Unusually negative nitrogen isotopic compositions (δ¹⁵N) of mangroves and lichens in an oligotrophic, microbially-influenced ecosystem

Abstract. Extremes in δ¹⁵N values in mangrove tissues and lichens (range =+4 to −22‰) were measured from a mangrove forest ecosystem located on Twin Cays, offshore islands in Belize, Central America. The N isotopic compositions and concentrations of NH₄⁺/NH₃ in porewater, rainwater, and atmospheric ammonia, and the δ¹⁵N of lichens, mangrove leaves, roots, stems, and wood were examined to study the biogeochemical processes important for establishing these unusual N isotopic ratios. Dwarfed Rhizophora mangle trees had the most negative δ¹⁵N, whereas fringing Rhizophora trees, the most positive δ¹⁵N values. Porewater ammonium concentrations had little relationship to N isotopic fractionation in mangrove tissues. In dwarfed mangroves, the δ¹⁵N of fine and coarse roots were 6–9‰ more positive than leaf tissue from the same tree, indicating different sources of N for root and leaf tissues. When P was added to dwarfed mangrove trees without added N, δ¹⁵N increased within one year from −12‰ to −2‰, approaching the δ¹⁵N of porewater ammonium (δ¹⁵N=+4‰). Isotopically depleted ammonia in the atmosphere (δ¹⁵N=−19‰) and in rainwater (δ¹⁵N=−10‰) were found on Twin Cays. We propose that foliar uptake of these atmospheric sources by P-stressed, dwarfed mangrove trees and lichens can explain their very negative δ¹⁵N values. In environments where P is limiting for growth, uptake of atmospheric N by Rhizophora mangle may be an important adaptive strategy.