Soil freeze-thaw with root litter alters N transformations during the dormant season in soils under two temperate forests in northern Japan

•Effect of freeze-thaw on N dynamics via increase in root litter was investigated.•Net nitrification and N mineralization were reduced by snow removal.•Response of DOC dynamics was different between soil under oak and larch.•Root litter reduced differences in net N mineralization under different vegetation.

Extreme soil freeze-thaw regimes caused by winter climate change can alter nitrogen (N) transformations through plant root mortality. In this study, we conducted a snow removal experiment to simulate extreme soil freeze-thaw to understand the effect of soil freeze-thaw on N transformations in soils under two northern temperate forests (oak and larch) in northern Japan. We also investigated the effect of root input on microbial biomass and transformations of dissolved organic carbon (DOC) and N by experimentally adding root litter to obtain mechanistic insight. Snow removal significantly reduced net nitrification and N mineralization, while it increased net ammonification in both soils. Root litter addition provided contrasting effects of vegetation type on net DOC change; it significantly decreased net DOC production in the oak soil, while it was increased in the larch soil. The contrasting responses of DOC change with vegetation were possibly related to specific microbial composition and physiology of the vegetation. Further, root addition eliminated the differences in net N mineralization and microbial biomass between the two vegetation types in the snow removal plots. Our results indicated that soil freeze-thaw could reduce the differences in vegetation type on soil N dynamics through root litter input.