Amino acid accumulation and growth of Sphagnum under different levels of Ndeposition

Nitrogen (N) is a critical nutrient for Sphagnum mosses dominating mire eco systems. We simulated N deposition hy adding doses of NH4NO3 (0, 1, 3, 5 an d 10 g m(-2) yr(-1)) to two Swedish mires with different levels of backgrou nd atmospheric N deposition, i.e., on Luttumyren in central Sweden 0.3-0.4 g N m(-2) yr(-1) and 0.7-1.1 g N m(-2) yr(-1) on Akhultmyren in south Swede n. After two years of NH4NO3 additions, free amino acid concentrations of S . fuscum, S, magellanicum and S. rubellum from the two nines were analyzed and length growth of the mosses were measured. N additions increased amino acid concentrations in Sphagnum capitula, whereas II decreased Sphagnum len gth growth. In general, we found that when Sphagnum amino acid N concentrat ions exceeded 2.0 mg amino acid N g(-1) dry mass, Sphagnum length growth wa s reduced. The decreased growth did not explain the variation in amino acid concentrations. Hence, increased Sphagnum N assimilation in N treated plot s was most likely the factor causing tissue amino acid concentrations to in crease. Significant differences among control plots between the two mires i n Sphagnum total amino acid N concentrations did not occur. Total amino aci d N concentrations of Sphagnum are thus not sensitive enough to reflect dif ferences in N deposition rates when they are below 1.0 g m(-2) yr(-1).