Effects Of Lead On Soil Nitrogen Transformation And Functional Microorganisms In Tea Garden Under Different Pollution Modes

Heavy metal pollution has become a major environmental problem due to its concealment,high toxicity,persistence and bioaccumulation.Due to the special physiological function of tea tree,soil acidification in tea garden is serious.As a cash crop for leaf use,tea plant has a great demand for nitrogen fertilizer,so it is often used to increase nitrogen fertilizer to ensure the yield of tea.Excessive application of nitrogen fertilizer will not only reduce fertilizer utilization,but also increase soil acidification.With the aggravation of soil acidification,the bioavailable content of risk elements such as lead in the soil increased,and the harm to the soil element cycle and related organisms increased.In this study,Shuchazao,a high-quality tea seedling variety,was used as the research material,and two lead pollution modes（cumulative and primary）and three lead concentrations（low concentration 100 mg/kg,medium concentration 300 mg/kg,and high concentration 900 mg/kg）were set up to study the transformation of lead into soil nitrogen in tea plantations under different pollution modes.The number of soil nitrogen cycling microorganisms,enzyme activity,community structure and soil nitrogen cycling functional gene abundance.To provide theoretical basis for studying the influence mechanism of lead on nitrogen conversion in tea garden soil and improving the bioavailability of soil nitrogen,the main research results are as follows:The total and available lead contents in soil were significantly different among different treatments,and the total and available lead contents in soil under cumulative pollution were higher than those under primary pollution（P<0.05）.Under the treatment of lead pollution,the total lead content in soil ranges from 11.73 to 976.83 mg/kg,and the effective lead content ranges from 6.39 to 724.68 mg/kg.Lead pollution reduced soil organic carbon content,and the reduction amount under single pollution treatment was higher than that under cumulative pollution.The soil pH decreased with the increase of lead concentration under accumulation and primary pollution.Soil moisture content,available potassium and cation exchange capacity increased with the increase of lead concentration.Lead had no significant effect on soil available P content（P>0.05）.Soil cation exchange capacity was significantly affected by different pollution modes,and cumulative pollution was higher than primary pollution.Total nitrogen and ¹⁵N abundance of tea seedling roots increased under high concentrations of cumulative pollution,while total nitrogen and ¹⁵N abundance of tea seedling above-ground parts decreased under lead treatment.The abundances of total nitrogen and ¹⁵N in root and overground parts of tea seedlings were significantly decreased under high concentration of Pb（P<0.05）.Lead pollution leads to the increase of residual nitrogen in soil.The contents of total nitrogen,¹⁵N abundance,alkali-hydrolyzed nitrogen,ammonium nitrogen and nitrate nitrogen in soil increased with the increase of lead treatment level.The contents of ammonium nitrogen and nitrate nitrogen were significantly affected by pollution mode,and the content of nitrate nitrogen in primary pollution soil was significantly higher than that in cumulative pollution（P<0.05）.The soil microbial biomass N was the highest under high concentration and primary pollution treatment,while the soil microbial biomass N under other lead treatments was lower than that under the control.The number of soil culturable ammoniating bacteria was higher under low concentration and high concentration of lead under accumulation and primary contamination.The number of culturable autogenous azotobacter,nitrous acid bacteria and nitric acid bacteria decreased under lead treatment,and the number of autogenous azotobacter and nitric acid bacteria significantly decreased under high concentration of lead,and nitrous acid bacteria significantly decreased under medium concentration and high concentration of lead treatment.The number of culturable denitrifying bacteria was increased under lead treatment,and significantly increased under accumulated high concentration of lead treatment（P<0.05）.Lead pollution inhibited the activities of urease,nitrosation reductase and nitrate reductase,activated the activity of hydroxylamine reductase,and had a phenomenon of"low promotion and high inhibition"on the activity of protease.Different pollution modes had significant effects on soil urease activity,and cumulative pollution was significantly higher than primary pollution（P<0.05）.Both PCoA analysis and NMDS analysis showed that the nitrogen cycling functional microorganisms in soil treated with high concentration primary pollution had the greatest difference from other treatments.In the soil nitrogen cycle,the microorganisms involved in nitrification were the most,followed by aerobic＿nitrite＿oxidation and urea decomposition.Nitrospirillum,Nitrococcus and Nitrospirillum are the dominant bacterial genera.Redundancy analysis showed that alkali-hydrolyzed nitrogen,available lead and total lead were the main influencing factors of soil nitrogen cycling microorganisms.Lead pollution decreased the abundance of nifH,amoA and nxrB genes in tea plantation soil.The abundance of amoA,nxrB,narG,napA,nirK,nosZ and norB genes was significantly affected by different pollution modes.For nirK,narG,nifH,nosZ,napA and norB gene abundance,primary pollution was significantly higher than cumulative pollution（P<0.05）.