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

van den Elzen, Eva; van den Berg, Leon J.L.; van der Weijden, Bas; Fritz, Christian; Sheppard, Lucy J.; Lamers, Leon P.M.. 2018 Effects of airborne ammonium and nitrate pollution strongly differ in peat bogs, but symbiotic nitrogen fixation remains unaffected. Science of the Total Environment, 610-611. 732-740. https://doi.org/10.1016/j.scitotenv.2017.08.102

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Abstract/Summary

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− 1 y− 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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1016/j.scitotenv.2017.08.102
UKCEH and CEH Sections/Science Areas:	UKCEH Fellows
ISSN:	0048-9697
Additional Keywords:	Sphagnum capillifolium, nitrogen deposition, biogeochemical processes, diazotrophs, amino acids
NORA Subject Terms:	Ecology and Environment
Date made live:	23 Nov 2017 12:26 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/518487

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