Differed Adaptive Strategies to Nutrient Status between Native and Exotic Mangrove Species

To rapidly rehabilitate mangrove forests, exotic mangrove species featured by high growth rates have been introduced in China, which would undoubtedly affect the nutrient status, nutrient acquisition and utilization strategies of mangrove plants but the mechanism remain unclear. Qi'ao island (Zhuhai city suburb) has the largest continuous exotic mangrove forests in China, where a mass collection of mangrove soils, plant tissues and tidewater were conducted. Ecological stoichiometric ratios and isotopic compositions were then analyzed to evaluate the ecosystem-scale nutrient status, and compare the nutrient acquisition and utilization strategies of native Kandelia obovate (KO) and exotic Sonneratia apetala (SA) species. Soil and foliar C:N:P stoichiometries indicated that there are high P availabilities but N limitation, while further isotopic evidence indicated that native KO and exotic SA responded differently to the N limitation status. First, native KO seemed to prefer NO3- , while exotic SA preferred NH4+ , according to Δ15Nleaf-root (leaf-root δ15 N difference) as well as relationships between foliar δ15N and soil extract NH4+ δ15 N or between N and heavy metal contents. This suggested possible inter-specific competition between native KO and exotic SA, leading to different N species preferences to maximize resource utilization. Next, native KO likely adopted the“conservative”strategy with reduced investments in N-rich growth components but root systems leading to lower growth rates and higher N use efficiency (NUE) and intrinsic water use efficiency (iWUE), while exotic SA advocated the“aggressive”strategy with heavy investments in N-rich growth components leading to rapid growth and lower NUE and iWUE, showing signs of invasiveness. Further, native KO is more responsive to aggravated N limitation by enhancing NUE. This study will provide insights into the adaptation of different mangrove species to nutrient limitation and the risks associated with large-scale plantation of exotic mangrove species