| Soil supports enormous microbial quantities and diversities,driving biogeochemical cycle of key elements across all scales.The extremely high diversity of soil microbial communities is largely owing to the complexity and heterogeneity of soil habitats on the micro-scale.With climate warming and reduced precipitation in middle and low latitudes,subtropical forests are likely to face increasingly severe arid climate in the future,which will profoundly impact soil microorganisms,and the related ecosystem functions and services.Although soil microbial community structures and the assembling mechanisms have been extensively studied at the macro-scale in two decades,up to now,we have not yet understood the mechanisms of the organization,distribution and assembly of soil microbial communities at the micro scale.It remains not answered how soil microbial communities respond to climate change at the micro-level,and how it may affect the macro-scale diversity distribution patterns and ecosystem functions.This study took the subtropical evergreen broad-leaved forest soils as the research object.First,at the local scale and based on the 20-hectare forest dynamics plot in Tiantong Mountain,the distribution patterns of microbial diversity was analyzed over the range of a few meters to several hundred meters;Next,based on the extreme drought experimental platform in Tiantong Mountain,the microbial diversities within individual soil aggregates(micrometer to millimeter)were explored,combined with the micron-scale soil pore geometry obtained by synchrotron-radiation-based X-ray microtomography(SR-μCT),to analyze the soil microbial community assembly mechanisms and the response to climate change at the micro-scale.Therefore,this study jointly discussed the soil microbial community diversities and their assembly mechanisms at both the macro-and micro-scales.The main findings are as follows:(1)There was a significant distance decay pattern for soil microbes on the macro-scale in this subtropical forest,hence,the microbial communities had significant turnover within a local scale of several hundred meters.This was mainly driven by geographical distances,with weak influence from soil physical-chemical properties.No significant correlation between soil physical-chemical properties and soil microbial diversity was found.(2)There existed rich microbial diversity within individual aggregates,and the diversities were similar in aggregates with different sizes(0.25-8 mm).Heterogeneity among microbial communities of individual aggregates was at a similar level with that among different communities at the macro-scale,and was mainly driven by the structure of abundant taxa.The microbial composition in smallest soil aggregates(0.25-1 mm)was the most distinct.At the micro-scale,microbial community structure was largely determined by characteristics of soil pore geometry,with weak influence from soil carbon and nitrogen contents.(3)At the micro-scale of individual soil aggregates,the diversity distribution of microbial communities had a species-weight relationship similar to the species-area relationship at the macro-scale.In other words,the microbial diversity increased with increasing aggregate mass,and conformed to the S=c W~z model(S refers to the number of microbial taxonomic units,i.e.OTUs;W is the mass of soil aggregates;c and z are the fitting constants).(4)Extreme drought resulted in reduced structural stability of soil aggregates,with significantly increased porosity and connectivity,and altered pore size distributions,which would affect carbon sequestration by soil aggregates and the internal microbial communities.There were no significant correlations between soil pore geometry parameters and soil total carbon or total nitrogen contents.(5)Drought significantly reduced microbialα-diversity in all aggregates.In large macroaggregates,microbialα-diversity was positively related to the total carbon and total nitrogen contents,indicating that microbial diversity was restricted by resource availability under drought.In large macroaagregates,drought also significantly changed theβ-diversity of the microbial communities,with increased heterogeneity,altered composition of rare taxa,and significantly reduced dispersal rates.These changes could be largely attributed to those in the physical structure of the soils under drought.In conclusion,this study compared microbial community distribution patterns and influencing factors at macro-and micro-scales,and suggested that elucidating the heterogeneity and assembly mechanisms of micro-scale soil microbial communities is the basis for understanding those at the macro-scale.In addition,the effect of extreme drought on soil microbial community structures was not only due to changes in soil chemical properties including nutrient availabiltiy,but also and to a greater extent,due to changes in soil spatial structure.Therefore,in-depth study of soil microbial communities at the micro-scale will facilitate better understanding of the formation and maintenance mechanisms of microbial diversities,as well as better predicting of their response to climate change. |
