Nutrient additions in pristine Patagonian Sphagnum bog vegetation: can phosphorus addition alleviate (the effects of) increased nitrogen loads

Abstract

Sphagnum-bog ecosystems have a limited capability to retain carbon and nutrients when subjected to increased nitrogen (N) deposition. Although it has been proposed that phosphorus (P) can dilute negative effects of nitrogen by increasing biomass production of Sphagnum mosses, it is still unclear whether P-addition can alleviate physiological N-stress in Sphagnum plants. A 3-year fertilisation experiment was conducted in lawns of a pristine Sphagnum magellanicum bog in Patagonia, where competing vascular plants were practically absent. Background wet deposition of nitrogen was low (≈ 0.1-0.2 g · N · m(-2) · year(-1)). Nitrogen (4 g · N · m(-2) · year(-1)) and phosphorus (1 g · P · m(-2) · year(-1)) were applied, separately and in combination, six times during the growing season. P-addition substantially increased biomass production of Sphagnum. Nitrogen and phosphorus changed the morphology of Sphagnum mosses by enhancing height increment, but lowering moss stem density. In contrast to expectations, phosphorus failed to alleviate physiological stress imposed by excess nitrogen (e.g. amino acid accumulation, N-saturation and decline in photosynthetic rates). We conclude that despite improving growth conditions by P-addition, Sphagnum-bog ecosystems remain highly susceptible to nitrogen additions. Increased susceptibility to desiccation by nutrients may even worsen the negative effects of excess nitrogen especially in windy climates like in Patagonia.

Effects of nitrogen fertilization on diazotrophic activity of microorganisms associated with Sphagnum magellanicum

“…Sphagnum magellanicum samples were collected from two geographical locations: (1) a low N deposition site located in the Andorra valley mire, Tierra del Fuego, Argentina (<0.5 kg N ha −1 year −1 , Fritz et al 2012) and (2) a high N deposition site, the Dwingelderveld, Dwingeloo, The Netherlands (>25 kg N ha −1 year −1 for more than 30 years; Lamers et al 2000;Patberg 2011; for details see Table 1). The pristine bog vegetation of the Tierra del Fuego site was dominated by S. magellanicum interspersed with few vascular plants (cover <2 %) such as Empetrum rubrum, Nothofagus antarctica, Carex magellanica, Marsipospermum grandiflorum.…”

“…From the Argentinean field site (low N deposition), S. magellanicum was collected from former experimental plots (Fritz et al 2012). The plots had been treated from 2008 to 2010 with increased levels of either P (P; 10 kg P ha −1 year −1 , as NaH 2 PO 4 d i s s o l v e d i n b o g w a t e r ) o r n i t r o g e n ( N ; 40 kg N ha −1 year −1 , as NH 4 NO 3 dissolved in bog water; see Table 2 for timeline and Fritz et al (2012) for further details). As a control, unfertilized mosses from the same site were collected at the same time.…”

“…Moreover, high N availability in the form of NH 4 + ultimately could become toxic to wetland plants, especially under P-limited conditions ). Yet, when phosphorus (P) availability is increased simultaneously, it is thought that the negative physiological effects that plants experience under high N loads can be alleviated, due to stimulation of primary production Fritz et al 2012), and that also their microbial community might benefit directly from non-limiting P-supply.…”

“…Next, P is known to be a directly limiting factor for diazotrophy and often becomes the limiting factor for primary production with excess N input (Vitousek et al 2002), however, it is not known whether indirect P availability, in the form of high P-content in plant-tissue, has the same effect. Therefore, we hypothesized that (3) high indirect P availability to stimulate N 2 fixation, and to alleviate the negative effects of N deposition (Smith and Bennett 1999;Fritz et al 2012). We expected (4) N 2 fixation to be induced by limited N availability, either by absolute N limitation or by relative N limitation, (i.e.…”

Long-term macronutrient stoichiometry of UK ombrotrophic peatlands

“…A key characteristic of ombrotrophic bogs is that N, P and other elements (e.g., Ca, K, Mg) vital to their biogeochemical functioning (Bridgham et al, 1998;Bubier et al, 2007;Damman, 1986) and plant assemblage structure (Baker and Boatman, 1990;Fritz et al, 2012;Gotelli et al, 2008) are almost exclusively supplied via the atmosphere (Damman, 1990;Kellogg and Bridgham, 2003;Malmer, 1988). Such inputs of N and P are estimated to be low (Tipping et al, 2014;Turunen et al, 2004), suggesting that their availability should limit peat bog primary production (Schlesinger and Bernhardt, 2013), an effect that has been demonstrated experimentally (Aerts et al, 1992;Aerts et al, 2001;Olid et al, 2014).…”