Radial oxygen loss is correlated with nitrogen nutrition in mangroves

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 [A. marina], Kandelia obovata and Rhizophora stylosa) were then selected to further explore the dynamics of N at the root-soil interface. The results showed that seaward pioneer mangrove species such as A. marina appeared to exhibit higher leaf N despite growing under poorer 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, rhizo-box data further showed that soil nitrification was also strongly correlated with ROL. A. marina, which had the highest ROL among the three mangrove species studied, consistently possessed the highest levels of NO3-, nitrification and ammonia-oxidizing bacteria and archaea 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 the root-soil interface. The implications of this study may be significant in mangrove nutrition and the mechanisms involved in mangrove zonation.