Effects Of Straw Returning In Cu-contaminated Soil On Soil Enzyme Activities,carbon Components And Growth Of Maize Seedlings

Cu is one of the major sources of heavy metal pollution,which not only accumulates in the soil and harms the soil environment,but also can be transmitted through the food chain,causing harm to animals,plants and human beings.Corn straw contains rich organic matter,and plays an important role in enhancing soil fertility,providing crop nutrition,storing moisture and increasing crop yield.Some organic matter produced by microbial degradation can be used as bioadsorptive and chelating materials to fix Cu elements in soil and reduce the toxic effect of heavy metals.In order to ensure the health of cultivated land,this experiment studied the effects of straw returning to farmland under different Cu concentration pollution on soil respiration,soil enzyme activity,soil organic carbon components and other soil biochemical indexes as well as the growth of maize seedlings,providing theoretical basis for exploring soil Cu pollution control and restoration,and having great significance for ecological environment protection in Cu-polluted areas.This experiment consists of soil culture experiment and pot experiment.Both experimental designs were completely randomized block experiments,and two factors were set up（Cu concentration M and straw returning S）,in which Cu concentration levels 0,1,2 and 3 were 0 mg kg^-1,100 mg kg^-1,300 mg kg^-1,600 mg kg^-1,respectively.Straw supplemental levels 0 and 1 were0g·kg^-1and 4g·kg^-1(equivalent to 0 kg·hm^-2,9000 kg·hm^-2),respectively.A total of 8 treatments were performed with 4 replicates,numbered as M0S0,M0S1,M1S0,M1S1,M2S0,M2S1,M3S0,M3S1.The results are as follows:1.Cu pollution can significantly inhibit soil respiration rate and accumulation,while straw addition can increase soil respiration rate and accumulation.During the whole culture time,except that the interaction between Cu and straw had no significant effect on CO₂rate on the 120th day of culture,Cu and straw and their interaction had significant or extremely significant effects on soil respiration rate and respiration accumulation,and soil respiration rate reached its peak on the second day of culture,among which the largest treatment was M0S1.The rate of 44.73 mg kg^-124h^-1,was 147.5%higher than that of M0S0.Under the condition of no straw returning,the inhibition degree of Cu on soil respiration rate was 10.21%～29.36%.Under the condition of straw returning,the inhibition degree of Cu on soil respiration rate was 5.21%～15.36%.The results indicated that straw returning to soil polluted by Cu could effectively reduce the inhibition of Cu on soil respiration.2.Cu pollution can significantly inhibit soil enzyme activity,and straw returning to the field can enhance soil enzyme activity to different concentrations of Cu pollution.But different soil enzymes have different sensitivity to Cu concentration,among which catalase and acid phosphatase showed inhibitory effects under low concentration of Cu pollution,and the inhibition degrees were15.31%～47.62%and 8.31%～33.62%,respectively.However,soil sucrase,urease and polyphenol oxidase were not sensitive to Cu pollution,and their activities increased first and then decreased with the increase of Cu concentration.At the same time,straw returning to the field can improve soil enzyme activity in different concentrations of Cu pollution,and the improvement degree of soil enzyme activity in different concentrations of Cu is 36.64%～7.14%,50.66%～3.87%,21.05%～4.55%,30.39%～2.88%,56.47%～9.09%.The interaction between Cu and straw had significant effects on the activities of sucrase,polyoxidase,acid phosphatase and urease.The results indicated that straw returning to soil polluted by Cu could effectively reduce the inhibition of Cu on soil enzyme activity.3.Cu pollution had significant inhibitory effects on soil total organic carbon,active organic carbon,soluble organic carbon and microbial biomass carbon,and the inhibitory effect gradually increased with the increase of Cu concentration.However,the effect on soil total organic carbon was not significant in the early stage of culture,and straw returning to the field could increase the contents of soil total organic carbon,active organic carbon,soluble organic carbon and microbial biomass carbon.Compared with M0S0,the contents of soil total organic carbon,soluble organic carbon,active organic carbon and microbial biomass carbon decreased by 20.25%,28.53%,20.10%and 33.54%.Compared with M0S1,the contents of total organic carbon,soluble organic carbon,active organic carbon and microbial biomass carbon in soil treated with M3S1 decreased by 22.18%,42.27%,32.27%and 43.54%.The interaction between the two had no significant effect on the active organic carbon cultured in the early stage.4.In the early stage of culture,straw can significantly increase soil p H value when Cu concentration is constant;In the treatment of straw returning to the field,soil p H value will decrease with the increase of Cu concentration.At the later stage of culture,there was no significant difference among all treatments,and it was similar to M0S0 treatment,with p H of 6.9.The content of available Cu in soil increased significantly with the increase of Cu concentration,and the content of available Cu in soil increased by 23.43 mg·kg-1,32.43 mg·kg-1 and 53.43mg·kg-1,respectively.When Cu concentration is low,straw returning to field can increase the content of available Cu in soil,while when Cu concentration is high,straw returning to field can reduce the content of available Cu in soil.5.Cu pollution can inhibit plant root development.The root dry weight and root length decreased with the increase of Cu concentration.Straw returning under Cu pollution could inhibit the growth of root system and promote the growth of aboveground part of plant.The effects of Cu,straw and their interaction on SPAD value of low concentration Cu were not significant.With the increase of Cu concentration,the accumulation of nitrogen,phosphorus and potassium in plants gradually decreased,and the addition of straw could increase the accumulation of nitrogen,phosphorus and potassium in plants under the addition of Cu concentration.Cu accumulation in maize seedlings increased with the increase of Cu concentration in the soil.Cu concentration had different effects on Cu enrichment in different parts of maize,and Cu accumulation in underground part was obviously higher than that in above-ground part.With the increase of Cu concentration,the proportion of Cu accumulation in the above-ground part of corn seedlings increased gradually,while the proportion of Cu accumulation in the above-ground part of corn seedlings decreased with straw returning to the field.Through principal component analysis,it can be concluded that when the Cu concentration is less than 300mgkg^-1,straw returning to the field can improve soil biochemical indexes,but when the Cu concentration is too high,straw returning to the field can not improve soil biochemical indexes and plant growth under Cu stress.