A major focus of ecological research is the study of plant responses to global change factors (Peñuelas et al., 2018; Pugnaire et al., 2019). While some of these factors, such as climate change, have been the focus of much research on plant communities, other novel stressors of anthropogenic origin have emerged only recently. Among those is plastic contamination in terrestrial environments (Baho et al., 2021; Rillig et al., 2021). Although the number of studies addressing the effect of plastic contamination on ecological communities is increasing, we have virtually no knowledge about the potential interactive effects of climate change factors, such as drought, and plastic contamination on soil and plants. This knowledge is urgently needed not only for understanding global change effects on natural ecosystems, and productivity of agricultural systems.
Since the 1950s, a steadily increasing amount of plastic waste has been introduced into the environment. The estimated global annual production of plastic has exceeded 300 million metric tons (Nizzetto et al., 2016; Geyer et al., 2017), and it is predicted to increase. Due to its high durability, plastic waste persists in the environment for a long time (Rillig et al., 2021), during which it breaks off in smaller particles largely variable in size (centimeters to micrometers), shape, and behavior (Barnes et al., 2009; Rillig, 2012; Souza Machado et al., 2019).
Although much knowledge on the effect of plastic contamination on the environment comes from research on aquatic systems (Baho et al., 2021), terrestrial ecosystems have been signaled as ‘major sinks’ of plastic debris (Buks et al., 2020; Evangeliou et al., 2020). A major input of plastic into terrestrial environments comes from agricultural practices, where the use of mulching foils is extensive (Steinmetz et al., 2016; Souza Machado, Kloas, et al., 2018; Qi et al., 2018). Mulching foils are large plastic sheets (chiefly composed of polyethylene) that are applied on the soil to reduce evaporation, stabilize temperature, hinder the growth of weeds, and optimize crop germination (Steinmetz et al., 2016; Qi et al., 2018; Weithmann et al., 2018). Though mulching foils are usually removed upon germination, they often break apart and leave behind many smaller fragments that are spread over space by wind and runoff (Allen et al., 2019; Evangeliou et al.,2020) or into deeper soil horizons by soil organisms (Kiyama et al., 2012; Helmberger et al., 2020). In agricultural areas, the annual consumption of mulching films varies between 5 and 35 kg ha-1, amounting to plastic residuals in the range of 72–260 kg ha-1 (Liu et al., 2018). Thus, agricultural areas are potential hotspots of plastic contamination from which plastic debris can be dispersed into adjacent natural communities.
Experimental studies indicate that plastic fragments can affect several soil properties, such as soil porosity and soil water content (Lehman et al., 2019; Souza Machado, Lau, et al., 2018; Kim et al., 2021). Although, such effects vary in strength and direction depending on the properties of plastic fragments, e.g. shape, size, concentration and chemical composition (Lehman et al., 2021), studies have indicated that higher concentration and larger sizes of plastic fragments are associated with lower soil water content due to either increased evaporation or percolation (Souza Machado, Lau, et al., 2018, Wan et al., 2019; Lozano et al., 2021). When plastic fragments are incorporated in soil aggregates, they may favor the formation of fractures lines and thus reduce soil aggregate stability (Lehmann et al., 2019; Souza Machado et al., 2019; Wan et al., 2019; Liang et al., 2021; Lozano et al., 2021). This, in turn, leads to increased water loss when the presence of plastic results into the formation of larger soil pores, or in a decreased water loss when plastic fragments mediate the formation of smaller pores through which water moves more slowly. Therefore, soil texture might interact with the manner in which plastic affects soil water content, but little is known about this interaction.
Soil texture determines the spatial arrangement of soil aggregates and pore networks, affects soil water content, and the movement of water and nutrients in soils (Kemper, 1965; Rose & Rose, 2004). Clay-rich soils are characterized by smaller pores relative to sand-rich soils, and under optimal watering conditions, water percolates faster in sand-rich soils compared to clay-rich soils. However, under drought conditions clay-rich soils form larger aggregates, leading to wider soil pores and faster water loss (Horn et al., 1994; Beven & Germann, 1982). Thus, the effects of plastic fragments on soil properties may be mediated by soil texture (e.g. clay vs. sand content) and by the degree of water availability, giving rise to high context-dependence in such interactions.
Soil structure and soil water content are known to affect plant growth and performance (De Vries et al., 2012; Kaisermann et al., 2017; Martorell et al., 2021), and plastic fragments may influence such responses to an extent that is so far not fully understood (Rillig et al., 2019). On the one hand, plastic-mediated shifts in soil water content can either mitigate or amplify the effects of drought on plants. On the other hand, plastic fragments may facilitate the growth of roots by increasing the size and number of soil pores, thus attenuating the potential decline of soil water content. These processes can have long-term consequences on plant species and communities, especially in the context of climate-change-induced droughts (Lozano & Rillig, 2020). This is interesting, because negative effects of plastic on soil properties may be accompanied by effects on plants that are not necessarily negative.
We present the results of two experiments, where we tested the response of soil water content and plant performance to two water treatments (optimal vs. reduced) and two soil textures (clay-rich vs. sand-rich) contaminated with plastic fragments of varying concentration and size. We hypothesized that 1) the presence of plastic fragments leads to a decreased soil water content, and this effect is more pronounced a) when plastic fragments are present in higher concentrations or larger sizes, b) in clay-rich soils compared to sand-rich soils, c) in low compared to high water treatments; 2) plastic fragments have an indirect effect on plant performance, which is mediated by soil water content. Namely, plant performance is lower in treatments combinations where we expect soil water content to be reduced.