Effect Of Ionic Rare Earth Ore Mining On Soil Ammonia Oxidation

Due to its distinctive mineral endowment pattern and mining method for southern ionic rare earths,the residual rare earth ions and leaching solution are migrating and diffusing as a result of surface runoff and rainfall erosion.This causes environmental pollution issues such as soil acidification and excessive rare earth content in the mine area and its surroundings,which not only damage the mine area’s ecological environment but also affect the soil nitrogen cycling processes and their functional microbial taxa.The main goal of this study was to examine the impact of ionic rare earth mining on soil ammonia oxidation.This paper used microcosm culture experiments in conjunction with modern molecular biology techniques like Real-time PCR and Illumina Miseq to examine the effects of rare earth content and pH on inorganic nitrogen conversion,ammonia-oxidizing archaea（AOA）,ammonia-oxidizing bacteria（AOB）,and complete ammonia oxidizers（Comammox）as well as key enzyme activities.Collecting soils from rare earth mining areas and using 3,4-Dimethylpyrazole phosphate（DMPP）as a nitrification inhibitor,microcosm culture experiment was conducted to further clarify the microbiological mechanism of ammonia oxidation and its functional microbial phylogenetic characteristics.This paper provides a scientific and theoretical basis for enhancing nitrogen biochemical cycling in rare earth mining regions and directing the directional regulation of microbial community structure and function.The main findings of this study are as follows:（1）Soil incubation experiments with exogenous addition of rare earth yttrium showed that low content of rare earth yttrium（0-90 mg/kg）was more likely to lower soil pH than high content treatments（Y-120,Y-200 and Y-300）,with a significant negative correlation between soil pH and rare earth content（r=-0.982,p<0.01）.It also showed a continuous decrease in soil pH with increasing incubation time.When the concentration of yttrium was low,there was no significant difference in soil ammonia oxidation compared with the control treatment.However,when the concentration was high（>120 mg/kg）,the process was significantly inhibited.The abundance of AOA amo A gene was higher than that of AOB,Comammox amo A gene was not detected in all soil samples.The abundance of AOA amo A gene was not significantly correlated with the content of rare earth yttrium,while the abundance of AOB amo A gene was negatively correlated with the content of rare earth yttrium during the same incubation time.（2）The study on the effect of soil pH on ammonia oxidation in yttrium-contaminated soil showed that the P-3.5 treatment with significantly higher content of rare earth yttrium in the weak acid extraction state,higher rare earth yttrium bioactivity,the soil NH₄⁺-N content decreased slowly,as well as relatively lower NO₃^?-N content,apparent nitrification rate,soil enzyme activities and ammonia oxidizing microbial abundance,indicating that low pH inhibited soil ammonia oxidation.Compared with the low pH treatments,the biological effectiveness of rare earth yttrium was lower at higher soil pH（P-5.5 and P-6.5）,with negligible effects on soil ammonia oxidizing microorganisms and associated enzyme activities.Ammonia oxidation was obvious in P-5.5 and P-6.5.The observed soil nitrification rate was 4.01,2.14and 1.06 times higher when the soil pH was 6.5 compared to the P-3.5,P-4.5 and P-5.5treatments,respectively.Soil AOA amo A gene abundance was significantly higher than that of AOB under low pH circumstances.When soil pH was higher,AOB abundance was not significantly correlated with incubation time,whereas AOA abundance increased with incubation time and had a greater abundance advantage in acidic environments.Comammox amo A gene was undetected in all soil samples.（3）The DMPP microcosm culture experiments of rare earth mining soils during various mining periods revealed that ammonia oxidation occurred in four mining soils,with TY sample being the most obvious,indicating that natural succession is advantageous for the restoration of soil microbial abundance and species diversity as well as the enhancement of soil ammonia oxidation.AOA dominates YK,KC and DT soil ammonia oxidation processes,while AOB and AOA work together to promote TY soil ammonia oxidation.For ammonia oxidation in mining soils,the AOA taxa Nitrososphaera and the unclassified Crenarchaeota,and the Nitrosospira taxa of the AOB are the prominent species.The majority of AOA species share a high degree of similarity with uncultured archaea or AOA amo A gene sequences.