| Soil bacteria are the most widely distributed and abundant group of soil microorganisms in the soil,accounting for 70-90%.Bacteria are the most active in the soil due to their small size,strong metabolism,rapid reproduction and large surface area in contact with the soil.They play a major role in the decomposition of organic residues.Various bacterial physiological groups exist in the soil and play an important role in the soil carbon,nitrogen,sulfur and phosphorus cycles.Therefore,understanding the distribution pattern of soil bacterial communities and their driving factors can help to understand the structure and function of soil ecosystems.The elevational distribution patterns of soil bacterial communities and their drivers in the context of climate change have been the subject of interdisciplinary research in the context of climate change.An in-depth study of the spatial pattern changes of soil bacterial communities in forest ecosystems and their dynamics is essential for a deeper understanding of the function of soil bacterial community patterns and dynamics in forest ecosystems and their potential physiological and ecological mechanisms.In this study,the spatial distribution patterns of soil bacterial communities in different stand types and soil depths along the elevation gradient were investigated,and the main driving factors affecting their community distribution were explored.Meanwhile,the regeneration of Larix principis-rupprechtii Mayr.was used as an example to reveal the influence mechanisms of diversity,community composition,and species interactions on stand regeneration.The main findings of the study are as follows:(1)Characteristics of changes in stand and light factors along the elevation gradient in forests of L.principis-rupprechtii Mayr.The importance values and diversity indices of different herbs and shrubs varied among different stand types in response to the elevation gradient.The importance values of herbs and shrubs in larch forests showed a "single hump","double hump" or irregular trend with increasing elevation.Among them,Poa annua L.,Fragaria orientalis Losinsk.,Carex duriuscula subsp.stenophylloides,and Rosa sp.,Spiraea salicifolia L.were distributed in all elevation gradients and were the main herbaceous and shrub species in the study site.Herbaceous diversity,the density of regenerative individuals,litter thickness and accumulation,and canopy opening in both stand types reached the highest values in the medium(1950m-2050m)and high(2150m-2250m)elevation regions,while the shrub diversity index showed a decreasing trend with increasing elevation.The herbaceous and shrub diversity indices tended to be higher under the mixed larch-birch stands compared to the pure stands.(2)Characteristics of changes in soil physicochemical factors and soil enzyme activities in larch forests along an elevation gradient.The effect of altitude gradient on soil physicochemical factors was more significant than that of stand type and soil depth.Elevation gradient had significant effects on the contents of maximum soil water holding capacity(SMax W),minimum water holding capacity(SMin W),soil bulk density(BD),soil organic carbon(SOC),and soil organic matter(SOM);stand type had significant effects on the contents of soil temperature(ST)and soil alkaline decomposition nitrogen(AN),among which,the ST and AN contents were significantly higher in pure stands than in mixed larch-birch stands;Soil depth had significant effects on ST,SOC,SOM,and soil fast-acting potassium(AK)contents,where ST and AK contents were significantly higher at 0-10 cm soil depth than at 10-20 cm soil depth.Compared to the low and middle elevation regions,higher contents of SOC,SOM,and AN were present in the 0-10 cm layer of soils in pure and mixed larch-birch stands at higher elevations,and similar trends were not evident in the 10-20 cm layer.The interaction of altitude,stand type and soil layer had a significant effect(P<0.05)on the contents of SOC and SOM,where the interaction of altitude and soil layer had a more significant effect(P<0.001)on the contents of both than the interaction of stand type and soil layer(P<0.05).The effects of both elevation and interaction with stand type on the activities of soil enzymes(including β-Dribofuranoside hydrolase,urease and alkaline phosphatase)were significant,and the effect of elevation(P<0.001)on soil enzyme activities was more significant than that of stand type(P<0.05).SMax W,ST and fast-acting phosphorus(AP)were the main factors affecting soil enzyme activities in larch understory.(3)Elevational distribution patterns and driving factors of soil bacterial communities in larch forests.Along the altitudinal gradient,Proteobacteria(29.53%),Actinobacteria(22.06%),Acidobacteria(18.96%)and Chloroflexi(13.43%)were the dominant bacterial phyla,Alphaproteobacteria Alphaproteobacteria(16.6%),Gammaproteobacteria(9.45%),Actinobacteria(9.21%)were the dominant bacterial classes,with significant differences in the relative abundance of elevation to the dominant phylum and classes.Overall,the α-diversity index of soil bacteria in both stand types decreased with increasing elevation.The effect of stand type(P<0.001)on β-diversity of soil bacterial communities was stronger than that of altitude(P=0.504)and depth of soil layer(P=0.37).Stand types had the greatest effect on soil bacterial community soil bacterial community β diversity and key phylotypes of soil microbiota,followed by elevation.Forest stand type had the greatest degree of influence on key system types of soil microbiota,followed by elevation.Soil bacteria were more complex in a mixed larch-birch forest in northern China.SOM,AN and TP contents in soil,Udir,and Dthick were the key factors driving the distribution of soil bacterial communities in the understory of larch.(4)Mechanisms affecting the distribution of soil bacterial communities at different elevations on natural regeneration of larch stands.The bacterial communities that contributed more than 5% to stand regeneration(phylum level)were ranked as Nitrospirae > Latescibacteria > Acidobacteria > Actinobacteria > Patescibacteria > Entotheonellaeota > Dependentiae.Metabolism was the core functional category of soil bacterial communities in larch forests.The results of structural equation modeling indicated that soil bacterial community composition and altitude had positive effects on stand regeneration with total effects of 0.84 and 0.39,respectively.The total effects of soil nutrients,litter thickness and herbaceous diversity on stand regeneration were negative with total effects of-0.87,-0.12 and-0.12,respectively.The amount of stand regeneration significantly reduced the avg K of the soil bacterial community with a standardized path coefficient of-0.70.Soil nutrients(the main factors included AN,SOM and PHO)had a significant negative effect on soil bacterial community composition with a standardized path coefficient of-0.70.The topological complexity of the soil bacterial community co-occurrence network was lowest in the highly regenerated larch stands,but the proportion of key species was highest. |
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