Influence of understory vegetation on soil bacterial communities and nitrogen cycling gene abundance in cool-temperate and sub-alpine forests along an elevational gradient

Abstract

Understanding how the community structures and functions of soil microbes respond to environmental changes is essential for predicting the dynamics of ecosystem functions. Elevational gradients are an ideal setting for “natural experiments” on the relationships between soil microbes and environmental factors. However, little is known about elevational patterns of soil microbial functions and their interactions with aboveground vegetation. We examined community structures and nitrogen cycling gene abundance of soil bacteria in 20 vegetation survey quadrats in cool-temperate and sub-alpine forests along an elevational gradient (1050–2000 m above sea level) in central Japan. The diversity of soil bacterial communities was investigated based on terminal restriction fragment length polymorphisms (T-RFLP) in the 16S rRNA gene. The abundance of soil bacteria and key functional groups involved in nitrogen cycling was determined by quantifying the 16S rRNA and bacterial genes related to nitrogen fixation (nifH), ammonia oxidation (amoA), and denitrification (nosZ). The terminal restriction fragment (T-RF) richness of soil bacteria showed no relationship with elevation, but was negatively associated with soil nitrate-nitrogen and the species richness of understory vegetation. The abundance of all bacteria and nitrogen functional groups showed hump-shaped relationships with elevation, and was positively associated with soil water content and understory stem density. These results highlight that community structures and nitrogen cycling gene abundance of soil microbes along the elevational gradient can be explained by soil properties associated with understory vegetation. This suggests that vegetation decline caused by environmental changes, such as climate warming, land use change, and increased herbivore density, may deteriorate soil ecosystem functions through aboveground–belowground interactions.