LAUSR

The present study aimed to explore the possible functions of radial oxygen loss (ROL) on mangrove nutrition. A field survey was conducted to explore the relations among ROL, root anatomy and leaf N in different mangrove species along a continuous tidal gradient. Three mangroves with different ROL (Avicennia marina > Kandelia obovata > Rhizophora stylosa) were then selected to further explore the dynamics of N at root-soil interface. The results showed that seaward pioneer mangrove species such as A. marina appeared to exhibit higher leaf N despite growing under worse nutrient conditions. Greater leaf N in pioneer mangroves coincided with their special root structure (e.g., high porosity together with a thin lignified/suberized exodermis) and powerful ROL. An interesting positive relation was observed between ROL and leaf N in mangroves. Moreover, the data from rhizo-box further showed that soil nitrification was also strongly correlated with ROL. A. marina, which represented the highest ROL among the three mangrove species studied, consistently possessed the highest NO3-, nitrification and ammonia-oxidizing bacteria and archaea (AOB and AOA) gene copies in the rhizosphere. Besides, both NO3- and NH4+ influxes were found to be higher in the roots of A. marina when compared to those of K. obovata and R. stylosa. In summary, greater N acquisition by pioneer mangroves such as A. marina was strongly correlated with ROL which would regulate N transformation and translocation at root-soil interface. The implications of this study may be significant in mangrove nutrition and the mechanisms involved in mangrove zonation.