Characteristics Of N₂O Emission And Ammonia-oxidizing Microorganisms In Different Tobacco-planting Soil Under Cadmium Stress

As one of the most important greenhouse gas,the N₂O has a powerful warming effect.Farmland soil is an important source of N₂O emissions.Meanwhile,it is also the most severely disturbed ecosystem by human activities.Soil environmental changes and agricultural managements,such as soil pollution,fertilization and tillage,could have an important impact on N₂O emissions.Heavy metal pollution,especially the cadmium pollution,is a serious environmental problem in tobacco planting soil.Cadmium pollution could affect the nitrogen supply capacitys and the N₂O emission in soil by influencing the biochemical process after it enters into the soil.However,the response of N₂O emissions might be inconsistent in different tobacco soils under the cadmium stress due to their different physic-chemical,biological properties and management measures.Therefore,to study of N₂O emission characteristics and its functional microorganisms under cadmium stress could clarify the N₂O emission behavior,environmental effect and the inherent biological mechanism in different tobacco soils,which has important scientific value and practical guidance for N₂O emission estimation,the reduction management and nitrogen fertilizer regulation in tobacco fields.Shandong Weifang,Fujian Nanping,Hubei Enshi and Guizhou Zunyi are the typical tobacco planting areas with high quality tobacco leaf in China,which representative soil types are brown,paddy soil,yellow-brown soil and yellow soil respectively.In this study,a 28-days indoor incubation experiment was carried out to study the soil N₂O emission characteristics and its functional microbial effects in the four types of soils under different levels of cadmium stress.The main results were as follows:1.The physic-chemical properties,soil respiration and N₂O emission characteristics were different in four tobacco soils.The organic carbon content,respiration rate,N₂O emission rate and C/N ratio of paddy soil were highest,but the pH was lowest,while the organic carbon content,respiration rate and C/N ratio of brown soil were the lowest,and its pH was highest among four tobacco soils.The carbon and nitrogen emission characteristics were closely related to soil physi-chemical properties.There were significant positive correlations between the soil respiration rate,cumulative emission of CO₂ and ammonium nitrogen,organic carbon and carbon to nitrogen ratio,and a significant negative correlation with soil pH and total nitrogen.The N₂O emission rate and cumulative emission had significant positive correlations with soil ammonium nitrogen,organic carbon and carbon to nitrogen ratio,and significant negative correlations with soil pH.2.The different concentrations of cadmium stress had different effects on soil nitrification ratio and N₂O emission rate in different tobacco soils.The cadmium stress with high concentration(100 mg·kg^-1)inhibited the nitrification rate of yellow soil,brown soil,and yellow-brown soil significantly,while promoted the nitrification ratio and N₂O emission rate of paddy soil to a certain extent.While the nitrification ratio of each soil under cadmium stress with low concentration(10 mg·kg^-1)had the intermediate disturbance effect,which was significantly higher than that of CK and the high cadmium stress treatments.3.Different concentrations of cadmium stress had different effects on soil N₂O emission rate at different culture stages.The N₂O emission rate of each soil was promoted at the early stage,inhibited at the medium stage and gradually decreased in the later incubation time under the low cadmium stress condition.While,the N₂O emission rate of tobacco soils in the early incubation stage were all inhibited under the high cadmium stress condition except for that of paddy soil,which was stimulated in the early incubation stage,and then the inhibitory effect of cadmium on N₂O emission were gradually weakened with the incubation time.4.The cadmium stress could significantly reduce the abundance of soil ammonia oxidizing bacteria?AOB?and archaea?AOA?in different tobacco-planting soils.And it had a greater effect on AOB than AOA.The cadmium stress mainly affected the N₂O emission in different soils by regulating the dominance of AOA and AOB in nitrogen transforming microorganisms.There was no significant correlations between NO₂ emission rate of yellow-brown soil and the abundance of amoA gene of AOA and AOB.There was a significant positive correlations between the N₂O emission rate and that of AOB in brown soil and paddy soil,and a significant positive correlation with AOA in yellow soil.And the N₂O emission rate of brown soil and paddy soil was positively correlated with the abundance of amoA gene of AOB,while that of yellow soil was positively correlated with the abundance of AOA.5.The addition of ammonium nitrogen could promote the nitrification ratio and N₂O emission of four types of soil under cadmium stress.And the dynamic characteristics of N₂O emission varied with the soil types.Low cadmium stress could promote the nitrification ratio of paddy soil and yellow-brown soil,while high cadmium stress could significantly reduce the nitrification ratio of paddy soil,and it had no significant effect on the other soils.The addition of ammonium nitrogen can increase the abundance of ammonia-oxidizing microorganisms in the soil in a short period of time,but the ratio of AOA to AOB varies with soil type.The correlation between the copy number of AOB-amoA gene and the release rate of N₂O in nitrogen application was greater than that of AOA.Under the dual effects of cadmium stresss and ammonium nitrogen addition,the four types of soil N₂O are jointly dominated by AOA and AOB.