Soil microbes in temperate forest ecosystems are able to cycle several hundreds of kg of N ha-1 yr-1 and are therefore of paramount importance for N retention. Belowground C allocation by trees is an important driver of seasonal microbial dynamics and may thus directly affect N transformation processes over the course of the year. Our study aimed at unravelling plant controls on soil N cycling in a temperate beech forest at a high temporal resolution over a time period of two years, by investigating the effects of tree girdling on microbial N turnover. In both years of the experiment, we discovered (1) a summer N mineralization phase (between July - August) and (2) a winter N immobilisation phase (November - February). The summer mineralization phase was characterised by a high N mineralization activity, low microbial N uptake and a subsequent high N availability in the soil. During the autumn/winter N immobilisation phase, gross N mineralization rates were low and microbial N uptake exceeded microbial N mineralization, which led to high levels of N in the microbial biomass and low N availability in the soil. The observed immobilisation phase during the winter may play a crucial role for ecosystem functioning, since it could protect dissolved N that is produced by autumn litter degradation from being lost from the ecosystem during the phase when plants are mostly inactive. The difference between microbial biomass N levels in winter and spring equals 38 kg N ha-1, and may thus account for almost one third of the annual plant N demand. Tree girdling strongly affected annual N cycling: the winter N immobilisation phase disappeared in girdled plots (microbial N uptake and microbial biomass N were significantly reduced, while the amount of available N in the soil solution was enhanced). This was correlated to a reduced fungal abundance in autumn in girdled plots. By releasing recently fixed photosynthates to the soil, plants may thus actively control the annual microbial N cycle. Tree belowground C allocation increases N accumulation in microorganisms during the winter which may ultimately feedback on plant N availability in the following growing season. Keywords: beech (Fagus sylvatica) forest, belowground carbon allocation, ectomycorrhiza, nitrogen cycle, nitrogen retention, plant soil interactions, soil microbial community, tree girdling.
