| As an important medium for the rapid exchange and dissemination of materials,energy and key information in the key interface between plants,root secretions play an extremely important ecological role in regulating the key ecology of forest soils,and have become an important topic in the study of forest underground ecology under current global changes.In recent years,with the continuous increase of atmospheric nitrogen(N)sedimentation content,the changes in soil N nutrient effectiveness caused by exogenous activity C input have profoundly affected the life activities of forest roots and their associated rhizosphere soil nutrient cycling processes,and further caused feedback processes of forest community structure and ecological function.However,there are still many uncertainties in the under standing of the characteristics of forest life activity pattern changes under the background of N sedimentation and the process of forest soil nutrient cycle change mediated by it,especially under the field in situ conditions,which to a certain extent seriously limits the in-depth under standing and scientific systematic under standing of the forest root-soil nutrient interaction process and its ecological importance under the influence of global change.To this end,based on the long-term simulated N sedimentation forest test plot established by the research group in Tangjiahe National Nature Reserve,taking the typical red birch forest(Betula albosinensis Burk)as the test subject,three horizontal N addition treatments(low N:25 Kg N/hm2·a-1:high N:50 Kg N/hm2·a-1:control:0 Kg N/hm·a-1)were collected in 2020 in the early(June),mid-term(August)and late(October)growth season.The changes in the C-N transformation process and related microbial metabolic activity characteristics of rhizosphere soil under different levels of B addition treatment were also analyzed indoors,and the changes in the characteristics of microbial metabolic activity of rhizospliere soil under different levels of N addition treatment were also determined to reveal the changes in the input characteristics of root secretions of birch forest and the response rules of the microbial process of soil nutrient transformation mediated by it to different N addition treatments.The main findings of this paper are as follows:1.The responses of different simulated N sedimentation treatments to the C input rate and annual flux input of root secretions of the birch forest differed.Overall,exogenous N addition led to a significant decrease in the C input rate and annual C input flux of birch root secretions,and the inhibition effect of high N addition treatment on the C input flux of birch root secretions was more obvious than that of low N.Specifically,compared with the control(CK),the low N and high N addition treatments reduced the root secretion rate(μg·g-1·h-1)of the birch forest by 16.62%and 36.86%,respectively,and the secretion rate of the unit root long root system(μg·cm-1·h-1)of the red birch forest was reduced by 37.50%and 50.00%,respectively,and the root secretion rate per unit root surface area of the birch forest(μg·cm2·h-1)was significantly reduced by 40.48%and 54.76%,respectively.In addition,in addition to significantly reducing the rate of birch root secretions,N addition also significantly reduced fine root biomass.Correspondingly,in terms of control,the annual C input flux of root secretions under exogenous N addition decreased significantly,and the annual C input flux(g·m-2·a-1)of root exudates of birch forest was reduced by 14.66%and 49.3 1%,respectively,by low N and high N addition treatments.2.Differences in the responses of rhizosphere soil C-N transformation processes to different simulated N sedimentation treatments in birchforest.Both the low N and high N addition treatments had a strong impact on the size of the rhizosphere effect of the nutrient conversion microbial process in the red birch soil,and the high N addition treatment had a stronger inhibitory effect on the nutrient conversion microbial process of the rhizosphere soil of the birch forest compared with the low N treatment.Specifically,compared with the control(CK),the low N and high N addition treatments reduced the rhizosphere effect values of soil carbon(C)mineralization rate in birch forest by 37.03%and 47.78%,respectively,while the rhizosphere effect values of soil nitrogen(N)mineralization rate in birch forest were reduced by 13.89%and 27.04%,respectively.3.The magnitude of rhizosphere effects of soil microbial parameters in the red birch forest differed in response to different simulated N sedimentation treatments.Similar to the response of the C-N transformation process to the N treatment in rhizosphere soils,the rhizosphere effect size and intensity of microbial parameters(MBC,MBN,BG,NAG,PPO,PER)closely related to soil nutrient transformation in birch forests were significantly reduced by both higli N addition treatments,and the extracellular enzyme characteristic parameters of these microorganisms were significantly inhibited under the high N addition treatment,and the low N addition treatment was about 2 times that of the higli N addition treatment.Specifically,the rhizosphere effectivity of soil microbial biomass C(MBC)in the low N and high N addition treatments decreased by 33.14%and 50.53%,respectively,compared with the control(CK),and the rhizosphere effectivity value of soil microbial biomass N(MBN)was reduced by 30.38%and 34.80%,respectively,and the rhizosphere effection value of β-D-glucosidase(BG)was reduced by 23.72%and 28.36%,respectively.The rhizosphere effectivity values of β-N-acetyl glucosamine glycosidase(NAG)were reduced by 18.66%and 38.29%,respectively,the rhizosphere effectivity values of polyphenol oxidase(PPO)were reduced by 23.20%and 38.36%,respectively,and the radial effect values of peroxidase(PER)were reduced by 14.90%and 9.83%,respectively.4.Coupling between root secretions of birch forests under N sedimentation and microbial processes of nutrient conversion in rhizosphere soil.The results of linear regression analysis showed that the C input flux of root secretions of birch forest under several N-addition treatments had a significant positive effect on the microbial process of nutrient conversion in rhizosphere soil(P<0.05),indicating that there was a close coupling effect between the core units of rhizosphere ecosystem(root-soil-micro organism).Overall,exogenous N addition reduced the C input flux of root secretions in birch forests and drove lower rhizosphere soil microbial activity and weaker soil nutrient conversion excitation intensity.This result provides strong experimental evidence to confirm that root secretions are important ecological factors that profoundly regulate the interaction process of plant-soil-microbial core units in changing environments.In summary,the exogenous N addition led to a significant decrease in the C input flux of root secretions of birch forest,and then had a significant consistent effect on its mediated rhizosphere microbial activity and its associated soil nutrient conversion excitation process,and with the increase of exogenous N addition level,the inhibitory effect of N addition on the microbial transformation process of rhizosphere soil nutrients further enhanced.It is speculated that the possible reasons for this phenomenon are as follows:exogenous N addition alleviates the growth N nutrient restriction of trees in the red birch forest in this area to a certain extent,and the root secretion is used as an energy input,and the root C source input is reduced by self-physiological regulation under the condition that the nutrient restriction is effectively alleviated,that is,the nutrient adaptation strategy of "low C input-low N return" is adopted under N sedimentation.This hypothesis is also strongly corroborated by the fact that the increase in exogenous N levels drives the intensity of the excitation effect of the microbial process of transforming weaker rhizosphere nutrients.In short,this study provides strong experimental evidence for the close coupling effect of the core units of rhizosphere ecosystems(root-soil-micro organisms)under global change,enriches and expands the scientific understanding of the forest root-soil-microbial interaction process and its ecological connotation under global change from the unique perspective of the rhizosphere,and also provides theoretical guidance for the adaptive management and ecological restoration practice of climate change in the alpine forests of Southwest China under global change. |
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