Previous research has shown that stand density reduction by thinning induce a decline in tree leaf litter quality and decomposability of sessile oak (Quercus petraea). The aim of this research project is to investigate how this plasticity in oak litter traits affect forest ecosystem functioning related to soil nitrogen (N) cycling, along with tree N acquisition and competition with understory plants. Combining an in situ study based on a national-scale experimental network with ex situ experiments and using innovative tools such as isotope labelling and metabarcoding, we will test the two following hypotheses: (i) the decline in leaf litter quality related to plastic responses of trees to stand density reduction induces a slowdown of soil N cycling, i.e. inhibiting N mineralization and promoting the dominance of organic N cycling associated with ectomycorrhizal (ECM) fungi; (ii) this slower soil N cycling promotes the fitness of ECM tree seedlings by improving their N acquisition at the expense of competing understory plants. The overall idea is that this decline in leaf litter quality by plasticity could be an adaptive strategy of ECM tree species consisting in short-circuiting nutrient cycling through plant-litter-soil feedbacks. This would preserve their pre-empted nutrient pool of the capture by competing decomposer microbes and understory plants in the context of forest regeneration occurring in treefall gaps or low stand density. Our project will be one of the first attempts to study the importance of phenotypic plasticity in litter quality for biotic interactions, including both plant-soil and plant-plant interactions. It will also allow to better understand the consequences of forest management adaptation to climate change (e.g. stand density reduction by thinning intensification to enhance stand resistance to drought) for the functioning and natural regeneration of forest ecosystems.