Effects Of Cadmium Contaminated Rice Stubble On Cd-contaminated Soil Quality And Cd Accumulation By Rice And Pakchoi

The cadmium pollution of farmland soil has become a hot issue in the field of environmental research.Cd in agricultural soil can be accumulated in rice roots via root.Generally,the rice stubble will remain in the soil after harvest,which decomposed by micro organisms.thus the Cd released into the soil as Cd source for crop.However,few reports have been reported on the migration of Cd and its effect on other plant growth in root stubble addition system.In response to this problem,taking the moderately and mildly Cd-contaminated soil as the object,no stubble CK and three Cd-contaminated stubble treatments were set(ST1: 0.24%,ST2: 0.48%,ST3: 0.72%,air-dried soil quality score)4 treatments,and a two-season pot experiment of rice-pak choy rotation was carried out.Root stubble was applied once before transplantation of the rice seedlings.In the process of rice planting,Diffusion Gradients in Thin-films(DGT)was used to analyze the variation characteristics of soil Cd availability in different growth stages of rice,and to explore the effect of Cd-contaminated stubble returning to the field on crops under the rice-pakchoi rotation condition.The effects of growth and absorption of accumulated Cd and their effects on soil properties are expected to provide theoretical basis and practical support for Cd migration and management.The main findings are as follows:(1)Root stubble addition increased soil fertility,improve soil buffer performance,and increase soil p H.Among them,ST3 treatment has the best effect,with p H increased from 6.41 to 7.87,an increase of 22.8%.After rice planting,different treatments had no significant effect on the availability of soil trace elements,but showed a decreasing trend after pakchoi planting.(2)Root stubble addition increased the concentrations of DGT-Cd and trace elements at the tillering and heading stages of rice.When the soil depth was-1 to-4 cm,the contents of DGT-Fe and DGT-Mn gradually increased,and the maximum values were1.5 mg/L and 0.3 mg/L,respectively;when the soil depth was-1 to-4 cm,the concentration of DGT-Cd gradually increased.DGT-Cd in the tillering stage was significantly higher than that in the heading stage.The highest DGT-Cd in the two stages were 1.5 g/L and 0.15 g/L,respectively,indicating that the tillering stage was the most active period for rice growth and absorption of Cd.With the passage of time,the concentrations of DGT-S and DGT-Ca gradually decreased,and the minimum values were 0.05 mg/L and 0.5 mg/L,respectively.There was no significant change in DGT-Zn and DGT-Cu in different rice stages.(3)Stubble returning significantly increased the total soil Cd content,and the total Cd content reached the maximum in ST3 treatment,which was 1.23 mg/kg,while the DTPA-Cd content did not change significantly.-The effect of crop system is not obvious;the proportion of different Cd forms in soil has no obvious change in the two seasons,and the overall performance is reducible Cd>exchangeable Cd>oxidizable Cd>residual Cd.(4)The application of stubble significantly increased the content of mineral elements in rice roots,which had a promoting effect on rice growth.Root stubble returning to the field significantly increased the fresh weight of the shoots of pak choi,and increased the accumulation of Fe and Zn in pak choi,but decreased the accumulation of Cu.Stubble application increased the Cd content in rice roots,but significantly decreased the rhizome-stem transport coefficient,indicating that stubble returning to the field has the effect of inhibiting Cd transport from roots to shoots.With the increase of the application rate of root stubble,the Cd content of rice grains and the shoots of pakchoi gradually increased,and the Cd content in rice grains did not exceed the standard.The Cd content in the shoots of cabbage exceeds the national standard limit,so further measures are needed to reduce the accumulation of Cd in subsequent crops in the stubble field.In conclusion,the return of rice stubble to the field increased the soil nutrient content,and also increased the total soil Cd content,and the removal of the stubble could reduce the Cd content in the soil.The return of stubble to the field had no significant effect on the Cd content of rice grains,and the Cd content of rice grains all met the limit requirements of "National Food Safety Standard-Limits of Contaminants in Foods"(GB2762-2017).The tillering stage is the key period to effectively reduce Cd uptake in rice.Root stubble returning to the field significantly increased the fresh weight of the shoots of pak choi,and also promoted the absorption of Cd by pak choi.It can be seen that further measures are needed to control the migration of Cd in subsequent crops.