Effects of airborne ammonium and nitrate pollution strongly differ in peat bogs, but symbiotic nitrogen fixation remains unaffected

Highlights: • N₂ fixation of moss symbionts is not down-regulated by increased N deposition. • Ammonium N deposition leads to N stress response in keystone spp. of bogs. • Nitrate N deposition, in contrast, leads to increased peat N mineralization. • Differential N effects on bog ecosystem functioning should be taken into account. Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (< 0.5 kg ha⁻¹y⁻¹). An important additional N source is N₂ fixation by symbiotic microorganisms (diazotrophs) in peat and mosses. Although the effects of increased total airborne N by anthropogenic emissions on bog vegetation are well documented, the important question remains how different N forms (ammonium, NH₄⁺, versus nitrate, NO₃⁻) affect N cycling, as their relative contribution to the total load strongly varies among regions globally. Here, we studied the effects of 11 years of experimentally increased deposition (32 versus 8 kg N ha⁻¹ y⁻¹) of either NH₄⁺ or NO₃⁻ on N accumulation in three moss and one lichen species (Sphagnum capillifolium, S. papillosum, Pleurozium schreberi and Cladonia portentosa), N₂ fixation rates of their symbionts, and potential N losses to peat soil and atmosphere, in a bog in Scotland. Increased input of both N forms led to 15–90% increase in N content for all moss species, without affecting their cover. The keystone species S. capillifolium showed 4 times higher N allocation into free amino acids, indicating N stress, but only in response to increased NH₄⁺. In contrast, NO₃⁻ addition resulted in enhanced peat N mineralization linked to microbial NO₃⁻ reduction, increasing soil pH, N concentrations and N losses via denitrification. Unexpectedly, increased deposition from 8 to 32 kg ha⁻¹ y⁻¹ in both N forms did not affect N₂ fixation rates for any of the moss species and corresponded to an additional input of 5 kg N ha⁻¹ y⁻¹ with a 100% S. capillifolium cover. Since both N forms clearly show differential effects on living Sphagnum and biogeochemical processes in the underlying peat, N form should be included in the assessment of the effects of N pollution on peatlands.