Nitrogen form governs nutrient retention and plant uptake in coral island soils: Evidence from a 15N-tracing experiment

Nitrogen (N) availability critically limits plant productivity in nutrient-depleted coral island ecosystems, necessitating substantial inputs of exogenous N fertilizer. However, excessive or unbalanced fertilization poses risks to environmental sustainability. In this study, we assessed how three N fertilizer forms, ammonium (NH4+-N), nitrate (NO3–-N) and amide nitrogen (NH2-N), affect soil properties and plant performance in coral sand environments. A 15N-labeled greenhouse experiment was conducted using two island-adapted species, Ficus microcarpa and Terminalia catappa. Results showed that NO3–-N markedly enhanced nitrogen retention, microbial biomass nitrogen, and overall plant growth, while NH4+-N promoted microbial biomass carbon. F. microcarpa and T. catappa both exhibited superior growth under NO3–-N, although T. catappa achieved higher leaf nutrient concentrations with NH2-N, reflecting differences in nutrient uptake preferences. Isotopic tracing revealed greater nitrogen retention in soils than in plant tissues, with NO3–-N fertilization yielding the highest nitrogen recovery efficiency. These findings highlight the importance of nitrogen form in shaping soil–plant interactions in sandy, alkaline soils and offer mechanistic insights for designing targeted, sustainable fertilization strategies for coral island ecosystems.