Beach-cast seagrass material fertilizes the foredune vegetation of Mediterranean coastal dunes

Stable nitrogen isotopes were used to assess the relevance of debris from the seagrass Posidonia oceanica as a source of inorganic nutrients for the members of three functional groups of plants (camephytes, geophytes, and C3 perennial grasses) in three coastal dune systems of Minorca (Balearics Archipelago, western Mediterranean). The δ15N of foredune camephytes and geophytes was usually higher than that of atmospheric nitrogen and often was not statistically different from that of P. oceanica debris. Conversely, the δ15N of camephytes and geophytes collected in hinddunes and matorral areas was not statistically different from that of atmospheric nitrogen and usually lower than that of P. oceanica. Two-way ANOVA revealed a strong and significant effect of habitat on the δ15N of camephytes and geophytes, without any significant effect of the functional type or a significant interaction term. Although the δ15N of the C3 perennial grasses from the foredunes was never statistically different from that of beach-cast seagrass material and that of perennial C3 grasses from the matorral was always significantly lower than that of beach-cast seagrass material, Student's t-test was unable to reveal statistically significant differences between the C3 grasses from the foredunes and the matorral, due to the high standard deviation. The foliar nitrogen contents of foredune plants other than grasses were higher than those of plants from the hinddune and matorral sites, and the foliar C:N ratios were much lower. However, the differences between the grasses growing in the foredunes and in the matorral were not statistically significant for foliar N content or the foliar C:N ratio. On the whole, this evidence strongly supports the hypothesis that beach-cast P. oceanica material is a relevant source of nitrogen for the vegetation of Mediterranean foredunes.