Study On Soil Nitrification,Denitrification And Functional Microorganisms Of Cunninghamia Lanceolata Forest At Different Ages

Soil nitrogen cycle plays an important role in the circulation of natural ecosystem elements,including nitrogen fixation,nitrification and denitrification.Soil microorganisms play an important role in the nitrogen cycle,driving and regulating the processes of the nitrogen cycle.The development stage of Cunninghamia lanceolate forest will change the soil and vegetation composition,and soil microorganism will change with it,which will affect the turnover and circulation of soil nutrients in the plantation.Therefore,the study of soil nitrification and denitrification processes and microorganisms in Cunninghamia lanceolate plantation at different age stages will be helpful to further understand the change law and its influencing mechanism of nitrogen turnover in Cunninghamia lanceolate plantation.The research was carried out in Baisha Forest Farm(25°08?N,116°39?E)in Shanghang County,Longyan City,Fujian Province.Chinese fir(Cunninghamia lanceolata)plantations of five forest stands with distances greater than 10 km and different forest ages(5-year-old,8-year-old,21-year-old,27-year-old,40-year-old)were used as the research object.To study the Potential Nitrification Rate(PNR)and Potential Denitrification Rate(PDR),the gene abundance of related nitrogen cycle microorganisms and the changes of soil nitrogen content in five forest stage stands.To reveal the changes of soil physical and chemical properties,soil nitrogen conversion rate and functional gene abundance with the growth of Cunninghamia lanceolata forest,and the relationship among the environmental factors,microbial functional gene abundance of nitrogen cycle and soil nitrogen cycle were study.The main conclusions of the study are as follows:(1)With the increase of forest age,the content of total carbon and total nitrogen showed a trend of decline after rising first,while the content of soil available phosphorus,available potassium and dissolved organic carbon and nitrogen in soil did not change with obvious regularity.The soil C/N ratio of 27-year-old Cunninghamia lanceolate stands was significantly higher than that of 8-year-old and 21-year-old stands.The contents of ammonium nitrogen(NH₄⁺-N)and nitrate nitrogen(NO₃^--N)in the soil of the 5-year-old forest were the highest,indicating that the concentration of nitrogen substrate in the soil of the 5-year-old forest was higher,and the nitrification and denitrification might be stronger.The content of available soil nutrients was higher in 5-year-old and 8-year-old stands,indicating that the soil nutrients in the young stands were in good condition.(2)There were significant differences in soil microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)contents of Cunninghamia lanceolate plantation at different forest ages.The content of MBC in 5-year-old Cunninghamia lanceolate forest was significantly lower than that in other stands,and the content of MBN in 21-year-old stands was the lowest.The results indicated that microbial biomass carbon-to-nitrogen ratio(MBC/MBN)of 21-year-old stands was the highest and that of 5-year-old stands was the lowest.The MBC in different forest ages had a significant positive correlation with soil total carbon(TC)and total nitrogen(TN)content,and a significant negative correlation with available phosphorus(AP)content.The differences of soil nutrient contents and soil physical and chemical characteristics in the undergrowth of Cunninghamia lanceolate at different forest ages had an indirect effect on soil microbial biomass.(3)The net nitrogen mineralization rate and net nitrification rate of different stand ages of Cunninghamia lanceolate showed a trend of"W".This indicated that the nitrogen availability of 8-year-old and 27-year-old Cunninghamia lanceolate forests was low.With the increase of forest age,the soil potential nitrification rate(PNR)was roughly showing a rising trend,but fell again in 40-year-old;Except for the 5-year-old stand forest,with the increase of the forest age,the soil potential denitrification rate(PDR)tends to increase gradually,but there was no significant difference among different forest ages.The soil net nitrogen mineralization rate and net nitrification rate were positively correlated with soil nitrate nitrogen(NO₃^--N)content.Soil nitrification potential rate was positively correlated with soil C/N ratio and dissolved organic carbon(DOC),and negatively correlated with soil total phosphorus(TP)content and available potassium(AK)content.Soil potential denitrification rate was significantly correlated with soil water content,DOC and dissolved organic nitrogen(DON).To some extent,the growth and development of stand will affect the soil nitrification and denitrification of Cunninghamia lanceolate forest.(4)The abundance of ammonia-oxidizing archaea(AOA)of 5-year-old and 40-year-old Cunninghamia lanceolata artificial forest was significantly higher than other stands.The abundance of ammonia-oxidizing bacteria(AOB)was highest in 5-year-old stand and lowest in 21-year-stand.Compared with AOB,AOA has an absolute advantage in quantity.There was no significant difference in gene abundances(nar G,nir K,nir S,and nos Z)of denitrifying microorganisms among the five stand forests,but the five-year-old stands had higher gene abundances than other stands.This suggested that the soil nitrogen cycle might be relatively rapid,and the nitrification and denitrification were active in the soil under the 5-year-old Cunninghamia lanceolata artificial forest.There was a significant positive correlation between soil NO₃^--N content and soil AOA gene copy number,but no correlation with soil AOB gene copy number.The soil p H was significantly positively correlated with the copy number of the nos Z gene which encoding nitrous oxide reductase in denitrifying microorganisms.