Evolution Characteristics And Formation Mechanism Of Soil Microbial Communities In Burned Areas Of Dahurian Larch Forests

Responses of soil to fire play key roles in clarifying the response of forest ecosystem to fire disturbance,they are also important contents of studying the underlying mechanism responsible for the ecological recovery of post-fire forests.Soil physical and chemical properties and microbial communities play key roles in ecological restoration at the landscape scale and are regulated by a complex interplay between abiotic and biological factors.This study aimed to identify the potential drivers for the changes in the physical and chemical properties and microbial communities of mineral soils in the Larix gmelinii forest of northeast China,to analyze the potential mechanism involved in soil microbial community responses to fire disturbances,and to provide a reference for understanding and predicting the boreal ecosystem response after fire disturbance.In this study,a fire chronosequence from 1900 to 2018 with 80 plots from 12 burned areas over a small region of the Greater Khingan Mountains was established;the study plots were selected through a supplementary line of the site condition,and that all study plots in different site condition are at the same small spatial scale.Thus,these ensured that this research applied a space-for-time substitution method.At each plot,topographic factors(slope gradient,slope aspect,slope position,altitude),arboreal variables(species,density,diameter at breast height,biomass)were measured,the time since fire was recorded,and 0-10 cm soil samples were collected.By quantifying the influencing factors of physical and chemical attributes and microbial communities of surface mineral soil.the correlations and path integrations of the time since fire,site condition,vegetation,soil habitat factors,water-soluble organic matter(WSOM),eco-enzymatic stoichiometric characteristics,microbial communities were clarified,the potential evolution characteristics and formation mechanism of the soil microbial communities in the burned areas of Dahurian larch forests were explored.The main results obtained are as follows:(1)Fire disturbance affects the importance of the drivers of soil habitatFire disturbance can change the importance and relative importance of biotic and abiotic variables on soil habitat factors in boreal larch forests.Time since fire and altitude are important ecological predictors of soil habitat factors,and their positive effects are far greater than negative effects,especially in terms of soil water holding capacity,organic matter and inorganic nitrogen concentration.The influences of slope position and slope gradient are greater than that of slope aspect,and the three have smaller positive influences and show similar patterns.The steeper,the more uphill,and the more facing sun the slopes are,the more barren soil is likely to appear.The relationship between average DBH and soil habitat factors.which are closer than that of arbor biomass and individual density,was obviously changed by fire disturbance,but the importance of DBH is not significantly changed.Although the site condition is more important than the arbor for the changes of post-fire soil habitat factors,with the progress of the post-fire succession,the importance of the arbor will gradually increase and ultimately exceed the importance of the site condition.(2)Fire disturbance,site condition and vegetation jointly determine the characteristics of soil WSOMFire disturbance can cause significant changes in the composition of soil WSOM in Dahurian larch forests.Nonetheless,WSOM derived from plants and soil organic matter is still the main source of WSOM in post-fire soil.Although its proportion has declined after fire disturbance,it will gradually increase with time since fire.Fire disturbance can increase the importance and relative importance of abiotic variables driving the variation of soil WSOM.The average molecular mass,stability and humification of WSOM will also gradually increase with the increase with time since fire,altitude,and slope gradient.The plastein and bioavailability of soil WSOM in the burned areas,which will restore to the pre-fire level within 28 years after the fire,are closely related to the arbors.Nevertheless,the degree of explanation and importance of the arboreal variable will be reduced by fire disturbance,and the mathematical correlation with soil WSOM characteristics will also be reversed.Although the importance of abiotic variables will gradually decrease with the time since fire,site conditions are still important driving factors for soil WSOM,especially altitude and slope gradient.