Effects Of Pb,La Single And Combined Pollution On Carbon Metabolism Microorganisms In Black Soil

With the large-scale mining and application of rare earth and heavy metal,both of them continue to accumulate in the environment and cause the soil to exhibit the characteristics of rare earth-heavy metal combined pollution,with the detection amount also showing a rising trend.As a major agricultural country,rare earth-heavy metal combined pollution in black soil grain producing areas has become one of the prominent issues that urgently need to be solved in China’s development of agricultural modernization.Therefore,it is of practical significance to understand the soil quality,microbial structure,and function of black soil after being contaminated by single and combined of rare earth and heavy metal,for the protection and rational utilization of black soil,and for the remediation and purification of soil resources.In this study,farmland surface（0-20 cm）samples were collected from the typical black soil area in Harbin,Heilongjiang Province.After artificial lead（Pb）,lanthanum（La）poisoning（50 mg/kg,200 mg/kg,500 mg/kg）,aging and cultivation,the content of rare earth,heavy metals and soil physical and chemical properties were analyzed,and the key enzyme activities involved in soil carbon cycle and the relationship between carbon content and pollutant characteristics were explored.EcoPlates^TM method and metagenomic sequencing were used to study the effects of Pb-La combined pollution on microbial community structure,carbon metabolism function and carbon source utilization differences in black soil.The main research results are as follows:（1）Pb-La had an elevating effect on soil pH,microbial biomass carbon（MBC）and β-glucosidase activity under single contamination in the lightly contaminated treatment groups（50mg/kg,200 mg/kg）and an inhibiting effect under high concentrations（500 mg/kg）of Pb,La and their combined contamination.All treatments had the potential to reduce soil dissolved organic carbon（DOC）content,and this increased with increasing pollution complexity.In addition,there was an irregular trend towards soil amylase and invertase activities under Pb-La contamination.（2）Microorganisms in black soil predominantly utilize carbohydrates and amino acids carbon sources.Biolog-ECO^TM results showed that microorganisms in the control and Pb500La50 also tended to use carboxylic acids,polymers and amines,while carbon source utilization of phenols and carboxylic acids was higher in the Pb500 and Pb500La200 contaminated soils.500 mg/kg Pb,200mg/kg La addition significantly reduced the soil microbial diversity index,showing Pb500La50 >CK > Pb500 > Pb500La200.（3）La（50 mg/kg）was able to mitigate the effects of 500 mg/kg Pb on black soil microbial diversity to some extent.The results of macrogenome sequencing showed that a total of 116 phyla,1846 genera and 8747 species were involved in the black soil carbon cycle,with bacterial domain microorganisms dominating the experiment.At the phylum level,the main microbial groups are Actinobacteria,Proteobacteria,Chloroflexi,Acidobacteria and Thaumarchaeota,which account for more than 80% of the total soil microorganisms.The dominant genera are unclassified＿c＿Actinobacteria,Nocardioides,Sphingomonas and unclassified＿c＿Chloroflexi.In addition,500 mg/kg Pb inhibited the growth of Actinobacteria,Chloroflexi,Acidobacteria,and Thaumarchaeota,while the addition of La increased the relative abundance of Chloroflexi and Acidobacteria on the basis of Pb stress.（4）Pb-La coexistence conditions affected the carbon cycle function of black soil,as evidenced by differences in carbon cycle pathways,modules and KO genes.Carbon metabolism,carbon fixation,and methane metabolism pathways were annotated to 247,76,and 128 KO genes,respectively.The Actinobacteria,Proteobacteria and Chloroflexi were the main contributors involved in carbon cycle function in Pb-La contaminated black soil,with the highest contribution among the three metabolic pathways.Furthermore,the addition of La at 50 mg/kg in Pb500 stimulated microbial community levels and functions converging to CK,while the synergistic effect of high La concentrations with Pb had an inhibitory effect on soil microbial abundance and functions.In summary,the coexistence of Pb-La has different effects on the carbon source preference,community structure and carbon cycle function of black soil microorganisms.In-depth discussions on these issues will help to provide reference values for the environmental risks of rare earth-heavy metal combined pollution in China.