Nutritional constraints in ombrotrophic Sphagnum plants under increasing atmospheric nitrogen deposition in Europe.

•  We studied the effects of increasing levels of atmospheric nitrogen (N) deposition on nutrient limitation of ombrotrophic Sphagnum plants. •  Fifteen mires in 11 European countries were selected across a natural gradient of bulk atmospheric N deposition from 0.1 to 2 g/m2  year-1 . Nutritional constraints were assessed based on nutrient ratios of N, phosphorus (P), and potassium (K) in Sphagnum plants collected in hummocks (i.e. relatively drier microhabitats) and in lawns (i.e. relatively wetter microhabitats). •  Nutrient ratios in Sphagnum plants increased steeply at low atmospheric N input, but above a threshold of N deposition of c. 1 g/m2  year-1 the N : P and N : K ratios tended to saturation. Increasing atmospheric N deposition was also accompanied by a reduced retention of Ca and Mg in Sphagnum plants and a decreased stem volumetric density in hummock Sphagnum plants. •  We suggest a critical load of N deposition in Europe of 1 g/m2  year-1 above which Sphagnum plants change from being N-limited to be K + P colimited, at N : P > 30 and N : K > 3.