(3)Fire disturbance,site condition and vegetation jointly determine the soil eco-enzymatic stoichiometric characteristicsFire disturbance can cause significant changes in the soil eco-enzymatic stoichiometric characteristics in Dahurian larch forests.Among them,soil carbon,nitrogen and phosphorus acquiring enzyme activities exhibited similar patterns,and their activities continued to decline over time for several years and then gradually rose,showed V-shaped change trends within 10 years since fire.In the early post-fire successional stage,soil microorganisms are highly restricted by carbon,and the relative restriction by nitrogen will also increase and last for decades.Slope gradient has significant impacts on soil carbon acquiring enzyme activities and microbial carbon,nitrogen and phosphorus nutrient limitations.Moreover,slope aspect and altitude are important ecological predictors of the soil eco-enzymatic stoichiometry in burned areas,but slope position is the least important topographic factor.The importance of arbor on soil microbial carbon,nitrogen and phosphorus nutrient limitations will be increased by fire disturbance,but arboreal importance to changes in extracellular enzyme activities will be reduced.The importance of altitude and slope aspect can be significantly enhanced by fire disturbance,while the importance of slope position will not be changed significantly.Fire disturbance will result in changes in the interpretation and prediction capabilities of the influencing factors of soil eco-enzymatic stoichiometric characteristics.Among them,the interpretation and relative importance of arboreal variables will be enhanced significantly along with post-fire succession.(4)Fire disturbance and site condition indirectly affect the soil microbial community through vegetation and soil physical and chemical propertiesThe absolute abundances of various soil microbial communities in Dahurian larch forests are mostly positively correlated with the time since fire.They decrease rapidly after fire disturbance,and show gradual upward trends within 12 years since fire,then they gradually decrease but the variation level increases,while the community composition structure has no obvious temporal pattern.Fire disturbance will significantly change the relationship between soil microorganisms and their influencing factors,and affect the abilities of biotic and abiotic variables to regulate and predict microbial absolute abundance and community composition.The soil microbial communities in the burned plots are not completely different from the control plot,and there may still be a high similarity.Altitude and slope gradient had obvious effects on soil microbial communities.Even in the burned plots,they were still the most powerful site conditions for predicting and explaining changes in soil microbial communities.Arboreal variables were always in close contact with changes in the microbial community,they w ere important biological predictors of the soil microbial community in the burned area though fire disturbance could change or reverse their effect on the microbial community to a certain extent.The soil habitat factor was the most obvious latent variable that directly affected the microbial community in the burned area,especially the impact on the microbial community composition.Among them,the soil organic matter content and water content well explained the changes in soil microbial biomass and were also important ecological predictors for the changes in microbial community.Although the ability of soil WSOM to explain or predict the changes in soil microbial community in burned areas was relatively weak,it had strong and significant direct impacts on the microbial community.Compared with other latent variables,the soil eco-enzymatic stoichiometric characteristics could explain the changes in microbial communities well.Moreover,they,especially sulfur acquiring enzyme activity,were also very important ecological predictors of microbial communities,while they had no significant direct impact on the microbial absolute abundance.In addition,changes in the characteristics of postfire soil microbial communities had different responses to various latent variables,and the impact of each latent variable was irreplaceable.Collecting more relevant variables may reveal or predict them more accurately.In summary,fire disturbance will cause more complicated changes in soil habitat factors,WSOM,eco-enzymatic stoichiometry and microbial communities,and they may take more than 28 years to fully restore to the pre-fire level.Fire disturbance can reduce the effect of arbors on the soil,but it will increase the impact of topographical factors on the soil.The importance of arbors will gradually increase with the time since fire,while the importance of topography will gradually decrease,and the relative importance of arbors and topography will gradually change,but topography has always been an important controlling factor.Consequently,wildfires can cause an on-going shift in the importance and the relative importance of topography and arbors on soil in boreal larch forest.Although the time since fire and topography have no significant direct impact on the soil microbial community,it has a significant direct impact on the arbors,soil habitats factors,WSOM,and extracellular enzymes that can directly affect the soil microbial community.Therefore,they are also key factors in the interpretation and prediction of changes in soil microbial community.Therefore,artificial intervention similar to the post-fire vegetation succession may be a powerful means to promote the post-fire soil ecosystem restoration,while artificial intervention that is contrary to the postfire vegetation succession is likely to delay the recovery rate.Overall,this work highlights that discriminating between the effects of the abiotic and biotic factors is necessary for better comprehending the long-term changes that occur in the post-fire ecosystem and anticipating the response of the boreal forest to wildfire and global change.