Effects Of Polystyrene Microplastics Combined With Cadmium Contamination On The Physiological Ecology Of Two Crops And The Changes In Their Soil Environment

The presence of microplastics and heavy metals in soils is a common compound contaminant,which is extremely difficult to degrade and easily forms a compound contamination,changing the soil environment and affecting the growth and physiological and ecological characteristics of crops.To study the effects of combined contamination of soil microplastics and heavy metals on soil environment and crop growth,two common crop seeds,rice（Oryza sativa）and pea（Pisum sativum）,were selected as test materials,and polystyrene microplastics（PS-MPs）and heavy metal cadmium（Cd）with particle size of 3μm were selected,and the exposure concentrations of PS-MPs(0 mg·kg^-1,10 mg·kg^-1,50 mg·kg^-1,100 mg·kg^-1,200mg·kg^-1and 400 mg·kg^-1)and combined with Cd contamination(0 mg·kg^-1,1.2mg·kg^-1,2.4 mg·kg^-1,3.6 mg·kg^-1and 4.8 mg·kg^-1)were used to determine the exposure of crop seeds to different exposure concentrations.The changes of seed germination index（germination rate,germination potential,germination index and vigor index）,seedling growth physiological index（biomass,plant height,chlorophyll content,antioxidant enzyme activity,malondialdehyde content,crop enrichment Cd coefficient and photosynthetic fluorescence parameters and fluorescence imaging）and inter-root soil environmental index（physicochemical properties and soil enzyme activity）under different exposure concentrations were determined to study the changes of soil environment and their effects on To investigate the effects of soil environmental changes on seed germination and seedling growth of crops.The study results are as follows:（1）PS-MPs stress had an inhibitory effect on both rice and pea seed germination indexes,and the inhibition was more pronounced with increasing PS-MPs concentration and was greater for pea seeds.PS-MPs in combination with Cd(1.2mg·kg^-1)showed synergistic effects on rice seed germination rate and vigor index,pea seed germination potential and vigor index;no significant effects on pea seed germination rate and vigor index;antagonistic effects on rice and pea seed germination index.PS-MPs in combination with Cd(2.4 mg·kg^-1)showed no significant effect on rice and pea seed germination indices.PS-MPs in combination with Cd(3.6 mg·kg^-1)showed synergistic effect on rice seed germination indices and antagonistic effect on pea seed germination indices.PS-MPs combined with Cd(4.8mg·kg^-1)had no significant effect on rice seed germination rate and germination index,and showed synergistic effect on rice seed germination potential and vigor index;and showed antagonistic effect on pea seed germination index.（2）PS-MPs stress had a negative effect on the growth and physiological indicators of both rice and pea seedlings,and the inhibitory effect was more pronounced with increasing concentration of PS-MPs,and the inhibitory effect was greater for pea seedlings.PS-MPs combined with Cd(1.2 mg·kg^-1to 3.6 mg·kg^-1)stress showed synergistic effects on rice and pea seedling biomass,MDA content and F_m;combined Cd(4.8 mg·kg^-1)showed synergistic effects on dry weight,MDA content and Fm of rice,antagonistic effects on dry weight and Fm of pea,and no significant effects on MDA content of pea;combined Cd showed synergistic effects on plant height of both rice and pea.PS-MPs combined with Cd(1.2 mg·kg^-1)showed synergistic effects on chlorophyll content of rice and pea The combined Cd(2.4mg·kg^-1)stress showed no significant effect on the chlorophyll content of rice and pea seeds;combined Cd(3.6 mg·kg^-1)showed an increasing and then decreasing trend on the chlorophyll content of rice and pea;combined Cd(4.8 mg·kg^-1)showed a synergistic effect on the chlorophyll content of rice and an antagonistic effect on the chlorophyll content of pea.The combined Cd stress showed synergistic effects on CAT and SOD activities and antagonistic effects on POD activities in rice and pea.PS-MPs combined with Cd stress showed synergistic effects on F_oand F_v/F_min rice and pea.The enrichment of pea plants with Cd(1.2 mg·kg^-1and 3.6 mg·kg^-1)was promoted by PS-MPs and alleviated by pea plants with Cd(2.4 mg·kg^-1and 4.8mg·kg^-1).（3）Both PS-MPs and combined Cd complex contamination increased soil pH,and the higher the concentration of PS-MPs,the higher the pH;the increase in rice soil pH was greater than that of pea pH.Both PS-MPs and combined Cd complex contamination stresses significantly reduced soil effective phosphorus,fast-acting potassium and alkaline digested nitrogen content.The PS-MPs stress promoted urease activity in pea soil,had no significant effect on rice soil catalase activity,and negatively affected the rest of the indicators in both rice and pea soils,and the inhibitory effect was more pronounced with increasing PS-MPs concentration,and was greater for pea seedlings（except for FDA hydrolase activity）.PS-MPs(10mg·kg^-1to 200 mg·kg^-1)combined with Cd stress showed antagonistic effects on soil organic matter content in rice and pea;PS-MPs(400 mg·kg^-1)combined with Cd showed synergistic effects on soil organic matter content in rice and antagonistic effects on soil organic matter content in pea.PS-MPs(10 mg·kg^-1and 50 mg·kg^-1)in combination with Cd showed antagonistic effect on the activity of FDA hydrolase in rice and pea soils,and in combination with PS-MPs(400 mg·kg^-1)showed synergistic effect.（4）The most significant correlation between the indicators of both rice and pea seedlings under PS-MPs stress and FDA hydrolase activity;the most significant correlation between the indicators of rice seedlings under PS-MPs combined with Cd stress and soil pH;and the most significant correlation between the indicators of pea seedlings under PS-MPs combined with Cd stress and soil organic matter content.In summary,the combined pollution of PS-MPs and Cd has different effects on different crop seeds,seedlings,and soil.Peas are more inhibited under single PS-MPs stress,and the inhibitory effect is alleviated under PS-MPs combined with Cd(4.8mg·kg^-1)stress,which may be related to the nodules and rhizobia of leguminous crops.This indicates that crops with different characteristics have different physiological responses to the combined pollution of PS-MPs and Cd,and this pollution affects the germination of crop seeds and the physiological and ecological characteristics of their seedlings by changing the soil environment.The research results can provide a certain reference for evaluating the ecological effects of microplastics and heavy metal composite pollution in the soil plant system.