising a meta-analysis on published 17 literature and a lab experiment focusing on microplastic shapes- and polymer-induced effects 18 on soil aggregation and organic matter decomposition. We here focus on fibers, films, foams 19 and fragments as microplastic shapes. 20 In the meta-analysis, we revealed a strong research focus on fibrous and particulate 21 microplastic materials, with films and foams neglected. 22 Our experiment showed that microplastic shapes are important modulators of responses in soil 23 aggregation and organic matter decomposition. Fibers, irrespective of their chemistry, 24 negatively affected the formation of aggregates. This supported the shape dissimilarity 25 hypothesis. However, for other shapes like foams and fragments, the polymer identity is clearly 26 an important factor co-modulating the soil responses. 27 Further research is needed to generate a data-driven foundation to build on our developing 28 mechanistic understanding of the importance and consequences of microplastic shapes added 29 to our soils. 30 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.06.02.130054; this version posted June 3, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 31 Introduction 32 Microplastic contamination of the environment is becoming an issue of increasing concern. 33 Proposed as a new global change driver with increasingly realized impacts on marine, aquatic 34 and terrestrial systems, microplastic-induced effects on ecosystem functions and processes are 35 starting to be revealed, as research shifts from a more ecotoxicological focus to more fully 36 embrace an ecosystem perspective (Rillig and Lehmann, 2020). But microplastics are not a 37 monolithic issue and neither are the effects they induce. Microplastics are a group of synthetic 38 polymers encompassing a wide diversity in sizes, shapes, chemistries and additives (Wagner et 39 al., 2014). In addition, plastic particles are exposed to weathering in the environment, further 40 enhancing this diversity in properties: microplastics are either produced as primary microplastics 41 or secondarily via degradation in the environment to fragments < 5mm (Hartmann et al., 2019). 42 Our knowledge about the effects of microplastics on soils is still limited, although they likely are 43 exposed to microplastic broadly and at increasing levels, especially agricultural and urban soils. 44 Microplastics enter soils via atmospheric deposition (Allen et al., 2019), plastic mulching 45 (Steinmetz et al., 2016), sewage sludge (Nizzetto et al., 2016) and compost (Weithmann et al., 46 2018). Microplastics are ubiquitous and persistent contaminants. After entering the soil matrix 47 (Rillig et al., 2017), they have the potential to alter soil properties and processes. 48 Microplastics can affect soils as a function of their chemistry, but also through their shape (Rillig 49 et al., 2019). Microplastics can resemble natural shapes, or they can be quite dissimilar from 50 naturally-occurring objects in soil. In soils, microplastic beads and fragments can be almost 51 indistinguishable from e.g. sand grains. Microplastic fibers, rigid and unbranched filaments, 52 diverge clearly from hyphae, plant roots or other natural filamentous, linear organisms or 53 structures. Other microplastic shapes like films and foams have no analogs in a natural system 54 due to their properties and size. Rillig at al. (2019) highlighted the importance of shapes in the 55 context of the population of naturally occurring particles at any given scale, suggesting that 56 shapes more dissimilar from naturally occurring particles might cause stronger effects than 57 plastic particles that resemble natural shapes. Research on terrestrial systems revealed that a 58 key feature of soil - its structure - is often negatively affected by microplastic fibers, while for 59 microplastic beads and fragments there is a range of positive to negative effects (e.g. Machado 60 et al., 2018; Boots et al., 2019; Machado et al., 2019; Zhang et al., 2019). The spatial 61 arrangement of primary particles into aggregates, and the resulting pore networks, together 62 strongly control soil processes, including biogeochemical cycles and soil carbon storage and 63 processing. For example, soil organic carbon can be physically protected inside of soil 64 aggregates. The protection of soil organic carbon can be affected by microplastic indirectly via 65 effects on soil structure or via effects on microbial activity, but observed effects are variable, 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.06.02.130054; this version posted June 3, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 66 shifting with microplastic concentration and properties (Liu et al., 2017; Machado et al., 2018; 67 Boots et al., 2019). To better understand effects of microplastics on soil aggregation and 68 organic matter processing, it is thus necessary to more systematically examine key microplastic 69 properties, for example their shape. 70 To this end, we here present a two-part study, consisting of a quantitative synthesis based on 71 available data, and a laboratory experiment systematically varying microplastic shapes. 72 Research on microplastic-induced effects on soil systems is accumulating and with it the need 73 for a quantitative evaluation of the consequences for soil structure and decomposition. Hence, 74 we here conducted a meta-analysis on published articles presenting microplastic-induced 75 effects on soil aggregation and soil organic matter loss. We identified overall 9 articles for which 76 we extracted data on soil aggregate formation, stability and soil organic matter loss and 77 information on microplastic shape and polymer. With the resulting datasets we investigated (1) 78 the overall impact of microplastic on soil aggregate formation and stability and soil organic 79 matter loss, irrespective of shape and polymer, and (2) how the effects are modulated by shape 80 and polymer. 81 The biggest limitation of available research on microplastic induced effects on soils is the focus 82 on just filamentous and particulate shapes. To add new insights on the importance of different 83 shapes we designed a laboratory study in which we tested the effect of four different shapes, 84 each represented by three different polymers, on soil aggregation and organic matter 85 decomposition. We hypothesize (1) that fibers, films and foams will affect soil structure and 86 organic matter decomposition more strongly than fragments due to their shapes which are more 87 dissimilar to natural shapes present in the soil; and (2) that the shape is more important than the 88 polymer. 89 90 Material and methods 91 Meta-analysis 92 First, we conducted a meta-analysis on published articles reporting microplastic effects on soil 93 aggregation and/ or soil carbon fractions. We followed the PRISMA guidelines (for further 94 information see supplementary information; Fig.S1). 95 96 Literature search. We conducted a three-step literature search on 2nd of April 2020 to identify 97 and collect articles for our database. First, we used the two search strings “(microplastic* AND 98 soil)” and “(((microplastic* OR microfiber*) AND (decompos* OR "organic matter" OR litter)) 3 bioRxiv preprint doi: https://doi.org/10.1101/2020.06.02.130054; this version posted June 3, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 99 NOT (marine OR sea OR water OR aquatic))” to run topic searches in Web of Knowledge 100 (Thomson Reuters) focusing on articles in English language. Both search outcomes were 101 combined with the “OR” option to generate one search outcome with 276 hits eliminating 102 duplicates (search 1: 174 hits, search 2: 120 hits). These articles were screened by title and 103 abstract to identify potential candidate articles that were then further investigated according to 104 our inclusion and exclusion criteria. (1) Articles had to present data on soil aggregation (e.g. 105 water-stability, size class weights) and/ or decomposition (i.e. mass loss, rate of substrate 106 degradation) of organic material (e.g. litter, cellulose stripes) for microplastic contaminated 107 samples and associated controls. (2) Only soil based systems were applicable.