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

Abstract

Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (<0.5kghay). An important additional N source is N 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, NH, versus nitrate, NO) 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 8kgNhay) of either NH or NO on N accumulation in three moss and one lichen species (Sphagnum capillifolium, S. papillosum, Pleurozium schreberi and Cladonia portentosa), N 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 NH. In contrast, NO addition resulted in enhanced peat N mineralization linked to microbial NO reduction, increasing soil pH, N concentrations and N losses via denitrification. Unexpectedly, increased deposition from 8 to 32kghay in both N forms did not affect N fixation rates for any of the moss species and corresponded to an additional input of 5kgNhay 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.

Microbial nitrogen fixation and methane oxidation are strongly enhanced by light in Sphagnum mosses

“…In addition, we supplied P to the mesocosms via injection just below the moss surface (capitula). Although this way of P-fertilization is dissimilar to other studies (Limpens et al 2004;Fritz et al 2012;van den Elzen et al 2017), it resembles the way increased P fluxes enter peat ecosystems in reality. P-fertilization is in most studies sprinkled on top of the mosses or added in the inflowing surface water.…”

“…For leguminous plant species it has recently been reported that the plants modulate their carbon allocation to symbiotic nitrogen-fixing Rhizobia in response to both light an N availability (Friel and Friesen 2019). For N 2 fixation associated with Sphagnum, it has been observed before that N 2 fixation rates are highest under illuminated conditions (Larmola et al 2014;van den Elzen et al 2017). This is true even though N 2 fixation is inhibited by presence of O 2 (Vitousek et al 2013).…”

“…P-fertilization is in most studies sprinkled on top of the mosses or added in the inflowing surface water. These often resulted in a clear P fertilization effect (Fritz et al 2012;van den Elzen et al 2017). Most of the surface water entering a peatland, does not reach the capitulum level but only reaches the lower part of the moss.…”

“…Short-term high N-fertilization has been shown to result in reduced N 2 fixation activity in peatland Sphagnum mosses (Kox et al 2016) as well as in forest mosses (Leppänen et al 2013). Yet, long-term N-fertilization has been shown to hardly affect N 2 fixation in Sphagnum (Kox et al 2016;van den Elzen et al 2018). P-fertilization, on the other hand appears to stimulate N 2 fixation activity of the associated microorganisms, but not necessarily Sphagnum moss growth (Fritz et al 2012;van den Elzen et al 2017;Rousk et al 2017).…”

“…The diurnal rhythm of Sphagnum mosses influences the conditions in and around the mosses. Although there is little specific literature available on this diurnal effect on the Sphagnum microbiome, it is highly likely that the daily light regime affects the Sphagnum-associated microbiome (Larmola et al 2014;van den Elzen et al 2017).…”