Reproduction consumes enormous resources, particularly in masting plants. Masting refers to intermittent and synchronous production of large seed crops and is accepted to be caused by resource storage depletion in masting years. However, the latest studies demonstrated that seed production relies exclusively on current-year photo-assimilate and not on stored carbohydrate. Here, we estimate to what extent seed production relies on nitrogen (N) storage and uptake from soil by tracing seasonal variation of ¹⁵N labeled N in fruiting and non-fruiting mature trees of Fagus crenata. The ¹⁵N (NH₄Cl) containing water (equivalent to 10% of soil ammonium N) was sprayed to the soil surface around six trees after leaf unfolding.

N concentration in neither leaves nor branches differed between fruiting- and non-fruiting trees. However, ¹⁵N abundance differed between the two trees. In current-year shoots practically, ¹⁵N was significantly higher early in the growing season in fruiting trees, suggesting additional uptake from the soil in response to stronger sink strength due to additional burden of fruits. The amount of newly incorporated ¹⁵N to seeds suggested that compensatory N uptake from the soil contributed greatly to masting events.