In order to prove if resource depletion is the proximate mechanism of mast seeding, we trace seasonal and inter-annual variations in nitrogen (N) concentration and estimate the N storage pool of individuals after full masting of Fagus crenata in two stands. In 2005, a full masting year, the amount of N in fruit litter represented half of the N present in mature leaves in an old stand and was about equivalent to the amount of N in mature leaves in a younger stand. Due to this additional burden, both tissue N concentration and individual N storage decreased in 2006; this was followed by significant replenishment in 2007. These results indicate that internal storage may be important and N may be the limiting factor for fruiting. In the 4 years following full masting, the old stand experienced two moderate masting events separated by two years, whilst trees in the younger stand did not fruit. This different fruiting behavior may be related to different “costs of reproduction” in the full masting year, thus providing more evidence that N may limit fruiting. This study suggests that masting affects N cycle dynamics in mature F. crenata and N may be one factor limiting fruiting.