Lignin methoxyl hydrogen isotope ratios in a coastal ecosystem

Stable hydrogen isotope ratios of plant lignin methoxyl groups have recently been shown to record the hydrogen isotopic composition of meteoric water. Here we extend this technique towards tracing water source variations across a saltwater to freshwater gradient in a coastal, subtropical forest ecosystem. We measure the hydrogen isotopic composition of xylem water (dDxw) and methoxyl hydrogen (dDmethoxyl) to calculate fractionations for coastal mangrove, buttonwood and hammock tree species in Sugarloaf Key, as well as buttonwoods fromMiami, both in Florida, USA. Prior studies of the isotopic composition of cellulose and plant leaf waxes in coastal ecosystems have yielded only a weak correlation to source waters, attributed to leaf water effects. Here we find dDmethoxyl values range from 230& to 130&, across a 40& range in dDxw with a regression equation of dDmethoxyl & = 1.8 * dDxw 178& (R = 0.48, p < 0.0001, n = 74). This is comparable within error to the earlier published relationship for terrestrial trees which was defined across a much larger 125& isotopic range in precipitation. Analytical precision for measurements of dD values of pure CH3I by gas chromatography–pyrolysis–isotope ratio mass spectrometry (GC–P–IRMS) is r = 6& (n = 31), which is considerably better than for CH3I liberated through cleavage with HI from lignin with r = 18& (n = 26). Our results establish that dDmethoxyl can record water sources and salinity incursion in coastal ecosystems, where variations sufficiently exceed method uncertainties (i.e., applications with dD excursions >50&). For the first time, we also report yields of propyl iodide, which may indicate lignin synthesis of propoxyl groups under salt-stress. 2013 Elsevier Ltd. All rights reserved.