Contrasting effects of ammonium enrichment on fen bryophytes

In the second half of the 20th century, frequency and diversity of brown mosses strongly declined in Dutch fens while species of Sphagnum and Polytrichum increased markedly. We hypothesized that high ammonium (NH4+) concentrations in Dutch precipitation have promoted these species changes. To test this hypothesis, we examined the effects of varying concentrations (1-1000 mu M) of NH4+ on a brown moss (Calliergonella cuspidata), two species of Sphagnum (S. contortum and S. squarrosum) and Polytrichum commune in a hydroponic experiment. A second goal was to identify possible interspecific differences in the mechanisms responsible for NH4+ detoxification. Dry matter production and tissue cation concentrations indicated decreasing sensitivity to NH4+ in the order C cuspidata>S. contortum>S. squarrosum and P. commune. In S. squarrosum and P. commune, the highest NH4+ treatments induced strong increases in tissue nitrogen (N) and amino acid concentrations. Sphagnum contortum showed a similar, but less marked response. In contrast, C cuspidata showed a negative relationship between external NH4+ concentrations and both tissue N and amino acid concentrations (except arginine). Our findings support the hypothesis that the present rates of NH4+ deposition in The Netherlands are detrimental to brown mosses, but not to fast-growing Sphagnum and Polytrichum species. The efficiency of the NH4+ assimilation apparatus, producing N-rich amino acids, probably plays a key role in determining sensitivity to increased NH4+ deposition.