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

Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (<0.5kgha^-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 11years of experimentally increased deposition (32 versus 8kgNha^-1y^-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 32kgha^-1y^-1 in both N forms did not affect N₂ fixation rates for any of the moss species and corresponded to an additional input of 5kgNha^-1y^-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.