Quantifying Nitrogen Uptake Preferences in Mature Tropical Plantation Trees with an In Situ Whole-Tree Paired 15N Labeling Method

The nitrogen (N) uptake preferences of mature tropical trees remain poorly understood, largely because traditional hydroponic methods fail to adequately simulate field conditions. To address this, we applied an in situ whole-tree paired 15N labeling experiment to quantify N acquisition strategies in three common species of tropical plantations in southern China: Hevea brasiliensis, Pinus caribaea, and Acacia mangium. The in situ whole-tree paired 15N labeling experiment revealed distinct species-specific nitrogen uptake strategies: Hevea brasiliensis preferentially absorbed nitrate (contributing 76% to total N uptake), Pinus caribaea relied more heavily on ammonium (61%), while Acacia mangium exhibited no strong preference for either N form but demonstrated the highest N uptake rate. These findings indicate the significant role of mycorrhizal type and biological nitrogen fixation in shaping N uptake patterns. Importantly, even when accounting for the dilution by the soil nitrogen pool, nitrate still contributed 42–99% of the total nitrogen uptake across the three tree species. All three species showed a substantial capacity for nitrate assimilation, challenging the conventional view of ammonium dominance in tropical trees and providing a mechanistic basis for refining nitrogen management practices in plantation forestry.