Characteristics Of Root Exudates And Their Mediated C、N Transformation Of Cupressus Funebris Plantation In The Hilly Area Of Central Sichuan Under Nitrogen Deposition

Root exudates,as an important medium for regulating the energy cycle of plant root-soil interface materials,play a key role in the function of forest soil and the process of soil biogeochemical cycling,and have become the current hotspot and frontier of underground ecology research.In recent years,with the increasing nitrogen（N）deposition,forest root C input and rhizosphere soil nutrient transformation mediated by it are deeply influenced by root activity and mycorrhizal characteristics,and deeply fed back to the stability of forest ecosystem community structure and ecological function.However,there is a lack of in-situ research on the change of forest root exudate input characteristics and the process of soil nutrient transformation mediated by it,which limits the in-depth understanding of forest rhizosphere ecological process and its theoretical system under the background of global change to a certain extent.Therefore,this study took Cupressus funebris Endl artificial forest,which widely distributed in the hilly area of Central Sichuan Basin as the research object.Simulate the atmospheric N deposition by N addition at different levels（low N:25 kg N/hm²·a;High N:50 kg N/hm²·a;l Control:0 kg N/hm²·a）.In 2020,we collected root exudates and rhizosphere/non-rhizosphere soils in the growing season（May,August and October）and non-growing season（January and December）.And analyzed the C input rate and flux of root exudates,the C/N transformation process and the microbial characteristics in rhizosphere soils.The main results of this paper are as follows:1.The response of cypress plantation root exudates C input rate and annual flux to different N treatments was different.The addition of N significantly reduced the C input rate and annual flux of cypress plantation root exudates,and the high N treatment had a stronger inhibitory effect on the C input of root exudates.Specifically,compared with the control,the low-N and high-N treatments reduced the rate of root exudation per root biomass(μg·g^-1·h^-1)of the cypress plantation by 14.10%and28.35%（P<0.05）;the low-N and high-N treatments reduced the rate of root exudation rate per root length(μg·cm^-1·h^-1)decreased by 13.12%and 27.29%（P<0.05）;the low-N and high-N treatments reduced the rate of root exudation rate per root surface area(μg·cm^-2·h^-1)decreased by 15.25%And 29.78%（P<0.05）.Compared with the control,the low-N and high-N treatments reduced the annual C input flux(g·m^–2·a^-1)of the root exudates of the cypress plantation by 10.15%and 30.64%（P<0.05）.In addition,the inhibitory effect of N treatment on the C input of root exudates was more obvious in the growing season than in the non-growing season.2.Effects of different nitrogen treatments on C、N process in rhizosphere soil.N addition significantly inhibited the rhizosphere effect of soil C、N transformation process of Cupressus funebris artificial forest,and the inhibition effect of high N treatment on C、N transformation process was stronger than that of low N treatment.Specifically,compared with the control,low N and high N treatments reduced the rhizosphere effect of soil C mineralization rate by 7.42%and 21.47%（P<0.05）,while the rhizosphere effect of soil N mineralization rate have decreased by 9.09%and23.00%（P<0.05）under low N and high N treatments.Similarly,N addition significantly reduced the rhizosphere effect of inorganic N in rhizosphere.Compared with the control,low N and high N treatments reduced the rhizosphere effect of NH₄⁺-N by 6.08%and20.88%（P<0.05）,while the rhizosphere effect of NO₃^--N have decreased by 7.05%and20.69%（P<0.05）under low N and high N treatments.3.The rhizosphere effects of different N treatments on soil microbial characteristics were different.Similarly,N addition significantly inhibited the rhizosphere effects of soil microbial biomass C and N（MBC,MBN）,β-D-glucosidase（BG）,β-N-acetylglucosamine glycosidase（NAG）,polyphenol oxidase（PPO）,and peroxidase（PER）in Cupressus funebris artificial forest.The inhibitory effect of high N treatment on the above rhizosphere microbial characteristics is more intense than that of low N treatment,and the former is about 2-3 times of the latter.Specifically,compared with the control treatment,the rhizosphere effect of MBC decreased by 13.43%and30.03%respectively under low N and high N treatments.The rhizosphere effect of MBN decreased by 16.28%and 31.08%（P<0.05）respectively.The rhizosphere effect of BG decreased by 8.77%and 26.22%（P<0.05）respectively.The rhizosphere effect of NAG decreased by 11.33%and 21.28%（P<0.05）respectively.The rhizosphere effect of PPO decreased by 12.02%and 42.39%（P<0.05）respectively.The rhizosphere effect of PER decreased by 24.80%and 42.50%（P<0.05）respectively.4.Coupling effect between C input from root exudates and microbial transformation of C、N in rhizosphere of Cupressus funebris artificial forest under N treatment.Under different N treatments,the C input rate of root exudates of Cupressus funebris artificial forest showed a significant positive correlation with the microbial process of C、N transformation in rhizosphere soil（P<0.05）.That is,with the addition of exogenous N,Cupressus funebris artificial forest reduced the input of C from root exudates,and induced lower extracellular enzyme activity in rhizosphere soil and excitation effect intensity of soil C、N transformation.The results also confirmed that root exudate C input is an important regulatory factor driving the change of microbial process of C、N transformation in forest rhizosphere soil.In summary,N addition significantly reduced the amount of C input from root exudates of Cupressus funebris artificial forest,and inhibited the activity of rhizosphere soil microorganisms and the related excitation process of soil C、N transformation,and the inhibition intensity of high N treatment on the above process was significantly higher than that of low N treatment.The above results indicated that different N levels had similar effects on the microbial process of soil C、N transformation（both inhibition effects）but different effects（inhibition intensities were different）,which provided experimental evidence for confirming that the coupling effect between root-soil-microbial core units was deeply regulated by N deposition.This study enriches the scientific understanding of soil biogeochemical cycle process of plantation in hilly area of central Sichuan under N deposition from the perspective of rhizosphere,and also provides theoretical guidance for vegetation restoration and adaptive management of large-scale plantation in hilly area of central Sichuan to cope with global climate change.