Effects Of Simulated Warming And Nitrogen Deposition On Soil Dissolved Organic Matter And Microbial Community Structure In Cunninghmia Lanceolate Plantations In Subtropical

Global warming and nitrogen deposition have become an indisputable fact.Increasing temperature and nitrogen deposition could cause the changes in the quantity and chemical structure of soil dissolved organic matter?DOM?and microorganisms of soil.However,the relationship between DOM and soil microorganisms remains unclear.In order to explore the effects of warming and nitrogen addition on the amount and composition of DOM and the structure of microbial communities in the soil of young subtropical Cunninghmia lanceolate plantations,total 30 experimental quadrats?2 m×2m?were set up in Chen Da,Sanming,Fujian Province,in 2014,including six treatments:control?CT,no warming and no nitrogen addition?,low nitrogen(LN,40 kg·N·hm^-1a^-1),high nitrogen(HN,80 kg·N·hm^-1a^-1),warming?W,+5°C?,warming+low nitrogen(WLN,+5°C+40 kg·N·hm^-1a^-1),warming+high nitrogen(WHN,+5°C+80 kg·N·hm^-1a^-1)with 5replicates in each treatment.In April 2018,soil samples were taken from 0-10 cm and10-20 cm soil layers in each sample quadrat to analyze the quantity and composition of DOM and microbial community structure of the soil.The results showed that compared with the dissolved organic carbon?DOC?concentration of CT(8.83 mg·kg^-1),the soil DOC concentration of all the treatments had a downward trends in 0-10 cm soil layer,among which LN,WLN and WHN decreased by 33.4%,37%and 31.3%,respectively;however,compared with the soil DOC concentration of CT(3.60 mg·kg^-1),the DOC concentration of each treatment increased in the 10-20 cm soil layer,and WHN increased by 49.4%.The results showed that warming and nitrogen addition treatments might enhance DOC leaching downward.In 0-10 cm soil layer,compared with the dissolved organic nitrogen?DON?concentration of CT(0.26 mg·kg^-1),the DON concentration of nitrogen addition treatment decreased,W increased 61.5%,WLN and WHN concentration increased slightly;the DON concentration of LN increased 57%,HN increased 35.3%,W increased100%,WLN and WHN increased significantly.The effects of warming,nitrogen and warming+nitrogen on the chemical structure of DOM are as follows:AI and HIX_synyn increased and E₂/E₃ decreased after treatment in0-20 cm soil layer,and W in 10-20 cm soil layer increased,indicating an increasing degree of aromatization and humification of soil associated with the high molecular weight and the complicated soil structure.The fluorescence peak A of aromatic substances and protein fluorescence peak B also increased after treatments,suggesting that microbial metabolism might be enhanced.FI showed that DOM derived from soil microorganisms in 0-20 cm soil layer after treatment had an increasing trend.Infrared spectra showed that the aliphatic substances derived from microorganisms in DOM increased after each treatment in 0-10 cm soil layer.Therefore,the combination of spectral techniques can reflect accurately that warming,nitrogen and warming+nitrogen will increase the compositions of DOM derived from microorganisms,leading to a more complex chemical structure of DOM.In 0-10 cm soil layer,nitrogen addition plays an important role in changing the microbial community structure,and different content of nitrogen addition caused different effects on the microbial community structure.Compared with CT microbial community?except ACT?in LN increased,while decreased in HN.Warming treatment resulted in decreases in all kinds of microbial community,except for GP.Therefore,we suggest that the structure of microbial community was affected by both warming and nitrogen addition.Cy:Pre ratio indicated that the most serious environmental stress occurs in the WLN.The results show that,the most important way to influence the structure of microbial community is to change the availability of nutrients in soil,which is in accordance with previous studies.The ratio of Iso:Ant increased significantly in the treatments of warming and warming+nitrogen addition,indicating that the microbial stress would be caused by warming at 5°C in this experimental area.Therefore,the influence of climate warming on soil microorganisms in subtropical forest should be paid more attention.In the 10-20 cm soil layer,compared with CT,LN made the microorganisms of each community structure decrease.The responses of warming and warming+nitrogen treatments were consistent.Compared with CT,except for GP and ACT of WLN,the microorganisms of other communities decreased significantly,indicating that warming and nitrogen addition would result in a significant suppression in soil microbial abundance in subtropical forests.The results from redundancy analysis showed that the chemical structure and quantity of DOM could make a better explanation for the variation of soil microbial community structure than traditional soil environmental factors?pH,C/N?.The effects of soil environmental factors on microbial community structure are as follows:in 0-10 cm soil layer,the first ordination axis is AI that explains 25.6%variation of soil microbial community structure.AI is the most important factors affecting soil microbial community structure.In 10-20 cm soil layer,E₂/E₃ value explains 17.0%variation,which is the main factors affecting soil microbial community structure.The values of AI and E₂/E₃represent the changes of macromolecular aromatic substances.Under each treatment,the increase of aromatic substances may decrease the PLFAs content of microbial communities.Therefore,the quantity,chemical structure and soil microbial community structure of soil DOM will change under the conditions of temperature increase and nitrogen application,and the change of aromatic substances in soil DOM has the greatest impact on the structure of microbial community.