Mangrove Isotopic (δ 15 N and δ 13 C) Fractionation across a Nitrogen vs. Phosphorus Limitation Gradient

Abstract

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 (Յ1.5 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 ␦ 15 N decreased from ϩ0.10‰ (fringe) to Ϫ5.38‰ (dwarf). The ␦ 15 N of N-fertilized trees also varied spatially, but the values were consistently more negative (by ϳ3‰) compared to control trees. Spatial variation in ␦ 15 N values disappeared when the trees were fertilized with P, and values averaged ϩ0.12‰, similar to that in control fringe trees. Neither variation in source inputs nor microbial fractionation could fully account for the observed patterns in ␦ 15 N. The results instead suggest that the lower ␦ 15 N 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 ␦ 13 C was unaffected by fertilization, values increased from fringe (Ϫ28.6‰) to transition (Ϫ27.9‰) to dwarf (Ϫ26.4‰) 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.

Annual cyclicity in high‐resolution stable carbon and oxygen isotope ratios in the wood of the mangrove tree Rhizophora mucronata

“…In general, a decrease in soil water potential will lead to an increase in the d 13 C value (Guy, Reid & Krouse 1980). Several studies have further demonstrated a relationship between mangrove growth forms (expressed in tree height) and carbon isotopic composition of leaves, with tall mangrove stands showing lower d 13 C values than scrub mangroves (Lin & Sternberg 1992b;Kao & Chang 1998;McKee et al . 2002).…”

“…2002). Although, the growth forms are the result of a synergistic effect of different stress factors, such as soil salinity, sulphide concentration and nutrient levels (Lin & Sternberg 1992a;McKee et al . 2002), only salinity affected the c i / c a .…”

Proceedings of the Smithsonian Marine Science Symposium

“…Several important contributions to an understanding of nutrient production, cycling, limitation, effects on organisms, and related ecophysiological phenomena have already been published (Feller et al, 2002 Lovelock et al, 2006; Cheeseman and Lovelock, 2004), but many more are expected in the near future now that the fi eldwork phase of the Biocomplexity Program has come to a conclusion. One surprising discovery, in a complementary project by CCRE postdoctoral fellow Amy Erickson, was that the intertidal tree crab Aratus, long thought to be a mangrove leaf eater, actually prefers an animal diet if given a choice (Erickson et al, 2008).…”

New perspectives on an iconic landscape from comparative international long‐term ecological research

“…However, rather than solely a function of marine P inputs, very high precipitation has resulted in a raised bog in the interior wetland, resulting in P limitation. Interestingly, increased P limitation occurring from the coast to the interior of Panama appears to be a key driver of plant and bacterial community structure, suggesting this upside-down phenomenon is not restricted to carbonate wetlands, but perhaps a more common feature of subtropical and tropical coastal wetlands (Anderson 1983, Bridgham and Richardson 1993, Chen and Twilley 1999, Paludan and Morris 1999, McKee et al 2002. These results suggest that the shift towards P limitation found in a coastal Panama peatland was a developmental characteristic associated with ombrotrophic conditions that developed as hydrologic inputs became dominated by precipitation (Anderson 1964(Anderson , 1983.…”