Introduction
Plastic contamination has become a widespread societal and ecological threat due to its global application, persistence, and adverse impact on biota. Global plastic production has reached 368 million tons, with production in China and Europe at 114.08 and 58.88 million tons in 2019, respectively; some predictions of global production to exceed 33 billion tons by 2050.

Soil systems have become a major sink of plastics due to agricultural activities, including through amendments of sewage/sludge and organic fertilizers, as well as through the use of plastic films and mulches. Specifically, the application of municipal sludge in agriculture adds approximately 7.76 million tons of synthetic fibers or sedimented microplastics (MPx) and nanoplastics (NPx) to soil systems each year. Furthermore, mulching use for crop production as part of weed control and irrigation strategies introduces approximately 4.4 million tons of plastics each year to soils. In addition, landfill sites have been identified as one of the most significant entry pathways of plastic pollution into the environment.

Objectives

Given the known and expected discharge of plastics into the environment, we conducted a meta-analysis using the published literature on effects of MPx/NPx on agricultural plant species. We also focused on evaluating the significance of size and concentration of plastics on the physiological and biochemical response of different plant species. We also highlight research gaps, discuss methods, and offer recommendations and perspectives for future research.

                                                                   Figure. Meta-analysis classes with regard to MPx/NPx.

Plastic type disturb the plant response

The meta-analysis overall results revealed that physiological endpoints were affected by MPx/NPx type; specifically, shoot biomass and root length were reduced by approximately 25%, with other decreases being evident for germination 13%, root biomass 13% and plant height 6%.

Exposure to MPx/NPx induced a negative response on photosynthetic pigments, with the most investigated reduction in photosynthetic rate (14%) and total chlorophyll content (9%). Overall, MPx/NPx had notable effects on the biochemical activities of plants, with the most reported endpoints on POD with an increase of 104%, 47% increase in SOD, 15% in CAT and 32% in MDA activity. Interestingly, taking into account the plastic size, we have found that CAT activity was reduced by 20% with MPx exposure, while 64% increase was observed under NPx exposure.

Figure. Eco-toxicological effects of plastic on plant biological functions according to their size (microplastics and nanoplastics, and overall effect)

Effects of MPx/NPx concertation on plants

The MPx/NPx concentration had a significant impact on plant germination. MPx/NPx inhibited germination at ≤1 mg L^-1 by 14%, 2-10 mg L^-1 (13%) and >10 mg L^-1 (9%). In case of root length, low concentrations (≤1 mg L^-1), exhibited a highly negative response (39% reduction), at medium concentration (2-10 mg L^-1) values were less effected (15% reduction), and interestingly, at high concentrations (>10 mg L^-1) a positive response was evident (239% increased). Exposure to microplastics and nanoplastics showed negative effects on plant height and root biomass as a function of concentration. Exposure to MPx/NPx had dose dependent negative effects on shoot biomass. Shoot biomass reduced by 4% at ≤1 mg L^-1concentration, and 6% at 2-1000 mg L^-1, although at higher concentrations (>1000 mg L^-1), increases (21%) in shoot biomass were evident.

By taking average at all concentration and classes, the total chlorophyll content was decreased by 8%. With increasing dose of MPx and NPx, the activities of SOD and POD enzymes were increased. At low concentration (≤10 mg L^-1), the increases in SOD and POD activities were 35 and 76%, respectively; at higher concentration (>10 mg L^-1), the increases were greater (54 and 135%), respectively. In general, the meta-analysis results highlighted that physiological indicators were reduced with concentration of MPx/NPx, excluding root length which was increased with concentration; and impacts on chlorophyll contents were generally minimal. Antioxidant enzymes were increased with MPx/NPx concentration, with the exceptions being CAT and MDA activities which were negatively correlated with concentration.

Figure. Mean effect of plastic concentration on germination (A), root length (B), plant height (C), root biomass (D), shoot biomass (E), total chlorophyll contents (F), SOD (G), POD (H), CAT (I) and MDA (J).

Key thinking

It is also important to note that much of the existing literature includes studies that have doses orders of magnitude beyond that expected in the environment. We need to focus on generating hard data that may increase our understanding of exposure and risk under environmental relevance conditions. Microplastics and nanoplastics in particular presents more complication, as the mitigation of their adverse effects are probably not targeting the microplastics and nanoplastics themselves, but rather on secondary aspects like plastic design, use and waste-management strategies. Solid waste management, green chemistry, and efforts to create a truly circular economy are independent fields and if integrated together with regulatory action, can be helpful to reduce the burden of plastics in the environment.

Reference

Azeem, I., Adeel, M†*., et al. (2022). Micro and Nanoplastics Interactions with Plant Species: Trends, Meta-Analysis, and Perspectives. bioRxiv.