Physiological and morphological effects of long-term ammonium or nitrate deposition on the green and red (shade and open grown) Sphagnum capillifolium

Sphagna are vulnerable to enhanced nitrogen (N) deposition. This article reports how the green (shade, under Calluna) and red (open grown) Sphagnum capillifolium respond to ammonium and nitrate additions of 56 kg N ha⁻¹ y⁻¹ over the background of 8-10 kg N ha⁻¹ y⁻¹ on an ombrotrophic bog in the Scottish Borders after seven years. Samples and measurements were made during a range of hydrated and desiccated conditions in the summer of 2009. Both ammonium and nitrate increased moss N concentration, but while ammonium decreased cross-sectional area of leaf hyaline cells and the leaf hyaline/chlorophyllose cell area ratio, nitrate increased both of them and capitulum pH. The changes in leaf morphology have not previously been reported to our knowledge. Especially the red S. capillifolium was affected by ammonium with significant changes in shoot N concentration (+71%) and the cross-sectional area of leaf chlorophyllose cells (+67%), and reductions in shoot dry weight (−30%) and fresh weight (−42%), the cross-sectional area of leaf hyaline cells (−24%), the leaf hyaline/chlorophyllose cell area ratio (−54%), as well as in chlorophyll fluorescence (measured as F_v/F_m) of desiccated capitulum (−65%) (all p < 0.05). These observations show that N deposition may affect moss physiology also through changes in leaf anatomy and morphology. The results also highlight potential sampling issues and causes of variability in N responses when collecting variably pigmented Sphagna.