Evaluating interactive effects of microplastics and biochar on legume growth and soil properties

Abstract

Growing population and their excessive use of plastic products have developed a serious environmental and health hazard of microplastics in ecosystem. Microplastics have various entry routes in agroecosystem where they cause negative impacts on plant and may enter food chain and have indirect impact on human health. This causes a serious threat to food security as well as to human health. Microplastics have shown to negatively impact plant growth and performance. Microplastics can act as vector for various other contaminants, and release toxic additives harmful for overall ecosystem. Biochar has been investigated and proven to be a potential mitigation strategy for different pollutants and improves soil health. However, there is little research information available on the use of biochar in microplastic contaminated soils. Therefore, this study investigated the interactive effects of microplastics (0, PVC=0.25%, PE=0.25% & PVC+PE=0.25%+0.25% w/w) and cotton stalk (Gossypium hirsutum) biochar (0, 0.5% w/w) on mung bean (Vigna radiata) growth, phytotoxicity, physiology, and antioxidant activity. Effects of these treatments were also tested on soil properties and nutrients. Glasshouse experiment was conducted in ASAB, NUST for 45 days to test the effects of these treatments on mung bean. Plant growth, physiology, antioxidant activity, soil pH, soil EC and soil nutrition were analyzed to test the effects of microplastics and biochar. MultispeQ device was used for measuring plant physiology. Spectrophotometric absorption was done for estimation of antioxidant activity using NBT photoreduction method, Guaiacol method and H2O2 method. Soil pH and EC was measured using pH and EC meter, meanwhile soil phosphorus and potassium was measured using spectrophotometer and flame photometer respectively. Results indicated xix that microplastics and biochar had no significant impact on plant length and fresh biomass. PVC-MPs+PE-MPs combined contamination decreased shoot dry biomass (from 0% to - 26.21%). Biochar decreased shoot dry biomass (from 0 to -37%). Biochar had no significant impact on the shoot-to-root ratio; however, SR ratio was increased in PE-MPs and PVC+PE-MPs contaminated soils (from 0% to 37%). Microplastics elevated root phytotoxicity, with PVC-MPs exhibiting highest phytotoxicity (from 0% to 24%), meanwhile biochar decreased phytotoxicity with highest decrease in PVC+PE-MPs contamination (from 20.2% to 1.2%), thus indicating mitigation of toxic effects of MPs by biochar. Biochar enhanced chlorophyll content (0% to 13%) yet PVC+PE-MPs decreased maximum fluorescence and chlorophyll content (from -27% to -10% and from -20.8% to 3.34% respectively). Additionally, biochar positively influenced non-photochemical quenching, linear electron flow, and leaf thickness in combined microplastics contamination (PVC+PE). Biochar also improved antioxidant activity for better response against oxidative stress. While PVC improved electrical conductivity (from 0% to 38.2%) yet biochar reduced it (from 38.2% to 3.6%), microplastics increased soil pH and available phosphorus, but biochar reduced them to the level of uncontaminated soil. To conclude, whereas microplastics, especially when combined, negatively impacted plant growth and soil properties, biochar amendment had an ameliorating effect, particularly in lowering phytotoxicity cause by microplastics and improving photosynthetic efficiency