Molecular-level traits of root exudates in tropical forest trees reflect nitrogen-fixation strategy and phenological shifts.

Root exudates are central to plant-soil-microbe interactions, yet their seasonal chemical dynamics and variation among tree types remain poorly understood in tropical forests with abundant nitrogen (N)-fixing plants. Here, we applied spectroscopic analysis combined with ultra-high-resolution mass spectrometry to characterize root exudates from four tropical tree species, including two N-fixers (Albizia lucidior and Erythrina subumbrans) and two non-N-fixers (Betula alnoides and Castanopsis echinocarpa), across four time points spanning the dry-to-wet season transition. Exudates contained both protein-like and humic-like fluorescent components, with abundant highly unsaturated and phenolic molecules. N-fixers, compared with non-N-fixers, released exudates with lower aromaticity and higher proportions of amino acids and peptides, reflecting enhanced N metabolism linked to the ability of the plants to acquire N directly through fixation. All species exhibited marked seasonal variability in exudate chemistry, with the greatest interspecific dissimilarity occurring during the rapid-growing period in the rainy season (July), likely resulting from the increased nutrient demand. Our findings highlight the significant roles of plant N acquisition strategies and phenological stages in shaping the molecular composition of root exudates in tropical forests. Such interspecific and seasonal variations likely influence rhizosphere microbial dynamics and nutrient cycling, underscoring the ecological significance of root exudate plasticity in forest ecosystems.