| Changting,Fujian is a typical red soil erosion area in southern China,and ecological restoration is an important environmental problem that needs to be solved urgently.The process of natural restoration of vegetation should be followed in the process of ecological restoration,otherwise it is difficult to achieve the effect of restoration,and it is easy to degrade or even deteriorate.The invasion of pioneer plants is the first step of vegetation restoration in degraded land,and its adaptation and change to the environment is of great significance to the process of ecological restoration.D.dichotoma is a pioneer plant for vegetation restoration in eroded degraded red soil.It can invade the degraded bare land first and finally withdraw from the ecosystem.D.dichotoma still grows dominantly under the condition that the environment such as water,nutrients and light has changed greatly in different vegetation restoration stages,which inevitably contains its response mechanism to different stress.In view of this,the pioneer plant D.dichotoma in ecological restoration of red soil degraded land was taken as the research object.The indoor control simulated stress test was used to set different gradients of water,nutrient and light intensity treatments.The morphological indexes such as growth,tillering,leaf traits,root architecture and microstructure of D.dichotoma under different stress treatments,physiological indexes such as photosynthesis,chlorophyll fluorescence characteristics and antioxidant enzymes,as well as rhizosphere soil nutrient content and microbial indexes were measured.The nitrogen isotope tracer method was used to analyze the nutrient transfer process and law in the rhizome expansion process of D.dichotoma,and the differences in morphological and physiological characteristics of D.dichotoma under different stresses were compared.Based on the indexes of morphology,physiology and rhizosphere soil microorganisms,the morphological and physiological responses of D.dichotoma to different stress were analyzed by partial least squares path model.Finally,the internal mechanism of D.dichotoma adapting to different stress in the process of vegetation restoration in degraded red soil was revealed.The main results are as follows:(1)With the increase of drought stress intensity,the total root length,total root surface area,spacer length,root canal cell density and stem cross-sectional area of D.dichotoma showed a significant upward trend(P<0.05),while plant height,number of new ramets,leaf area,root diameter,root cross-sectional area,root canal cell area and stem sieve area showed a significant downward trend(P<0.05).With the increase of nutrient stress intensity,the spacer length,specific leaf area,total root length and root tracheid area of D.dichotoma increased significantly(P<0.05),while plant height,leaf length,leaf width,leaf area,number of new ramets,leaf dry matter content,leaf tissue density,root cross-sectional area,tracheid density,stem cross-sectional area and leaf vascular bundle area decreased significantly(P<0.05).With the decrease of light intensity,the plant height,spacer length,leaf area and specific leaf area,total root length,total root surface area,root canal cell density and leaf vascular bundle area of D.dichotoma showed a significant upward trend(P<0.05),while leaf thickness,root cross-sectional area,root canal cell area and stem cross-sectional area showed a significant downward trend(P<0.05).Them show that there are some differences in the morphological response of D.dichotoma to different stress.(2)With the increase of drought stress intensity,the distribution of underground biomass of D.dichotoma increased,the root-shoot ratio,root phosphorus and potassium content increased significantly(P<0.05),and the carbon content of leaves and stems decreased.With the increase of nutrient stress intensity,the distribution of aboveground D.dichotoma decreased.The contents of phosphorus and potassium in roots showed an upward trend,the nutrient content in stems showed an upward trend,the potassium content in leaves showed an upward trend,and the carbon content showed a downward trend.With the decrease of light gradient,the total biomass,total leaf weight and root-shoot ratio of D.dichotoma increased significantly(P<0.05),while the root carbon and nitrogen contents decreased.(3)With the increase of drought and nutrient stress intensity,the activities of SOD,POD and CAT in leaves of D.dichotoma increased significantly(P<0.05),while transpiration rate(Tr)and stomatal conductance(Gs)decreased significantly(P<0.05).There was no significant difference in antioxidant enzyme activity in leaves of D.dichotoma under different shading treatments(P>0.05).With the decrease of light intensity,stomatal conductance(Gs),transpiration rate(Tr),intercellular carbon dioxide concentration(Ci),Fo and Fm increased,and net photosynthetic rate increased significantly(P<0.05).(4)The nitrogen isotope tracer test showed that the two-way transfer of nutrients could be carried out between different ramets of D.dichotoma,and there was no significant difference between the forward and reverse transfer ratios(P>0.05).The nutrient transfer ratio increased significantly with the increase of nutrient concentration(P<0.05).The distribution ratio of root nutrients to D.dichotoma under different drought,nutrient and shading treatments was significantly higher than that of leaves(P<0.05).Under drought and light stress,the roots and leaves of D.dichotoma had a preference for nitrate nitrogen absorption.(5)The relative abundance of Proteobacteria in the rhizosphere soil of D.dichotoma decreased significantly with the increase of drought stress intensity(P<0.05).The relative abundance of Basidiomycetes,Agaricales and Archaeales increased significantly(P<0.05),while the relative abundance of Ascomycota decreased significantly(P<0.05).Available potassium and nitrate nitrogen had a positive effect on Planctomycetes,Gemmatimonadetes and γ-Proteobacteria,and had an inhibitory effect on Mucor,Glomeromycota,Tremella and Eurotium.The effects of different nutrient stresses on the abundance of bacterial and fungal species in rhizosphere soil did not reach a significant level(P>0.05).Under nutrient stress,carbon and organic matter had positive effects on soil bacteria Patella and Acidobacteria,and available potassium,ammonium nitrogen and nitrate nitrogen had positive effects on soil fungi Basidiomycota,Copromycota and Agaricomycetes.With the decrease of light intensity,the relative abundance of bacterial Proteobacteria and fungal Tremella decreased significantly(P<0.05).The contents of ammonium nitrogen and nitrate nitrogen had positive effects on soil bacterial Acidobacteria,Bacteroidetes,Planctomycetes,Fibrobacteres and Verrucomicrobia.The contents of organic matter,phosphorus and potassium had positive effects on soil fungi Mortierella,Mucor,Glomeromycota,Tremella and Mortierella.(6)Based on morphological,physiological and rhizosphere soil microbial indicators,partial least squares path model analysis showed that under different drought,nutrient and light stress,D.dichotoma increased the spacer distance,reduced the number of new ramets,and nutrient transfer between ramets was an important way for D.dichotoma to adapt to different stress.Under drought and nutrient stress,D.dichotoma adapted to stress by reducing aboveground biomass investment and increasing underground biomass investment,such as increasing fine root length and root surface area,and increasing efficient use of soil moisture.Under low light stress,D.dichotoma can also increase plant height upward growth,increase leaf area and net photosynthetic rate,improve the efficient use of limited light resources,and adapt to adversity stress. |
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