Soil warming decreases inorganic and dissolved organic nitrogen pools by preventing the soil from freezing in a cool temperate forest

We report on the seasonal responses of soil nitrogen (N) pools to soil warming in a cool temperate forest where mild freeze–thaw cycles occur during winter. Artificial soil warming of 2–5 °C was implemented to prevent freezing, making it possible to evaluate the effects of soil freezing on soil characteristics. At control sites, the dissolved organic N and NH4–N pools were largest in winter. Soil warming decreased these solute pools to 17–25% of control levels during winter, but not in other seasons. These results confirm that soil freezing is the driving force of N dynamics during winter, and is easily lost by a few degrees of warming at this study site. The substantial reduction of solute N pools may reduce N availability in the cool temperate forest.