Reproduction affects partitioning between new organs of a pulse of 15N applied during seed ripening in Fagus crenata

Abstract

Seasonal internal nitrogen (N) cycling is an important strategy for trees to uncouple growth from N acquisition. While N uptake, allocation and storage has been intensively studied in association with leaf phenology and vegetative growth, influence of reproduction on these key processes is still poorly understood. Therefore, we applied pulse 15N labelling to three fruiting and three non-fruiting trees in a 92-year-old Fagus crenata forest on 18 July and traced 15N content per organ dry mass (15Nexcess) in all new shoot organs from the upper crowns periodically until leaf fall. The amount of 15Nexcess recovered in the whole new shoots in fruiting trees did not differ from non-fruiting individuals, although up to 70% of 15Nexcess was recovered in fruits of fruiting trees but 87% in leaves of non-fruiting individuals. In addition, dramatic increase in 15Nexcess amount in nuts was accompanied by about twofold increase in nut N content. These results indicate that new N uptake from the soil contributed greatly to seed ripening, which in turn resulted in less allocation to leaves in fruiting trees. In non-fruiting individuals, on the other hand, 15Nexcess allocated to leaves was not accompanied by concomitant increase in leaf N content because biomass growth had ceased when 15N was applied. These results suggest that N uptake in the late growing season contributed to internal N storage in non-fruiting trees. These reproduction-related variations in seasonal N cycle have implications for N dynamics in the plant–soil system during environmental change.