Changes in Bacterial Diversity and Bacterial DNA Load in The Soil Beneath Deadwood of Various Tree Species Following Windthrow in Old-Growth Broadleaved Forests

Abstract Soil bacteria play a key role in the functioning of forest ecosystems by regulating many biochemical processes, such as transformation and mineralization of organic matter, nitrogen fixation, and carbon sequestration. Deadwood is an important element of forest ecosystems, but the impact of windfalls and deadwood on the diversity and structure of soil bacterial communities remains poorly understood. Accounting for the diversity of habitats associated with deadwood of different tree species allows assessing the spatial patterns of abundance, composition, and functional specificity of bacterial communities. The bacterial DNA load quantified by qPCR and the diversity and composition of soil bacteria in Podzols, Retisols, and Phaeozems of broadleaved forests were studied in areas of mass and single windthrows, under coarse woody debris of seven tree species, and in the background forests. Microbial community profiling by 16S rRNA sequencing was made for the first time in multispecies old-growth broadleaved forests demonstrating a high diversity of soil bacterial communities. The predominant bacterial groups were Alphaproteobacteria and Verrucomicrobiae and in Podzols, Planctomycetia as well. Soil texture was found to be the main factor influencing the bacterial DNA load as well as the diversity and composition of bacterial communities. The bacterial DNA load and bacterial diversity in loamy soils were higher than in sandy soils. The findings confirmed the hypothesis that the species of a fallen tree significantly affects the forest soil bacterial communities. At the same time, the structure of bacterial communities was quite conservative; the most pronounced changes were observed under fallen trees with specific wood properties (Quercus robur, Picea abies, and Acer platanoides). A substantial number of unique taxa were found in windthrow areas and beneath deadwood, accounting for more than a half of the total number of identified amplicon sequence variants (ASVs). It was concluded that both single and mass windthrows, as well as the simultaneous presence of deadwood of different tree species significantly increase the bacterial diversity of forest soils, thereby contributing to the functional sustainability of forest ecosystems.