High soil microbial activity in the winter season enhances nitrogen cycling in a cool-temperate deciduous forest

The soil microbial community plays essential roles in bioavailable nitrogen (N) cycling, and microbial dynamics during the plant dormant season could affect N cycling in the subsequent growing season. Despite frequent observations of seasonal shifts in microbial community composition in forests, the biogeochemical consequences for bioavailable N cycling have not been well characterized. Here we examine the relationship between microbial community dynamics and bioavailable N dynamics throughout a year in a cool-temperate deciduous forest, with a focus on the dormant season. Specifically, we analyzed temporal changes in abundances of N-cycling microbial populations and N concentrations and transformations in soils. We also assessed temporal changes in bacterial community composition by deep sequencing of 16S rRNA genes. Even though the bacterial community composition was stable throughout the year, we found a drastic increase in abundances of total bacterial and fungal populations in the mid-winter, followed by a subsequent decrease during the snowmelt period in the early spring. We also found a temporary increase in abundances of NH3-oxidizing bacterial population in the mid-winter and of denitrifying bacterial and fungal populations during the snowmelt period. Correspondingly, soil NH4+, NO3−, and dissolved organic N concentrations sequentially peaked in the winter and early spring. These results suggest that successive growth of litter degraders, ammonifiers, nitrifiers, and denitrifiers in the dormant season drives the subsequent bioavailable N transformations. High microbial N metabolic activities during the dormant season could also support plant growth during the growing season and increase the efficiency of annual N cycling in the forest via the seasonal partitioning of N between plants and microbial communities.