Mangrove islands in Belize are characterized by a unique switching from nitrogen (N) to phosphorus (P) limitation to tree growth from shoreline to interior. Fertilization has previously shown that Rhizophora mangle (red mangrove) fringe trees (5-6 m tall) growing along the shoreline are N limited; dwarf trees (???1.5 m tall) in the forest interior are P limited; and transition trees (2-4 m tall) are co-limited by both N and P. Growth patterns paralleled a landward decrease in soil flushing by tides and an increase in bioavailable N, but P availability remained consistently low across the gradient. Stable isotopic composition was measured in R. mangle leaves to aid in explaining this nutrient switching pattern and growth variation. Along control transects, leaf ??15N decreased from +0.10??? (fringe) to -5.38??? (dwarf). The ??15N of N-fertilized trees also varied spatially, but the values were consistently more negative (by ???3???) compared to control trees. Spatial variation in ??15N values disappeared when the trees were fertilized with P, and values averaged +0.12???, similar to that in control fringe trees. Neither variation in source inputs nor microbial fractionation could fully account for the observed patterns in ??15N. The results instead suggest that the lower ??15N values in transition and dwarf control trees were due to plant fractionation as a consequence of slower growth and lower N demand. P fertilization increased N demand and decreased fractionation. Although leaf ??13C was unaffected by fertilization, values increased from fringe (-28.6???) to transition (-27.9???) to dwarf (-26.4???) zones, indicating spatial variation in environmental stresses affecting stomatal conductance or carboxylation. The results thus suggest an interaction of external supply, internal demand, and plant ability to acquire nutrients under different hydro-edaphic conditions that vary across this tree-height gradient. The findings not only aid in understanding mangrove discrimination of nitrogen and carbon isotopes, but also have implications for identifying nutrient loading and other stress conditions in coastal systems dominated by mangroves.
Additional Publication Details
Mangrove isotopic (??15N and ??13C) fractionation across a nitrogen vs. phosphorus limitation gradient