Effects Of Nitrogen Addition On Arbuscular Mycorrhizal Fungi And Nitrogen Uuptake Of Chinese Fir Seedlings

With the development of economy,the amount of nitrogen deposition increased gradually,which had significant impacts on the nitrogen cycle of forest ecosystem.Arbuscular mycorrhizal fungi(AMF)are the symbiotic fungi which link closely to terrestrial plants.Mycorrhizal hyphal formed mycelial network,get the nutrition for plants from the soil which roots can not reach.AMF improve plant disease and drought resistance.In return,plants transmit the photosynthesis product to the mycorrhiza for its growth.As the donminant species in southern China,Chinese fir plantation area continues to expand in recent years,accounting for 18%and 6%of the country and the world plantation area respectively.China has become the world’s third largest nitrogen deposition area.Arbuscular mycorrhizal fungi as indigenous fungi of Chinese fir,change of soil environment induced by nitrogen deposition will cause the change of soil microorganisms especially fungi,and then change the growth of Chinese fir.This study is based on the Fujian Sanming forest ecosystem and global change research station.The site is characterized by a subtropical monsoonal climate,with the mean annual temperature of 19.1 ℃,and and a relative humidity of 81%.The mean annual precipitation of 1749 mm,which mainly occurs from March to August.The soils developed from well weathered granite are classified as mountain red soils in Chinese soil classification.Three levels of nitrogen addition were applied in the Chinese fir plantation,to research the impact of nitrogen deposition on arbuscular mycorrhizal fungi and the growth of Chinese fir.Experimental treatments were control(CT),low nitrogen(40 kg N hm-2 yr-1,LN),high nitrogen(80 kg N hm-2 yr-1,HN)with 5 replicates per treatment.The NH4NO3 dissolved in 800 mL deionized water was sprayed from the seedling of above canopy area.The same amount of deionized water was sprayed in the control area.Results were showed as follows:(1)Soil microbial biomass carbon varied significantly in different seasons.Microbial biomass C changed dramatically in January;the content of microbial biomass nitrogen was lower than that of the control;nitrogen addition decreased soil total carbon content,but there was no significant difference between different months.There were significant differences in soil dissolved nitrogen in different months,and significantly lower in April and the highest in August.The contents of available P in April and August were significantly higher than in other months.The effect of nitrogen addition on AMF colonization increased at first and then decreased.Under different treatments,the colonization of AMF increased in low nitrogen addition treatment,and decreased in the high nitrogen treatment.Nitrogen application increased the AMF spore density and increased with the increase of nitrogen application rate.The extraction of the soil related protein from the different months showed an increasing trend with the extension of the test time.The content of glomalin-related soil protein showed approximately U pattern,with a decrease from January 2015 to August 2015 and increase from August 2015 to the following year in April.The application of nitrogen reduced the overall incidence of glomalin-related soil protein content.(2)Soil microbial biomass,actinomycetes,fungi and bacterial biomass showed the first increase and then decrease with the amount of nitrogen application.Compared with 2015,the microbial biomass reduced significantly,and the microbial community structure changed significantly in the year of 2016.The proportion of gram-negetive bacteria and actinomycetes decreased in April 2016 compared to April 2015 and November 2015,the proportion of gram-negetive bacteria,fungi and arbuscular mycorrhizal fungi increased in total microbial community.The biomass of arbuscular mycorrhizal fungi decreased after nitrogen addition,especially the high nitrogen significantly inhibited the growth of arbuscular mycorrhizal fungi.The abundance of arbuscular mycorrhizal fungi was also affected by nitrogen application,and the nitrogen application increased different species of fungi compared with the control.(3),Nitrogen addition improved the growth of tree height,basal diameter and crown of Chinese fir seedlings.Leaf nitrogen content showed a decreasing trend,and reached the lowest value in April.There was no significant difference in leaf P content between treatments.There was a significant positive correlation between arbuscular mycorrhizal fungi colonization rate and height and leaf carbon content of Chinese fir seedlings.Arbuscular mycorrhizal fungi could promote photosynthesis of Chinese fir seedlings.There was a significant positive correlation between the total extraction of glomus and the content of nitrogen.Thus arbuscular mycorrhizal fungi could promote the formation of soil aggregates,improve the ability of water and fertilizer conservation,and then increase the nutrient contents of plant leaves.